We find that seabird burrows buffer nest microclimates against seasonal extreme hot and cold temperatures, thus providing a thermally-stable environment for chicks. During seasonal extreme cold weather, Atlantic puffin and Leach’s storm-petrel burrows were warmer than the external temperatures, while burrows were cooler during seasonally extreme hot weather. Indeed, our results align with other studies on burrow-dwelling seabirds. For example, Cassin’s auklet (Ptychoramphus aleuticus) burrow temperatures remained stable on Farallon Island, California, despite large fluctuations in outside ambient temperature (Manuwal, 1974). Likewise, in the high arctic, little auk (Alle alle) burrows were warmer than ambient air temperatures (Kulaszewicz & Jakubas, 2018). Burrows also provide ectotherms, such as insects and lizards, thermal protection from extreme temperatures by buffering lethal hot and cold exposure (Sunday et al., 2014; Moore, Stow & Kearney, 2018). Given that climate extremes are predicted to increase in frequency and intensity in the future (Wingfield et al., 2017; Harris et al., 2018), burrows may therefore provide a direct line of defence for seabird chicks against current and future extreme cold and warming events, as predicted for other species (Moore, Stow & Kearney, 2018).
Although burrows generally buffered environmental extremes, the response of burrows was species specific. Atlantic puffin burrows had greater buffering capacity during cold weather extremes compared to warm weather extremes. This is likely because Atlantic puffin burrows are located on grassy slopes, in close proximity to the ocean, and fully exposed to the summer sun, therefore do not buffer well to heat extremes. Moreover, in both hot and cold extremes, Atlantic puffin burrows with smaller volumes had greater buffering capacity. Similar to the present study, in Wilson’s storm-petrel (Oceanites oceanicus) nests in Antarctica, smaller nest dimensions (entrance area), greater insulation, and burrow orientation were important for establishing a favourable thermal environment (Michielsen et al., 2019). Thus, our findings suggest that smaller burrows may be relatively insulated by the ground therefore less hot and cold air can enter and circulate through the burrow. By contrast, Leach’s storm-petrel burrows buffered temperatures more during warm weather. Compared to Atlantic puffins, their burrows have an additional layer of protection from foliage, such as ground-covering ferns and trees. Forests act as thermal insulators by cooling the understory when ambient temperatures are hot (Ewers & Banks-Leite, 2013; de Frenne et al., 2019; Zellweger et al., 2019). Therefore, it is likely that the forest cover is further insulating the burrows by offering shading from solar heating. This is supported by our data because during warm extremes, greater canopy cover was associated with greater buffering for Leach’s storm-petrel burrows. Comparably, in East Africa, naked mole-rat (Heterocephalus glaber) burrows located under vegetation had lower temperatures than those under unshaded bare earth (Holtze et al., 2018).
We further find that the thermal response of individual burrows depends on specific extreme events. For example, most Leach’s storm-petrel burrows showed a significant change in temperature during Extratropical cyclone Odette, while there was no change in temperature for the majority of burrows during Hurricane Larry. These responses are likely because the ambient temperature did not change dramatically during Hurricane Larry, while Extratropical cyclone Odette brought the coldest temperatures of the season. Future extreme events may present additional challenges and likely complex interactions will need to be considered when evaluating the response of burrows.
Since burrows are less effective at buffering temperatures during extreme weather, the chicks may face problems in the future given warming temperatures driven by climate change. Consequently, habitat management approaches may be needed to reduce nest temperatures during extreme events. Adapting conservation methods from other endangered species, such as sea turtles, could hold the key. For example, conservationists use ‘nest shading’ to reduce turtle nest temperatures by building small shade structures over egg clutches (Jourdan & Fuentes, 2015; Mutalib & Fadzly, 2015). Alternatively, artificial nest boxes show great promise for improving the breeding success of burrowing seabirds (Libois et al., 2012; Sutherland, Dann & Jessop, 2014), and may offer thermal-protection from heat events. Although, special consideration of the nest box design will be imperative to prevent overheating or excessive cooling (Lei, Green & Pichegru, 2014; Kelsey et al., 2016; Fischer et al., 2018). This could be a promising avenue for protecting chicks from future extreme events and merits future investigation.
The interplay between air temperature and wind speed emerged as a key driver for the internal thermal microclimate of Atlantic puffin and Leach’s storm-petrel burrows. This finding is consistent with other studies. For example, Cassin’s auklet burrow temperatures fluctuated in proportion to the changes in ambient temperatures, and temperatures were buffered more within soil burrows, compared to rock crevice nests (Manuwal, 1974). Similarly, external air temperature, wind speed, and wind direction determined the internal temperature of Wilson’s storm-petrel nests (Michielsen et al., 2019).
Thus, given the complex responses of burrows microclimates to extreme events, quantifying how changes in a variety of external (wind, temperature, precipitation) and internal (temperature, humidity) environmental conditions will impact burrow-nesting seabirds is a key future direction. This will be particularly critical in Newfoundland since the frequency of extreme windy days have increased over the past decade (Government of Canada, 2022). Moreover, precipitation presents an additional layer of complexity and may pose challenges for seabird chicks. Wet burrows can be lethal to seabird chicks because they are covered in non-waterproof down until they grow adult feathers. Consequently, Atlantic puffin chick body temperature has been observed to significantly decrease during periods of high precipitation (Vongraven, Aarvik & Bech, 1987). Moreover, burrowing seabird colonies are often vulnerable to flooding and collapse during heavy rainfall, particularly in unvegetated areas, which can lead to breeding failure (Tiller et al., 2000; Glencross, Lavers & Woehler, 2021). For example, a mass mortality of Atlantic puffin chicks was recorded following extreme precipitation and cold weather in Witless Bay Ecological Reserve(Wilhelm et al., 2013).
The temperature variations identified in this study may also translate into breeding performance. Burrow thermal variations have previously been documented to influence breeding success and growth rate in seabirds (Kulaszewicz & Jakubas, 2018). Similarly, higher nest temperatures can impede growth, as observed in blue tit (Cyanistes caeruleus) and eastern kingbird (Tyrannus tyrannus) chicks (Murphy, 1985; Andreasson, Nord & Nilsson, 2018). Therefore, given predicted future warming and increases in extreme events frequency, the next steps may be to investigate how temperature variations influence the growth rates and energy budget of these threatened seabird chicks. Furthermore, the thermal optimum for Atlantic puffin and Leach’s storm-petrel chicks is presently unknown. Therefore, research investigating seabird thermal optima is needed to understand whether these burrows are providing a thermal refuge for chicks from extreme events.