The resources available to breeding seabirds within their foraging ranges can influence productivity, either directly through the quality and quantity of food consumed by chicks, or indirectly by affecting the foraging behaviour and efficiency of parent birds. Where local resource availability is low, or the quality of resources are poor, species with flexible time-energy budgets can increase their foraging effort to provide adequate energy and nutrients to their chicks, although this may come at the expense of nest attendance. We investigated provisioning rates and nest attendance in European Herring Gulls Larus argentatus from seven colonies across southwest Scotland and Northern Ireland during two chick-rearing periods (2013 and 2014) in relation to the food resources used by these colonies. We observed variation in provisioning rates and nest attendance between colonies, and variation between years in nest attendance. We found no significant relationships between these behaviours and the proportion of intertidal prey consumed, suggesting that provisioning rate and nest attendance did not differ between resource types at the colony level. We also found no evidence that variation in behaviours was related to breeding success. Our results suggest that, within this region, the type of resources consumed had a greater influence on Herring Gull breeding success than differences in two proxies of foraging efficiency (provisioning rate and nest attendance), although other factors may also have influenced breeding success. Our work highlights the benefit of determining what food resources are provided to chicks, in addition to measuring foraging behaviours, to fully understand the consequences of consuming different resources on the breeding success of generalist foragers.

To understand drivers of population change it is important to establish how speciesuse the resources in their environment (Johnson 1980). Food is a particularly important resource and individuals will select food sources within their foraging range based on its availability and profitability, ensuring the maximum benefit for the lowest cost (Pulliam 1974). Changes in food resources can therefore affect population dynamics (reviewed in White 2008). For example, local food resources can be particularly limiting for breeding seabirds that are generally colonial and constrained to a central nesting site (White 2008), resulting in intra- and inter-specific competition for local food resources, which can influence a population’s demography (Ashmole 1963; Furness & Birkhead 1984; Birt et al. 1987). Learning how seabirds use local food resources under different conditions can help us betterunderstand how food resources influence population size.

Behaviours associated with a species’ foraging strategy can show great plasticity, influenced by the availability, quality and distribution of food resources (Pyke1984). As well as variation in availability, and energetic and nutritional quality, different resources may also differ in the time and energy it takes to obtain them, which may influence parental foraging behaviours (Tremblay et al. 2005; Burke & Montevecchi 2009; van Donk et al. 2019). The ability to flexibly adjust their foraging in response to changes in resources can allow individuals to buffer their breeding output, making foraging behaviour a good candidate through which to identify environmental changes (Cairns 1987). As the distribution and availability of resources can change with a changing environment, parents may have to switch to alternative food sources to provision their young. This is especially true of generalists, which can forage on multiple resources within their foraging ranges.Alternative food sources may be less profitable in energy or nutrients, or more costly for parents to gather due to longer search, capture and/or handling times(Burger & Piatt 1990; van Donk et al. 2019). This may necessitate longer foraging trips and result in reduced provisioning rates (Hamer et al. 1993; Quintana 2008; Rishworth & Pistorius 2015). Increasing foraging time can also reduce nest changeovers, with fewer instances of both parents attending the nest simultaneously, and increase the overall nest attendance of at least one parent. Reduced nest attendance can result in higher predation risk and the exposure of chicks to unfavourable weather, and hence lower productivity (Uttley et al. 1992; Hamer et al. 1993; Wanless et al. 2005; Ashbrook et al. 2008; Chivers et al. 2012). Differences in the main food resource exploited can therefore affect how long adults spend away from the nest, and hence variation in nest attendance and provisioning rates(Bijleveld & Mullers 2009). In seabirds, adult behaviours related to offspring care, including provisioning rates and nest attendance, can be readily observed at the nest (Uttley et al. 1992; Wanless & Harris 1992; Kitaysky et al. 2000; Chivers et al. 2012). Therefore, as foraging behaviours can influence chick survival and productivity, they can be useful in revealing the pressures acting on the resources and habitats that seabirds rely on (Berger-Tal et al. 2011).

The Laridae gulls form a group of generalist seabirds of which several species typically consume a wide variety of resources from both marine and terrestrial environments (Hunt & Hunt 1973; Kubetzki & Garthe 2003). Large gulls generally forage on the most available resources within their foraging range and the type ofresources provisioned to chicks can subsequently influence their breeding success (Pons 1992; Annett & Pierotti 1999; van Donk et al. 2017). For example, in southwest Scotland and Northern Ireland, colonies of European Herring Gulls Larus argentatus (hereafter ‘Herring Gull’) that consumed higher proportions of intertidal prey had larger brood sizes than colonies where individuals consumed more terrestrial resources (predominantly grain as well as invertebrates and anthropogenic food items; O’Hanlon et al. 2017). Discrepancies in breeding success were attributed to intertidal resources being of higher quality than the available terrestrial resources (O’Hanlon et al. 2017). Although the energy density and lipid content of intertidal prey may not be as high as that of fish and some domestic refuse, it can contain specific nutrients that are important for chick growth such ascalcium (Annett & Pierotti 1989; Noordhuis & Spaans 1992; van Donk et al. 2017).Alternatively, intertidal prey may have been more abundant or accessible to the gulls from colonies that consumed a higher proportion of this resource, resulting in higher provisioning rates or nest attendance (Lamb et al. 2017).

Here we investigate variation in nest attendance and provisioning rates among Herring Gull colonies across southwest Scotland and Northern Ireland over two breeding seasons in relation to the food resources used by these colonies (based on the results of pellet analysis reported in O’Hanlon et al. 2017). Specifically, we hypothesise that if intertidal resources are more abundant or accessible, allowing gulls to forage more profitably, adults in these colonies will have increased provisioning rates and/or higher nest attendance. If, however, it is the quality of intertidal resources that affects breeding success, the relationship between the proportion of intertidal prey in the diet and a colony’s breeding success would not be affected by provisioning rates and/or nest attendance. These results will add to our understanding of what factors influence gull breeding success across a range of colonies.

This work was supported by funding from the European Union’s INTERREG IVA Programme (project 2859 ‘IBIS’) managed by the Special EU Programmes Body. We thank Julie Miller, Sinead Sheriden, Angus Lothian, Karen Hotopp, Maureen Ellis, Richard Thompson and Chris Siburn for all their help with conducting nest watches. Thanks to the editorial board member and two anonymous reviewers for their constructive comments. Contributions: NJO’H and RGN originally formulated the idea, developed the methodology and wrote the manuscript. NJO’H co-ordinated and conducted fieldwork and analysed the data.

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