Too salty for you? Changes of diet in the laughing gull nestlings during the growing period

In many seabird and waterbird species, salinity can impose physiological stress on recently hatched chicks because they have a limited capacity to excrete salt loads. In response, parents can select low‐salt food for their nestlings in the first stage of their growth. We determined the growth‐related variation in the diet of laughing gull Leucophaeus atricilla nestlings by using stable isotope analysis of feathers. Isotopic measurement of hatchling down reflects the mother's diet before laying eggs. At the same time, the primary feather tip indicates the food intake during the initiation of feather growth (one week of age), and the primary feather base indicates the food provided by parents during the period immediately before fledging. δ¹⁵N and δ¹³C values among the feather types showed a spatiotemporal shift in the nestlings’ diet. Younger chicks consumed more terrestrial prey sources (weevil beetles Sphenophorus sp.; ~51%) than chicks at fledging (~2%). However, fledging chicks and females before egg‐laying consumed mostly marine prey sources (Pacific white shrimp Litopenaeus vannamei and anchovy Anchoa sp.; > 90%). The base and primary tip portions had similar δ¹⁵N values, but both sections were less enriched than the hatchling down. This change was diet‐related, as the nestlings showed a lower percentage of anchovy consumption (13.3–14.7%) compared to females before egg‐laying (30.4%). Younger chicks had a different spatial niche than fledglings and females before egg‐laying, indicating a diet shift through the chick growth period. Assuming that the availability of food in the terrestrial or marine foraging areas did not vary through the nestlings’ growth period (five weeks), our findings support food resource selectivity by parents for their offspring during the first days of life, to reduce the physiological stress caused by high salt loads.