The impact of beachface morphology on habitat suitability and breeding success of least terns and snowy plovers

doi:10.21203/rs.3.rs-5003427/v1

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The impact of beachface morphology on habitat suitability and breeding success of least terns and snowy plovers

https://doi.org/10.21203/rs.3.rs-5003427/v1

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Beach-nesting birds rely on predictable and stable beach conditions for successful breeding, yet the complex impacts of climatic events, wave action, and variation in beach morphology on nest site selection remain underexplored. This study examines how variations in beach morphology influence nest site selection and breeding success in two conservation-priority species —the least tern (Sterna antillarum) and the snowy plover (Anarhynchus nivosus)—at Barra del Estero Beach, Baja California, Mexico. This beach, formed in the mid-to-late 1990s, likely due to alongshore sediment transport from neighboring beaches during energetic El Niño events, reveals that nest abundance correlates strongly with supratidal beach morphology. Both species exhibited significantly higher survival rates when nesting on berms compared to other areas. They presented highest nest densities on stable profiles featuring prominent berms extending approximately 40 m horizontally from the dune toe at elevations exceeding 2.8 m, 0.5 m above the highest spring high-tide level. While berms erode in winter due to high-energy waves, they serve as secure nesting platforms in summer, protecting nests from flooding. Thus, the ability of the beach to rebuild wide berms after the winter is crucial for nesting success. As sea-level rise and extreme weather events increasingly alter beach morphology, species with greater flexibility in nest site selection may more successfully identify higher-quality nesting sites. This research highlights the urgent need for comprehensive solutions to mitigate the growing risk of nest flooding in beach-nesting birds.

Beach berm

ENSO

beach erosion

shorebirds

beach-nesting birds

nest selection

beach morphodynamics

Climate change poses a widespread, rapid, and intensifying threat to coastal ecosystems, with extreme weather events that historically happened once every century potentially becoming an annual occurrence by the end of the 21st century (IPCC, 2021). Sandy beaches are among the most dynamic coastal ecosystems. They are capable of undergoing drastic morphological changes within relatively short time periods, spanning from hours to days, in response to extreme storm events that can, in turn, influence longer term trends extending from seasons to years (Senechal & Ruiz de Alegría-Arzaburu, 2020). A single storm sequence can cause a beach to lose 2 m of sand vertically over a matter of weeks, compromising its recovery time for years and affecting interannual shoreline and sand volume trends (Ruiz de Alegría-Arzaburu et al., 2022). Additionally, sea-level rise has the potential to amplify the erosive impacts of extreme climatic events on exposed coastlines (Harley et al., 2022), potentially altering the ecological processes within coastal ecosystems (Day et al., 2008; Fox-Kemper et al., 2021; Klingbeil et al., 2021; Passeri et al., 2015).

Sandy beaches are habitats of significant ecological importance for biodiversity (Sclader et al. 2007). Beach-nesting species face heightened vulnerability to beachface morphological changes, which can limit nesting areas (Maslo et al., 2016). The suitability of nesting sites during the breeding season depends on an appropriate beachface morphology, which offers adequate conditions to mitigate nest flooding. In turn, beach morphology relies on the capacity of the beach to restore its intertidal and supratidal morphology after winter storms. In the Pacific Ocean, interannual variation in beach morphology is primarily governed by the El Niño Southern Oscillation (ENSO; Vos et al., 2023), with the Eastern Pacific coast experiencing the largest winter waves during the El Niño phase of ENSO and leading to large beachface erosion. As a consequence, the recovery capability of beaches diminishes during the spring and summer seasons following El Niño years, typically needing 3–5 years for a full recovery (Ruiz de Alegría-Arzaburu and Vidal-Ruiz, 2018). For this reason, many beach-nesting species may experience a significant loss of nesting habitat during El Niño years. This holds particular relevance for species displaying high nest-site fidelity, as some individuals may nest within < 2 m of their previous season’s nest (Gonzales-Solis, Wendeln and Becker, 1995). Species with little flexibility may consequently be forced to select suboptimal nesting sites, which can result in nest flooding during years of high erosive conditions (Lauro and Burger, 1989).

Variability in beachface morphology can make the selection of nesting location a critical step to ensure breeding success in beach-nesting species. For instance, sea turtles (e.g., species in the suborder Cryptodira) require a specific beachface slope and elevation for incubating eggs that provides the necessary combination of temperature and moisture for optimal sex ratio determination and hatchling survival (Stokes et al. 2023). Similarly, beach-nesting birds must carefully evaluate nest location, weighing factors such as resource accessibility and risks such as predation, flooding, and overheating (Medeiros et al., 2012). In recent decades, nest failure attributed to flooding events has emerged as a significant driver of reproductive success in beach-nesting species (Denmon et al., 2013; Schulte & Simmons, 2015, Maslo et al., 2016; Van de Pol et al., 2010), surpassing predation in importance in many instances (Sabien et al., 2006; Van de Pol et al., 2010). While the likelihood of nest loss to flooding events can vary depending on the season and site, the combined effects of storm surges, wave setup, and sea level rise are likely to exacerbate this threat (Van de Pol et al., 2010). Yet, our current understanding of how climatic events, beachface morphology, and nest selection influence reproductive success is limited by the absence of long-term, high-resolution topographic data at sites where beach-nesting species are monitored. Access to such datasets would enable conservationists and coastal scientists to collaborate and investigate the role of beachface morphology in mediating nesting habitat availability. Ultimately, this could lead to the development of solutions to enhance breeding conditions for threatened species on sandy beaches in the face of rapid global change.

