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.

Mercury Uptake Affects the Development of Larus fuscus Chicks

“…This was likely related to feather growth, which allows Hg sequestration from the blood into the growing feather, a key mechanism of Hg detoxification in birds (Condon and Cristol 2009; Whitney and Cristol 2017). Chicks of L. fuscus start growing their primary feathers at approximately 10 d after hatching (Sotillo et al 2019), which coincided with the period when Hg in blood was highest in all treatments. The Hg sequestration into the growing feathers is further evidenced by the positive relationship between Hg in blood and Hg in primary feathers, the latter thus constituting an appropriate noninvasive matrix to assess dietary exposure of chicks during rearing.…”

“…As a generalist species, the lesser black‐backed gull ( Larus fuscus ) has a broad dietary spectrum at the population level, from predominantly marine, over urban to terrestrial foraging (Camphuysen et al 2015; Sotillo et al 2019), thus varying in their degree and type of contaminant exposure. As marine foraging increases, the risk of Hg exposure is also likely to increase (Polito et al 2016; Santos et al 2017a), with potential negative fitness consequences.…”

Time and energy costs of different foraging choices in an avian generalist species

“…We assessed whether, and to what extent, time- and energy costs vary between marine and terrestrial foraging. Marine fish have often been assumed to be the preferred chick diet for this species, given the ubiquity of this resource use by Lesser Black-backed Gulls in the North Sea [48–50] and its positive effects on chick growth and survival [51, 52]. However, given the highly competitive conditions to obtain food at fishing vessels [53, 54], we expect marine foraging to imply higher costs.…”

Adapting to urban ecosystems: unravelling the foraging ecology of an opportunistic predator living in cities

“…Once the chicks are larger, adults can move greater distances (more than 50 km) from the nests and exploit marine resources, a more nutritional resources for the growth of the chicks (Sotillo et al 2019).…”