Traditionally, the study of bird feeding habits has relied on direct observations and analysis of regurgitated pellets. However, in this study, we opted for stable isotope analyses, a method that has proven to be a robust tool wildlife research (Monti et al., 2021; Chiffard et al., 2023).
Our findings revealed significant variations in the isotopic values of δ¹³C, δ¹⁵N, and δ³⁴S among the different prey species of Falco eleonorae. These variations in isotopic signatures can be attributed to the diverse trophic levels of the prey (Post, 2002), which include granivores, insectivores, and a seabird (European storm petrel). Additionally, nitrogen values may be influenced by anthropogenic pollution or agricultural practices (Stewart et al., 2002; Shin et al., 2018; Kendall, 1998).
Despite these differences, the δ¹³C range suggests that these birds primarily feed on prey with a diet strongly influenced by C3 terrestrial plants − 23 to -31‰ (Kohn, 2010). The European storm petrel (Hydrobates pelagicus) exhibited distinct isotopic values, with the highest δ¹³C value (-21.26 ± 0.31‰, N = 3), indicating its unique marine-based diet (O’Lary, 1988; Boutton, 1991). Interstingly, the European storm petrel δ¹³C values we measured in the Galite Archipelago are lower than those measured in the Wetsren Mediterranean basin as predicted by Martinez et al., (2019) showing a south to north and from west to east carbon isotopic ratio depletion (see Table 2). δ¹⁵N average values of the storm petrel of Tunisia are also lower compared to the Western Mediterranean likely due to different factors as lower trophic level preys zooplankton or smaller fish rather than pelagic fish (Ferrier-Pagès and Leal, 2019).
Table 2
Stable Isotope Ratios of Prey Species: Current Study vs. Previous Study
| Actual study (n = 1) | Bibliography |
Preys | δ13C‰ | δ15N‰ | δ34S‰ | δ13C‰ | δ15N‰ | δ34S‰ |
Luscinia megarhynchos | -23.68 ± 0.54 | 5.75 ± 0.09 | 8.27 ± 0.77 | -- | -- | -- |
Muscicapa striata | -28.6 ± 0.72 | 12.17 ± 0.48 | 2.59 ± 1.01 | -23.09 ± 1.11 (n = 2) (Franzoi, 2016) | 6.96 ± 5.18 (n = 2) (Franzoi, 2016) | 3.63 (n = 2) (Franzoi, 2016) |
Upupa epops | -24.02 ± 0.16 | 5.51 ± 0.1 | 7.15 ± 0.25 | -23.85 ± 1.1 (n = 115) (Reichlin, 2009) | 8.41 ± 1.8 (n = 115) (Reichlin, 2009) | --- |
Sylvia communis | -24.36 ± 0.3 | 4.63 ± 1.32 | 9.12 ± 0.91 | -24.45 ± 0.59 (n = 4) (Franzoi, 2016) | 7.78 ± 2.03 (n = 4) (Franzoi, 2016) | 3.87 (n = 4) (Franzoi, 2016) |
Hydrobates pelagicus | -21.26 ± 0.49 | 9.04 ± 0.76 | 17.3 ± 0.41 | --- | H.p pelagicus :15‰ (Martinez et al., 2019) | --- |
Coturnix coturnix | -23.8 ± 0.43 | 6.41 ± 0.07 | 14.5 ± 0.23 | -- | -- | -- |
Chloris chloris | -24.27 ± 0.88 | 6.75 ± 1.56 | 7.62 ± 3.37 | -- | -- | -- |
The spotted flycatcher (Muscicapa striata) displayed the lowest δ¹³C and δ34S suggesting a full terrestrial diet habit. The low δ¹³C can result from a canopy effect, reducing sunlight exposure and depleting the CO2 isotopic composition for plants, ultimately consumed by insects and then by the birds (Lawrence and Novak, 2001). The highest δ¹⁵N value can reflect indirect impacts on instects and the food chain by the animal waste and nitrogenous fertilizers in agricultural soils (Craig et al 2001; Kendall, 1998). Further investigation is required to confirm these findings, especially regarding the δ¹⁵N values of spotted flycatchers in reproductive areas.
The European greenfinch (Chloris chloris) showed high intrafeather amplitudes for all isotopes (1.62‰, 3.12‰, and 6.52‰ for δ¹³C, δ¹⁵N, and δ³⁴S, respectively) suggesting dietary changes during molting despite being predominantly granivorous. This variation could be attributed to insect consumption, especially during chick rearing as suggested in Ordóñez-Delgado et al., (2022). The high δ³⁴S may be related to marine protein consumption, exposure to sea spray carrying enriched δ³⁴S molecules, or to the highly δ³⁴S enriched evaporites areas transferred along the food web.
The same high signatures were also recorded in the common quail Coturnix coturnix with an average δ34S of 14.50 ± 0.17‰, N = 3. According to the literature, a high δ34S can be explained either by consumption of marine protein or by plants that absorb molecules enriched in δ34S brought by sea spray (Thompson et al., 2010), or by a geological explanation, meaning the presence of a bedrock rich or depleted in sulfur in certain locations (Hebert and Wassenaar, 2005). A study with a representative sample of feathers from this specie need to take place to better understand their dietary habits during their breeding season.
For the Eurasian Hoopoe, a difference of -0.17‰ for δ13C is observed between our current study and the results obtained in the Reichlin (2009) study (Table 2). However, considering the standard deviation of ± 0.16 in the former study and ± 1.1 in the latter, this difference can be attributed to measurement uncertainties. This similarity suggests that the birds likely maintained relatively consistent food sources in terms of carbon isotopic composition between the two studies. The observed variations are likely within the limits of measurement error and do not necessarily represent a significant change in their environment or feeding behavior.
For δ13C, there is a slight difference between the current study and the previous one, with values of -24.36‰ and − 24.45‰ (Franzoi, 2016), respectively (Table 2). However, both values fall within the expected range of variation considering the measurement uncertainties. The δ15N values show a more significant difference. In the current study, the value is 4.63 ± 1.32, while in the previous study, it is 7.78 ± 2.03 (Franzoi, 2016) (Table 2). This indicates a notable disparity in the nitrogen isotopic composition of Sylvia communis between the two studies. For δ34S, the values are 9.12 ± 0.91 in the current study and 3.87 in the previous study (Table 2). Once again, this demonstrates a significant difference between the sulfur isotopic compositions reported in the two studies (Franzoi, 2016). Overall, the comparison of these results suggests potential differences in the diet or habitat of Sylvia communis between the two studies. The variation in nitrogen and sulfur isotopic compositions particularly indicates potential changes in trophic level or environmental conditions. Further investigation may be necessary to understand the reasons behind these differences.
This study has proven that multi-isotope approach on prey feathers of falcon’s can be very informative for depicting dietary ecology of wildlife. However, the limited sample size in our study may not fully capture the dietary variation within each prey species. Therefore, expanding the sample size and analyzing a greater number of feathers per species would enhance the reliability and representativeness of the findings. This would enable a more comprehensive investigation of stable isotope signatures, providing a clearer understanding of prey dietary patterns. Moreover, an increased sample size would contribute to enhanced statistical power, allowing for the detection of more subtle variations in isotopic signatures among individuals of the same species. This is crucial for elucidating potential intraspecific dietary variations, influenced by factors such as age, gender, or regional distinctions.