Here, we investigate the effects of variation in beachface morphology on the nest site selection and reproductive success of two beach-nesting bird species of conservation concern at a site in Baja California, Mexico: least tern (Sternula antillarum) and snowy plover (Anarhynchus nivosus). First, we provide an overview of the long-term morphological variation at the site spanning from 1984–2021, aiming to contextualize historical changes in beachface morphology. Then, we analyze intertidal and supratidal morphological changes along the beach, with a particular focus on seasonal to interannual beachface slope variation in relation to incoming wave energy. Finally, we investigate the role of beachface morphology on nesting habitat availability and nest survival for snowy plovers and least terns. We hypothesize that the morphology of nesting habitats, particularly the presence of a berm, significantly influences the reproductive success of both species. Specifically, we expect that (1) birds selecting higher elevations with the presence of a berm will experience greater reproductive success, as they are less vulnerable to flooding events, and (2) during extreme weather events, wave action will lead to erosion and a steepening of the beachface, reducing nesting habitat and diminishing reproductive success. To the best of our knowledge, this study represents the first comprehensive investigation into the implications of beachface morphological variation on the reproductive success of beach-nesting birds, thereby elucidating the potential consequences of ongoing global change.

Barra del Estero Beach is a ~ 7 km sand barrier that delimits the outer portion of the Punta Banda Estuary within Todos Santos Bay along the Pacific coast of the Baja California, Mexico (Fig. 1). The barrier beach comprises sand dunes where the dominant vegetation is composed of red sand verbena (Abronia maritima), as well as invasive species such as iceplant (Carpobrotus edulis) and sea fig (Carpobrotus chilensis; Johnson, 1977). The inner and outer barrier beach also holds critical ecological importance for a variety of marine mammals, birds, and benthic invertebrates (Maimone-Celorio and Mellink, 2003; Jimenez-Perez et al. 2010). Consequently, it has been designated as a Ramsar Wetland (Ramsar, 2024), a site of regional importance for shorebirds (WHSRN, 2024), and an Important Bird Area (Birdlife International 2024).

Our study area comprises the northernmost 1.3 km of the Barra del Estero Beach, situated adjacent to the mouth of the Punta Banda Estuary (Fig. 1b). This area is notable for hosting a breeding population of both least terns (hereafter, ‘terns’) and snowy plovers (hereafter, ‘plovers’). Terns typically form colonies of dozens to hundreds of nests, exclusively located in the outermost edge of the beach, where they lay eggs in shallow depressions directly on the sand (Fig. 2a,c). In contrast, plovers prefer nesting along the barrier beach, typically selecting locations between the upper intertidal zone and the dune foot (Fig. 2b,d). Their incubation periods vary: terns typically average 19–25 days (Cornwell, 1986), while plovers generally range from 26–32 days (Warriner et al., 1986). In case of nest failure, both species have the ability to re-nest, although the probability of success may diminish as the breading season progresses (Massey and Atwood, 1981).

The Barra del Estero Beach is composed of siliceous medium sand (D₅₀ ∼ 0.2 mm), presents an average gentle beachface slope of ∼ 1/50, and serves as natural barrier against coastal erosion and flooding. The tidal regime is semi-diurnal and mesotidal, with mean tidal range varying between 0.5–2.3 m across neap and spring tides, respectively. The nearshore typically features one to two subtidal sandbars that form during the fall and winter months, migrating onshore during spring and summer, ultimately merging with the intertidal beach (Vidal-Ruiz & Ruiz de Alegría-Arzaburu, 2020). The beach is predominantly exposed to northwesterly swell waves and experiences seasonal erosion in winter and accretion in summer; however, these morphological variations are exacerbated during El Niño conditions (Ruiz de Alegría-Arzaburu & Vidal-Ruiz, 2018). In summer, the predominant southwesterly swell waves are partly sheltered by the Punta Banda headland, located to the south within Todos Santos Bay (Fig. 1a). The offshore annual mean significant wave height (H_s) is 1 m, with an associated spectral peak wave period (T_p) of 11 s. During winter storms, H_s can exceed 4 m, whereas during summer, it typically remains below 1 m under calm conditions (Ruiz de Alegría-Arzaburu et al., 2017, 2022). The wind conditions are characterized by prevailing northwesterly winds of ∼4 ms^− 1 and the presence of sporadic easterly wind events between October and March, known as Santa Ana events, that last 2–6 days with speeds exceeding 10 ms^− 1.

