Foraging niche shift maintains breeding parameters of a colonial waterbird during range expansion

Abstract

Relating the effects of foraging niche variation to reproductive dynamics is critical to understand species response to environmental change. We examined foraging niche variations of the slender-billed gull (Chroicocephalus genei), a nomadic colonial waterbird species during its range expansion along the French Mediterranean coast over a 16-year period (1998-2013). We investigated whether range expansion was associated with a change in chick diet, breeding success, and chicks body condition. We also examined whether breeding success and chicks body condition were explained by diet and colonial characteristics (number of pairs, laying phenology, habitat, and locality). Diet was characterized using dual-stable isotopic proxies (δ ¹³C and δ ¹⁵N) of feather keratin from 331 individuals subsampled from a total of 4,154 chicks ringed and measured at 18 different colonies. δ ¹³C decreased and δ ¹⁵N increased significantly during range expansion suggesting that chicks were fed from preys of increasing trophic level found in the less salty habitat colonized by the end of the study period. Niche shift occurred without significant change of niche width which did not vary among periods, habitats, or localities either. Breeding success and chick body condition showed no consistent trends over years. Breeding success tended to increase with decreasing δ ¹³C at the colony level while there was no relationship between stable isotope signatures and chick body condition. Overall, our results suggest that even if range expansion is associated with foraging niche shift toward the colonization of less salty and more brackish habitats, the shift had marginal effect on the breeding parameters of the Slender-billed gull. Niche width appears as an asset of this species, which likely explains its ability to rapidly colonize new locations.

Keywords: Chroicocephalus genei; body condition; dispersal; isotopic niche; limit of distribution; stable isotopes.

Figure 1

Geographic distribution of the Slender‐billed gulls breeding sites between 1967 and 2013 along the French Mediterranean coast

Figure 2

Box plot of the variation of δ ¹³C and δ ¹⁵N along the study period with the median and the upper and lower quartiles represented, respectively, by the bold line and the limit of the boxes. Differences between periods are shown by colors and compact letter display as the result of Tukey's post hoc tests (box plot that share the same letters are not significantly different)

Figure 3

Standard ellipse area (SEAc) of Slender‐billed gull colonies along the four time periods considered. Each dot corresponds to the isotopic signature of an individual. SEAc did not differ significantly from one another

Figure 4

Body condition of chicks (expressed as scaled mass index in g—see Section 2) among the four periods. We found no difference between time periods during the range expansion of the Slender‐billed gull