Recently-adopted foraging strategies constrain early chick development in a coastal breeding gull

Abstract

Human-mediated food sources offer possibilities for novel foraging strategies by opportunistic species. Yet, relative costs and benefits of alternative foraging strategies vary with the abundance, accessibility, predictability and nutritional value of anthropogenic food sources. The extent to which such strategies may ultimately alter fitness, can have important consequences for long-term population dynamics. Here, we studied the relationships between parental diet and early development in free-ranging, cross-fostered chicks and in captive-held, hand-raised chicks of Lesser Black-backed Gulls (Larus fuscus) breeding along the Belgian coast. This traditionally marine and intertidal foraging species is now increasingly taking advantage of human activities by foraging on terrestrial food sources in agricultural and urban environments. In accordance with such behavior, the proportion of terrestrial food in the diet of free-ranging chicks ranged between 4% and 80%, and consistent stable isotope signatures between age classes indicated that this variation was mainly due to between-parent variation in feeding strategies. A stronger terrestrial food signature in free-ranging chicks corresponded with slower chick development. However, no consistent differences in chick development were found when contrasting terrestrial and marine diets were provided ad libitum to hand-raised chicks. Results of this study hence suggest that terrestrial diets may lower reproductive success due to limitations in food quantity, rather than quality. Recent foraging niche expansion toward terrestrial resources may thus constitute a suboptimal alternative strategy to marine foraging for breeding Lesser Black-backed Gulls during the chick-rearing period.

Keywords: Anthropogenic food; Discard ban; Early development; Foraging strategies; Gulls; Opportunistic feeders; Scavengers.

Figure 1. Distribution of diet compositions inferred from field chick feather samples.

(A) Isospace for values of δ¹⁵N and δ¹³C in Lesser Black-backed Gull feather samples relative to average (±1SD) values of food sources. Stable isotope ratios in food sources are corrected using trophic enrichment factors. (B) Frequency histogram for the estimated proportion of terrestrial food in chick diet.

Figure 2. Asymptote and growth rate of body mass in relation to diet composition.

(A) Estimated asymptotic body mass (g) of male and female field chicks plotted in relation to the estimated proportion of terrestrial food in the diet. Regression lines are plotted for significant relationships. (B) Box plot of the asymptotic body mass (g) in aviary chicks, separated by sex and dietary treatment. (C) Estimated growth rate (days⁻¹) plotted against the estimated proportion of terrestrial food in diet of field chicks, separated by sex. (D) Box plot of the estimated growth rate (days⁻¹) of body mass in aviary chicks, separated by sex and treatment. Boxes correspond to median, first and third quartile, and whiskers to 1.5 times the interquartile range outlier points. ***, P < 0.001; **, P < 0.01.

Figure 3. Asymptote and growth rate of head length in relation to diet composition.

(A) Estimated asymptotic head length (mm) of male and female field chicks plotted in relation to the estimated proportion of terrestrial food in the diet. Regression lines are plotted for significant relationships. (B) Box plot of the asymptotic head length (mm) in aviary chicks, separated by sex and dietary treatment. (C) Estimated growth rate (days⁻¹) plotted against the estimated proportion of terrestrial food in diet of field chicks, separated by sex. (D) Box plot of the estimated growth rate (days⁻¹) of head length in aviary chicks, separated by sex and treatment. Boxes correspond to median, first and third quartile, and whiskers to 1.5 times the interquartile range outliers. ***, P < 0.001.

Figure 4. Chick residual body mass in relation to diet composition.

(A–C): Residual body mass (g) of field chicks plotted in relation to the estimated proportion of terrestrial food in the diet, during the first (A), second (B) and third (C) 10-day growth periods. Solid lines are plotted for instances where the relationship was found statistically significant. (D–F): Box plot of the residual body mass (g) in aviary chicks, separated by sex and dietary treatment, in the first (D), second (E) and third (F) 10-day growth periods. Boxes correspond to median, first and third quartile, and whiskers to 1.5 times the interquartile range outliers. **, P < 0.01.