3.1. Topographic surveys 2015–2023

We carried out a total of 82 topographic surveys between August 2015 and August 2023 along Barra del Estero Beach. For this study, we focused on the northernmost transects, labeled T1-T7, which were spaced 200 m apart and covered approximate 1.3 km of beach (Fig. 1a). Data were collected on foot by two people utilizing an RTK-GPS antenna affixed to a two-wheeled trolley, and elevations were relative to the mean low low-tide level (MLLT; +36.135 m from ellipsoidal heights). The measurements covered the intertidal and supratidal beach up to the nearest dune crest, following the methodology outlined by Ruiz de Alegría-Arzaburu et al. (2017).

To analyze variation in beach morphology, Digital Elevation Models (DEMs) were generated for each survey by linearly interpolating transects T1- T7 at 5-m intervals alongshore and 0.1-m in the cross-shore direction. Cumulative differences in DEMs over time provided us with information on the morphological evolution of the site over the 8-year period. Additionally, topographic profiles T1-T7 were individually analyzed to assess variation in high-water shoreline positions (at 2 m relative to from MLLT), as well as changes in supratidal slopes and volumes per survey time. The berm is a morphological feature of the upper beach that forms between spring and summer during low-energy wave conditions and presents a quasi-planar surface that serves as a secure nesting platform, protecting nests from flooding. It typically extends seaward form the dune toe and is often separated by a distinctive change in slope from the steeper beachface (Hine, 1979). The berm erodes during energetic wave conditions and rebuilds during spring and summer, once the sandbar has reconnected with the shoreline (Kono-Martínez et al., 2023). Figure 1c contrasts a typical summer topographic profile with a berm, which provides significant supratidal beach width, and a bermless winter profile, which is much narrower at > 50 m in the cross-shore.

3.2. Long-term shoreline variations 1984–2021

We retrieved a time series of satellite-derived shoreline (SDS) positions from four CoastSat transects for northern Barra del Estero Beach (source: http://coastsat.wrl.unsw.edu.au). The extracted shoreline positions were obtained from the CoastSat toolkit developed by Vos et al. (2019a, 2019b) from publicly accessible optical satellite data via Google Earth Engine, specifically Landsat 5, 7 and 8 (L5, L7, L8) with a spatial resolution of 30 m, as well as Sentinel-2 (S2) images with a finer 10-m spatial resolution. For a comprehensive overview of the CoastSat toolkit, refer to Vos et al. (2019b). In brief, this toolkit utilizes a Neural Network classifier to categorize RGB (+ infrared) satellite images into four distinct classes. Coupled with a global threshold on the Modified Normalized Difference Water Index (MNDWI), it then employs a sub-pixel resolution contouring algorithm to derive a precise waterline from the image.

We retrieved a time series of shoreline deviation from the mean for each transect from the CoastSat-derived shoreline position. CoastSat-derived shorelines were compared against shorelines extracted from the topographic transects T3, T4, T5, and T7 between 2015 and 2021 (see Fig. 1). A total of 82 in-situ shorelines were extracted as the intersection of the beach profile with elevation Z = 1 m (referred to MLLT), a close reference to the local mean tide level (MTL of 0.82 m). Figure 3 shows a comparative time-series of topography-derived shorelines and SDS for the different profiles along the beach (see orange transects in Fig. 1). The time-series shows close agreement between the two datasets (Fig. 3a), a conclusion reinforced by linear correlation analyses, with R² values ranging from 0.61–0.74 for the comparative 6-year period of available overlapping SDS and topographic data (Fig. 3b).

3.3. Wave conditions and ENSO

The extent of beachface morphological change is typically influenced by the amount of wave energy reaching the coast, which, we hypothesize, may affect the quality of nesting habitat for birds. The incident wave energy can be represented as the total wave power, P_t, calculated using the time series of integral wave parameters, such as the spectral peak wave period (T_p), significant wave height (H_s), and mean wave direction (e.g. Kono-Martinez et al., 2023). For this study, the longest available time-series of integral wave parameters was obtained from the closest node (32ºN, 117ºW) of the ERA5 ECMWF global reanalysis model (European Centre for Medium-Range Weather Forecasts; https://cds.climate.copernicus.eu). P_t was estimated by applying linear wave theory as s follows:

$$\:{P}_{t}=nEC$$

with n being the wave group-to-incident celerity ratio, $\:n=\frac{1}{2}(1+\frac{4\pi\:d/L}{sinh4\pi\:d/L}$); E the wave energy density, $\:E=\frac{1}{16}\rho\:g{H}_{s}^{2}$; C the wave celerity, $\:C=\frac{g{T}_{p}}{2\pi\:}tanh\frac{2\pi\:d}{L}$; L the wavelength; and $\:\alpha\:$ the angle between the coastline orientation and the wave direction. Integral wave parameters and wave energy fluxes were averaged between periods of morphological surveys.

