The most common diet results in low reproduction in a generalist seabird

Abstract

Dietary specialization has been described across a wide range of taxa in the animal kingdom. Fitness consequences are, however, not well documented. We examined the reproductive consequences of different dietary specializations in the herring gull Larus argentatus, an omnivorous seabird, using an extensive dataset which includes breeding and dietary data of 10 successive years. We hypothesized that pairs that focused on prey of higher energetic value would yield higher fledging rates. An alternative hypothesis is that pairs that relied on more resources simultaneously would reproduce better. The novelty of this study is that we used continuous measurements representing dietary composition and degree of specialization rather than restricting our analysis to predefined categories. By relating these two continuous measurements for diet to several proxies for reproductive success, we show clear consequences of dietary choice. Most pairs concentrated on bivalves, a prey type not particularly rich in energy. Pairs feeding on energy‐rich prey (e.g., “domestic refuse and fishery discards”) during chick rearing were found to have a higher reproductive success, supporting the first hypothesis. Pairs that used more resources did not clearly have a higher reproductive success. The majority of the pairs did not switch to energy‐rich prey during chick rearing, despite low breeding outcome. We discuss how trade‐offs between factors such as resource availability, predictability, and the time and energy needed to obtain certain prey species may influence resource selection.

Individual differences in foraging site fidelity are not related to time‐activity budgets in Herring Gulls

“…Nevertheless, the time spent in other foraging habitats could influence time‐activity budgets, especially when animals forage in an ‘urban area’ which contains prey types of high caloric value that are important for reproductive output and condition (Pons , Van Donk et al . ). However, we did not find evidence for this in our data, as there was no relationship between the time our selected animals spent on breakwaters and either the level of site fidelity (Table ) or the time‐activity budget (Fig.…”

“…Herring Gulls, like many other Larus species, are dietary generalists on the species level, but individuals have different foraging strategies (Morris & Black 1980, Gorke & Brandl 1986, McCleery & Sibly 1986, Pierotti & Annett 1990, Irons 1998. Herring Gulls breeding on the island of Texel, the Netherlands, forage during incubation mainly on a predictable and abundant resource, Blue Mussels Mytilus edulis (Camphuysen 2013, Van Donk et al 2017). These mussels grow on breakwaters which are human-made coastal defence structures.…”

“…First, we focus on incubation to control for possible changes in demands during different breeding phases. Second, we can control for differences in diet by selecting individuals with a similar dietary specialization, as gulls in this system feed primarily on Blue Mussels during incubation (Camphuysen 2013, Van Donk et al 2017. Finally, incubation is an important period for increasing energy reserves, as Herring Gulls recover from the physical impacts of egg-laying (females) and extra food provisioning in the prelaying phase (males to females) (Hario et al 1991).…”

Intercolony variations in movement patterns and foraging behaviors among herring gulls (Larus argentatus) breeding in the eastern Wadden Sea

“…Pelagic prey is energetically rich but unpredictable, due to its patchy distribution (Garthe & Hüppop, 1994; van Donk et al., 2017; Weimerskirch, 2007), while terrestrial and intertidal habitats are characterized by more stable feeding conditions (reliability, low handling and flight costs). The logger data implied that predictable and productive intertidal habitats, such as the cockle bed, were well known to the breeding herring gulls and were thus used intensively for foraging.…”

“…Individuals from some colonies also spend a large amount of time foraging in terrestrial habitats on earthworms or anthropogenic refuse (Kim & Monaghan, 2006; Sibly & McCleery, 1983; van Donk, Camphuysen, Shamoun‐Baranes, & van der Meer, 2017). Similar observations have been made for lesser black‐backed gulls ( Larus fuscus ), previously known as a predominantly marine species, but which is increasingly adopting a dual foraging strategy utilizing both marine and terrestrial habitats (Isaksson, Evans, Shamoun‐Baranes, & Akesson, 2016), possibly as a result of food depletion at sea (Garthe et al., 2016; Votier et al., 2004).…”