The El Niño Southern Oscillation (ENSO) serves as the primary indicator of global climate disruptions along the northeastern Pacific coast, including Baja California (e.g., Ruiz de Alegría-Arzaburu & Vidal-Ruiz, 2018). The ENSO index integrates both atmospheric and oceanic parameters (MEI.v2 version; https://psl.noaa.gov/enso/mei/) to assess the El Niño phase (positive values) or La Niña phase (negative values). During El Niño events, characterized by warmer-than-average sea surface temperatures in the central and eastern Pacific Ocean, changes in atmospheric circulation patterns often result in more energetic winter wave conditions, leading to increased beach erosion in northern Baja California (Vidal-Ruiz & Ruiz de Alegría-Arzaburu, 2020).

3.4. Bird nesting monitoring

We monitored a tern colony located on the northern beach during the breeding season from May to August each year from 2018 to 2022. We visited the colony twice a week and used an intensive search protocol described by Ralph et al. (1996). Incubating adults (Fig. 3a,c) were located by observation or tracking their flight patterns, with their incubation behavior confirmed after they landed again. Similarly, plover nests (Fig. 2d) were monitored in June and July from 2018–2022. These were located by systematically walking along the outer barrier beach and conducting observational sweeps every 100 m using binoculars and telescopes. Plovers were followed until they exhibited signs of incubation behavior and had settled onto a nest (Fig. 2b).

The fate of the nests was monitored while parents foraged. We numbered tern nests using a ~ 30 cm shell marked with an individual number. Location coordinates of all nests were recorded using a handheld GPS. The clutch initiation and hatch dates of plovers were estimated using the egg floatation methodology described by Szekely et al. (2011). The fate of eggs was recorded during subsequent visits, categorizing nests as successful if at least one chick hatched, flooded if the nest disappeared due to tidal inundation, predated if eggs were consumed by predators, abandoned if parents permanently left the nest unattended, or unknown if the reason for nest failure could not be determined.

3.5 Combining beachface morphological changes and bird nest locations

The positions of surveyed bird nests were overlaid onto the DEMs to analyze preferred nesting locations within the study area over the period from 2018–2022. Additionally, we projected nest locations onto the nearest individual beach profile (up to a distance of 50 m) over time to visualize their positions relative to the mean high-water spring (MHWS) level. The analysis of alongshore variations in beach profile characteristics focused on changes in beachface slope, shoreline position, and sand volume.

We calculated beachface slopes for the upper intertidal beach, ranging from the MHWS of 1.62 m up to 2 m of elevation. Higher elevation slope calculations were omitted to avoid interference from the presence of the berm. Time-series of shoreline positions were calculated for 2-m elevation positions, allowing for the quantification of variations in beach width over time. Additionally, supratidal sand volumes were calculated by integrating sand elevation from 2–4 m using the trapezoidal method. These beach profile characteristics were utilized to analyze alongshore variations in the presence or absence of nests over time.

3.6. Assessing the influence of berms on nest survival

To evaluate the hypothesis that nesting on a berm enhances nest survival in inundation-prone areas, we filtered our data to include only successful nests and those that failed specifically due to tidal flooding, thereby excluding all nests that failed due to predation, abandonment, or other unknown causes. We used the ‘nest survival’ procedure in RMark (v.3.0; Laake 2022), a known-fate model designated to estimate the probability of nest survival as a function of a suite of covariates. We implemented a hierarchical approach to assess the influence of beachface morphology and nest elevation on the likelihood a nest failed due to pre-hatch flooding.

Considering the potential impact of species identity on nest survival, we first tested for differences in baseline nest survival rates between terns and plovers. We detected no significant differences (β = 19.36, SE = 18,062.82, CI = -35383.7, 35422.5; Appendix 1); thus, species identity was not included as a variable in subsequent models. We then examined the effects of nest elevation and the presence or absence of a berm on nest survival using both using additive and interactive models. Akaike’s information criterion corrected for small sample sizes (AIC_c) was applied to compare models, with the model yielding the lowest AIC_c score deemed the most well supported model. In cases where models exhibited similar AIC_c scores (i.e., < 2), the simpler model was favored to minimize the inclusion of non-informative parameters (Arnold, 2010; Appendix 1; Table A2). Daily survival rates were calculated using the output of the selected logistic regression model (Appendix 1, Table A3) based on the following formula:

$$\:S\:=\:\frac{\text{exp}({\beta\:}_{0}{+\:\beta\:}_{1\:}\bullet\:{\text{B}\text{e}\text{r}\text{m})}_{}\:}{1+\text{e}\text{x}\text{p}({\beta\:}_{0}+\:{\beta\:}_{1}\bullet\:{\text{B}\text{e}\text{r}\text{m})}_{}}$$

We calculated overall nest survival rates according to the following formula:

$$\:{S}_{Overall}=\:{S}_{d}^{n}$$

where overall survival rate, S_overall, is equal to the daily survival rate, S_d, elevated to the power of the average incubation period, n. Because models incorporated more than one species, we averaged the incubation periods of both species to arrive at a mean n for the two species.

4.1. Morphological evolution of Barra del Estero Beach

The northern section of Barra del Estero barrier beach exhibited significant morphological variation over the past four decades (Fig. 4). In 1985, the northernmost three beach profiles (T1-T3) were nonexistent due to erosion (Fig. 4a); concurrently, a spit was visible at Estero Beach, located further north of the inlet. By 2002, the northern barrier beach had undergone significant morphological growth, possibly linked to the disappearance of the Estero Beach spit (Fig. 4b). The inlet mouth is very dynamic and exhibited substantial morphological variation over time (Fig. 4). The photographic sequence, however, demonstrates that the overall shape of the beach remained largely unchanged from 2002–2022 (Figs. 4c, d, and e).

4.2. Long-term shoreline trends: 1984–2021

Long-term shoreline variation was examined using validated SDS data spanning almost four decades (Fig. 5c). SDS data were extracted for T4, T5, and T7 from 1984–2021, while data for T3 were only available from 1994–2021, since it did not exist before 1994 (see Fig. 4). Thus, the earliest available shoreline data for T3 date back to 1994, marking the onset of the northernmost barrier beach growth. This growth was particularly large during the periods of 1994–1995 and 1997–1998, with the shoreline reaching its maximum width in 2001–2002 (Fig. 5d).

SDS variations (de-meaned raw in Fig. 5c and smoothed Fig. 5d) were compared with the ENSO index (Fig. 5a) and the total wave power (Fig. 5b). These data indicate beach growth (shoreline accretion) between 1984 and the early 2000s (Fig. 4 and Fig. 5c). Since the mid-1990s, the shoreline has exhibited relative stability, with fluctuations of approximately ± 25 m, except for the northernmost section (T3), which experienced variation of about ± 50 m (Fig. 5d). Overall, shorelines responded seasonally to the incoming wave conditions, with advancements and retreats occurring in summer and winter, respectively (Figs. 5); and shoreline retreats were generally larger during intense El Niño events (positive ENSO index).

4.3. Morphological beach changes and nest choice

The morphological changes of the beachface were examined during the months of June and July, periods with available nesting data for both terns and plovers. From 2018–2022, we monitored 411 tern nests and 69 plover nests along transects T1 to T4. Of these, 28% of the nests were successful, 18% failed due to tidal flooding and 54% failed due to predation, abandonment, or unknown causes. Significant variation in beachface morphology was observed between these two months from 2018–2022. Figure 6 presents the morphological conditions of the beach for each month and year; to enhance clarity, the intertidal beach is colored in blue and demarcated by the mean high-water spring level (MHWS), which corresponds to an elevation of 1.6 m. Topographic changes were notable throughout the six-year period, with large erosion observed in June 2019, marked by significant narrowing of the beach (indicated by the retreat of the 0.5 m contour line), and attributed to the preceding El Niño winter conditions (Fig. 5a). While intertidal erosion was evident in June 2019, notable growth of the supratidal beach was observed by 2022, marked by the widening of the 3-m contour line (Fig. 6).

Bird nests were located in the supratidal beach (above MHWS) and primarily concentrated along the northernmost barrier beach, between T1 and T4 (Figs. 1b and 6). The presence of nests depended on the supratidal beachface morphology. Figure 6 illustrates how nest locations varied in response to changes in beach topography; the highest number of nests was observed in June 2022, when the upper beach exhibited maximal width and elevation. In 2018, many terns were identified between T1 and T2; however, many were predated or flooded. Across all years, nests were found at elevations above MHWS, although nests situated below 2 m were frequently flooded (as indicated by crosses in 2021 and 2022 in Fig. 6). The majority of successful nests for terns and plovers (depicted by circles in Fig. 6) were located at elevations exceeding 3 m above the mean low low-tide level.

The morphological shape of the beach above MHWS played a defining role in nest site suitability (Fig. 7). Predominantly, nests in the study area were found adjacent to profiles T2 and T3, whereas profiles further south (T4-T6) showed no nesting activity (Figs. 1b and 7). Profile T1, located at the northernmost end, exhibited large morphological variation compared to T3 and T5, potentially rendering it less suitable for nesting. Notably, in June 2018, a wide berm at this profile facilitated a successful tern nest. Instead, in June 2021, a nest of the same species located above 2 m of elevation was flooded due to significant shoreline retreat and beachface erosion (see T1 in Fig. 7).

The area adjacent to profile T3 emerged as the preferred nesting site for birds, as evidenced by the abundance of nests, whereas no nests were observed near profile T5 (Fig. 7). While T5 appeared relatively stable over the study period, maintaining a consistent shape each year with shoreline variability limited to 20 m for Z = 1 m, the supratidal beach exhibited steep slopes and lacked prominent berms. In contrast, T3 represented a more dynamic profile, experiencing significant beachface erosion and berm disappearance in 2019 following the El Niño winter (Fig. 5a). However, it was capable of forming wide berms, that extended seaward from the dune toe providing optimal flat and elevated (typically close to Z = 3 m) conditions for successful nesting. Successful nests were observed in June 2020 and 2022, where the majority of tern and plover nests were located atop berms (see T3 in Fig. 7).

Our survival analyses demonstrated that nesting on a berm significantly increased daily nest survival (β = 1.85, SE = 0.56, CI = 0.75, 2.95) across 162 nests, including 137 tern and 25 plover nests, located adjacent to topographic profiles T1-T4 within 50 m of an alongshore buffer. Among these nests, 35% were successful, 11% failed due to tidal flooding, and 44% were either predated, abandoned or their fate was not known. Thus, the presence of a berm was associated with a 76% overall nest survival rate, in contrast to 18% survival rate in the absence of a berm (Fig. 8).

Birds exhibited a preference for more stable profiles that foster the formation of prominent berms, extending approximately 40 m flat from the dune toe at elevations above 2.8 m. This elevation corresponded to 0.5 m above the highest spring high-tide level of 2.3 m at the site. These berms provide a secure platform for nests, protecting them from flooding caused by tidal effects and wave set-up. Despite the dynamic nature of T3, and the occasional lack of a berm following highly-energetic winter wave conditions, it demonstrated the ability to recover its shape and reconstruct berms for the subsequent summer nesting season (Fig. 7). The capability of the beach to rebuild wide berms is considered a crucial factor in nesting success. While stable shorelines (such as T5 in Fig. 7) may exist, they do not necessarily equate to suitable conditions for bird nesting; rather, the presence of a substantial berm (as seen in T3 in Fig. 7) is paramount.

To elucidate the morphological characteristics of T3 as a preferred beach profile for bird nesting compared to adjacent profiles, Fig. 9 provides an overview of selected profiles from north to south, illustrating their morphological evolution (Fig. 9I), upper intertidal slope (Fig. 9II), shoreline variation (Fig. 9III), and supratidal volume changes (Fig. 9IV). While T1 exhibited a prominent berm in June 2018, it displayed significant morphological variation, including steep slopes (> 4º), large shoreline fluctuations (> 100 m), and supratidal volume changes (> 80 m³/m). In the mid-beach, T5 showed slope variation of 2 and 3º, as well as shoreline and volume changes of up to 42 m and 65 m³/m, respectively. The southernmost profile, T7, exhibited remarkable stability, with relatively minor slope, shoreline, and volume variation of approximately ± 1º, 25 m and 25 m³/m. The preferred beach profile for bird nesting, T3, displayed 2–4 º slope variation, and shoreline and volume changes of up to 65 m and 65 m³/m.

In this study, we provide robust evidence to support the hypothesis that beach morphology influences the breeding success of two beach-nesting bird species of conservation concern: the least tern and the snowy plover. Shoreline stability across seasons did not equate to suitable nesting habitat. Instead, portions of the beach that included a wide berm — a gently sloping sand platform above the mean high high-water line — had the lowest nest loss due to tidal flooding. Thus, the ability of the beach to recover and rebuild wide berms between seasons influences the extent and suitability of the habitat each season, as well as the nest choice and reproductive success of birds. These findings highlight the critical role of berms as morphological features that sustain breeding habitats for beach-nesting birds, and should urge conservationists, land managers and coastal engineers to clarify the processes that generate them, focus conservation on areas with berms, and mimic these features when restoring beach-nesting habitat.

Beach evolution and historical use by birds

Historical records of tern colonies at Estero Punta Banda suggest that nest site choice has changed over time. In the 1970s, terns bred on a salt flat at the southern end of Estero Punta Banda (Massey, 1977). By the 1990s, terns had established their colony on the northern section of Barra del Estero (Palacios, 1992; Zuria and Mellink, 2002). Currently, this area holds the only tern colony, with the highest nest density in the colony concentrated near the spit at the northernmost part of Barra del Estero — an area that did not exist prior to the 1990s. The long-term satellite-derived shoreline data confirms that this stretch of coastline formed in the mid-to-late 1990s, consistent with the historic establishment of the tern colony. This formation is likely attributed to the alongshore transport of sediment originating from neighboring beaches such as El Faro and Mona Lisa (Lizarraga-Arciniega and Fischer, 1998), which was potentially triggered by energetic El Niño wave conditions. In line with other wave-dominated environments, our study site displays wave-driven seasonal morphological variations: beachface erosion during high-energy periods and accretion during low-energy ones (Shepard, 1950; Ruiz de Alegría-Arzaburu et al., 2017). However, evident alongshore differences exist. The northern half (T1-T3) of the study site is highly dynamic and typically has berms in summer, except for years following El Niño winters. In contrast, the beachface morphology of the southern half (T4-T7) remains stable, characterized by a steep profile and lacking berms throughout the year.

Nest choice and beach morphology

The abundance of tern and plover nests correlated directly with beach morphology rather than with long-term beach stability, with the stable (but bermless) southern section (T3-T7) not supporting any nests. Our results are consistent with those of Medeiros et al. (2012), as the highest nest numbers were observed when the upper beachface was protected by a berm and where it exhibited its maximum width and elevation. Moreover, birds exhibited a preference for profiles featuring prominent berms that extend approximately 40 m horizontally from the dune toe at elevations exceeding 2.8 m, which corresponds with areas 0.5 m above the highest spring high-tide level. Consequently, berms function as a secure nesting platform, protecting nests from flooding. In times when berms were narrow or nonexistent, nests placed in the supratidal beach were flooded, and those located in the dunes were at risk of higher predation (JVV pers. obs.). The highest concentrations of successful nests also occurred on prominent berms. As berms are dynamic features that erode in winter and rebuild during spring and summer (Kono-Martínez et al., 2023), the ability of beaches to rebuild wide berms after the winter is a critical factor determining the amount of suitable nesting space.

Nest choice across time

Nest choice is influenced by myriad factors that include, among others, anthropogenic disturbances (Palacios, 2002; Zuria and Mellink, 2002; Ratcliffe et al., 2011), predation risk (Amat and Masero, 2004) and site fidelity (Atwood and Massey, 1988; Gonzales-Solis, Wendeln and Becker, 1995). At Barra del Estero, our data suggests that terns and plovers also take into consideration the morphology of the beach when choosing nesting locations. During years with berms, these species nested primarily on T1-T3 — areas that exhibited berms. But, during bermless years, they chose the northernmost spit of Barra del Estero, where the highest density of nests was typically found. While topographic data are unavailable for this area, historic satellite imagery and numerous field visits reveal that the northernmost section of Barra del Estero is highly dynamic, prone to dramatic changes in morphology and vulnerable to erosion during the breeding season. In this area, local land managers are frequently forced to relocate the perimeter of a protection fence around the colony as waves erode the previous location (L.Ortiz pers. comm.) and cause many nests to be flooded. Similar to terns, plovers avoided nest flooding by nesting above berms.

Nest choice in an environment of rapid change

Age and experience can play an important role when selecting successful nest sites (Dittman and Becker, 2002; Davis et al., 2017). Older birds are likely to select sites of higher elevation and reduce the risk of flooding compared to younger, less experienced birds (Nisbet, Winchell and Heise, 1984). It remains unclear, however, if birds glean information from previous seasons to select nest locations with appropriate beachface morphology or whether these environmental cues are determined in situ during the season. In many species, juvenile birds prospect their surroundings to gather information about habitat quality to be used during the next breeding season (Dittman and Becker, 2002; Davis et al., 2017). In highly site faithful species such as terns, this strategy should benefit fitness over their lifetime (Catlin et al., 2019), particularly when beaches are stable. However, as the magnitude and frequency of extreme weather events increase (Fox-Kemper et al., 2021), this strategy can lead to an ecological trap (Van de Pol et al., 2010) if beachface morphology significantly changes from one season to the next. For instance, if a bird gathers habitat information during a season prior to an El Niño event — when berms and shorelines are wide — the likelihood of flooding on that same location may be higher the next season because a narrower, bermless beach has been shown to be a recurrent consequence of El Niño.

Typically, when nest failure occurs, the breeding strategy and life history of a species should have a strong influence on the timing and location of the second nesting attempt, if they renest at all (Caitlin et al., 2019). Terns are colonial birds that can glean public information, such as the presence of more experienced individuals, bird density, and chick presence. At Barra del Estero, terns can renest as close as 30 cm to their conspecifics (L. Ortiz, pers. comm.). Tern chicks are also semi-altricial, relying on their parents to feed them until they can feed for themselves (Thompson et al. 1997). As such, nesting close to the colony offers protection against predators (Medeiros, et al. 2012). Contrastingly, plovers are territorial and their nests are more cryptic and loosely dispersed along the beach. Plovers attempting to renest, thus, face a suite of spatio-temporal pressures: they need to find a location with appropriate beachface morphology, time nest initiation with the spring tides to avoid flooding (Plaschke et al., 2019), and accurately choose a suitable elevation and distance between the water and dune to renest. Plover chicks are precocial and begin feeding themselves immediately after hatch. Thus, nesting near the waterline offers easy access to their foraging habitat but has a high risk of flooding, while nests closer to the dunes are safe from flooding but are further from foraging habitat and closer areas with higher risk of predation (Catlin et al., 2019). In this study, predation was a substantial cause of failure, particularly when nests were located behind the dunes where vegetation is more abundant and predation risk higher (Medeiros et al., 2012). Future analysis should investigate whether a rapidly changing coastal environment is squeezing beach-nesting birds between low elevation flooding pressure and high elevation predation pressure.

Conservation implications

The conservation of beach-nesting bird species has seen great advancements in reducing predation through predator control and disturbance through symbolic fences and stronger governance. However, sea level rise, in combination with the effects of wave action and extreme weather, are growing threats that are already causing negative outcomes for many beach-nesting birds and their associated habitats (Van de Pol et al., 2010). For instance, the impact of a single storm or hurricane can cause the decline or even failure of an entire colony (Seavey, Gilmer and McGarigal, 2010; Raynor et al., 2013), particularly if it occurs late in the incubation period when there is little time for renesting. Natural resource managers are thus faced with complex challenges to reduce the risk of nest flooding. Elevating the position of nests before a spring tide or storm has been attempted in the past but concrete evidence of its effectiveness is lacking (Koennel and Russel, 1996). The use of dredge spoils to create new island habitat (Allen et al., 2008) or targeted beach nourishment to restore degraded habitat (Smith et al., 2020) has been used successfully and could be useful to recreate berms, but more research is required to understand the responses of breeding birds and the cost-effectiveness of such interventions. As the effects of climate change increase the risk of flooding, growing the limited toolbox to address this threat is an urgent research and conservation priority.

This study investigated how variations in beachface morphology affects nesting selection and breeding success for two beach-nesting bird species of conservation concern: the least tern and snowy plover. The research was conducted along a 1.3 km stretch of the northern end of Barra del Estero Beach, Baja California, and utilized a multidisciplinary approach and a comprehensive dataset that included concurrent morphological, wave and bird-nesting data. This approach allowed us to assess the role of beach morphology in influencing nesting habitat availability and breeding success for both bird species.

We found a significant influence of beachface morphology on habitat availability, nest selection, and breeding success in snowy plovers and least terns. The presence of a wide berm played a crucial role in providing optimal nesting habitat, as it reduced the vulnerability of nests to flooding and contributed to greater hatching success for both bird species. Interestingly, beach stability does not necessarily correlate with breeding success; stable beach sections with steep beachfaces tended to offer reduced nesting habitat. Instead, our findings underscore the importance of a prominent berm — ideally several tens of meters wide and situated approximately 0.5 m above mean high-tide level — for successful nesting. It is worth noting that while berms may erode during winter months due to high-energy waves, the ability of the beach to rebuild wide berms post-winter is likely as a critical factor for nesting success.

Acknowledgments

The morphological data collection at Barra del Estero Beach was led by the Coastal Morphodynamics Research Group (www.mordics.org) from Universidad Autónoma de Baja California, and supported by the CONACYT CB-2014-238765 and INFR-2013-205020 research projects and complemented by UABC 636 IMTENS (Conv. 18) and 11720 projects. We want to thank Laura Martinez Rios, Thomas Ryan, Raquel Manriquez, Hans Sin, Fauna del Noroeste, Terrapeninsular, Red de Monitoreo de Chorlo Nevado en Mexico and dozens of volunteers who have been integral part of the least tern and snowy plover monitoring program. Both bird monitoring programs have been funded by the US Fish and Wildlife Service, Will J. Reid Foundation, Papoose Foundation, Marisla Foundation, California Department of Fish and Wildlife, Tracy Aviary, Manoment and Coastal Solutions Fellows Program.

Author contributions

A.R.AA. and J.G.W. conceptualized the study and led the writing of the manuscript. A.R.A.A. obtained the funding for the morphological survey data collection and undertook the data analysis. L.O.S and J.V.V. led the bird data acquisition. A.R.A.A and J.G.W. carried out the bird data analysis. N.R.S. contributed to the bird data analysis. All authors contributed to the writing of the manuscript.

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The datasets are available from the corresponding author upon reasonable request.

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The impact of beachface morphology on habitat suitability and breeding success of least terns and snowy plovers

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Abstract

Figures

1. Introduction

2. Field site and studyspecies

3. Methods

3.1. Topographic surveys 2015–2023

3.2. Long-term shoreline variations 1984–2021

3.3. Wave conditions and ENSO

3.4. Bird nesting monitoring

3.5 Combining beachface morphological changes and bird nest locations

3.6. Assessing the influence of berms on nest survival

4. Results

4.1. Morphological evolution of Barra del Estero Beach

4.2. Long-term shoreline trends: 1984–2021

4.3. Morphological beach changes and nest choice

5. Discussion

6. Conclusions

Declarations

References

Supplementary Files

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