Landscape‐mediated variation in diet is associated with egg size and maculation in a generalist forager. O’Hanlon N.J., Alonso, S., Miller, J.A.O., McGill R.A.R. & Nager R.G. 2019. IBIS. DOI: 10.1111/ibi.12739. VIEW
We often think of generalists as being able to buffer themselves from changes in their environment – as if the availability of one food item is reduced they can generally swap to an alternative food source. However, this assumes that there are available alternative food sources, and that these are comparable in energy and essential nutrients. In this study we show that although Herring Gulls Larus argentatus choose the resources most abundant to them, a shift to alternative resources leaves its traces on the Herring Gull’s eggs.
Many gull species, such as the Herring Gull, are opportunistic generalists, foraging on a huge variety of resources – from food items found in intertidal and farmland habitats, to fishery discards and domestic waste scavenged from towns and cities. During 2013 and 2014, we studied the diet of breeding Herring Gulls from seven colonies across south-west Scotland and Northern Ireland. During the breeding season, Herring Gulls are constrained to the colony, therefore they are restricted to how far they can forage, typically within about 50 km. In our study colonies, we found that Herring Gulls were foraging on the resources most available to them within their foraging range (O’Hanlon et al. 2017). This consequently influenced the gulls breeding success at the colony level, with higher final brood sizes in colonies where the gulls consumed more marine (predominantly intertidal prey, with some fishery discards) than terrestrial items (largely grain, but also some terrestrial invertebrates and domestic waste). We attributed this to intertidal prey containing more essential nutrients and energy for the growing gull chicks than grain (O’Hanlon et al. 2017). Therefore, even for generalists, the availability of different resources within the foraging range during the breeding season can influence productivity, and at least in our study area the gulls do best on a diet from marine intertidal habitats.
To explore whether diet also influenced earlier stages of breeding we then focused on the gulls’ eggs. Egg laying is particularly demanding, both in energy and nutrients, for female birds. Therefore, bird eggs have the potential to reflect information on the quality of the female that laid them as well as the local environment. We generally assume larger eggs are better quality as they will contain more nutrients for the growing embryo – with evidence that larger eggs produce larger chicks. Eggshell colouration can also reveal information on the quality of the egg or female, which the local environment can influence, particularly food availability. We can measure two aspects of eggshell colouration – the background egg colour and the maculation (the intensity and distribution of spotting. Figure 1). Variation in egg shape among individuals of a species can also provide information, for example, in some birds, young breeding females lay more pointed eggs.
>Figure 1 Contrasting egg maculation and shape with (a) a highly maculated, pointy egg and (b) a less maculated, oval egg © Nina O’Hanlon
In 2014, we visited seven Herring Gull colonies during late incubation and photographed clutches of eggs on a specially designed tray (Figure 2). This allowed us to extract data on egg size, shape, background colouration and maculation from the images, whilst causing minimal disturbance to the gulls. To relate the gulls’ diet to variation observed in the egg traits, we took feather samples from recently hatched chicks (less than 7 days old). Chicks grow these feathers when still in the egg, therefore their stable isotope signature reflects the resources the female Herring Gull consumed whilst producing the egg (along a marine and terrestrial gradient, and low to high trophic level).
Figure 2 Egg holder to photograph Herring Gull eggs in the field with the graph paper background to give a scale bar for the size measurements and a standardised colour panel (QPcard+201 colour checker panel) to allow measuring colour. For egg maculation we used NaturePatternMatch (Stoddard et al. 2014), to compare the similarity of the Herring Gull eggs to an unmaculated reference (duck) egg, which takes into account the intensity and size of the spots © Nina O’Hanlon
Interestingly, we found larger eggs in colonies where females consumed predominantly marine or predominantly terrestrial resources, and smaller eggs in colonies where the gulls consumed a more mixed diet of both marine and terrestrial items. We expected eggs to be larger where the gulls foraged on more marine (intertidal) prey, as if available to them the gulls do seem to have better breeding success in our study colonies. That gulls predominantly foraging on terrestrial items also had larger eggs was more surprising, as being largely grain we might expect this resource to provide less energy and nutrients. However, resources from farmland habitats are arguably more abundant and available to the gulls near the colonies we investigated; therefore, this abundant availability may make up for any lack of nutrients. Conversely, gulls in colonies without an abundant availability of terrestrial resources or good intertidal habitat consumed a mixed diet, which is likely less efficient than specialising on one diet, resulting in the female gulls producing smaller eggs.
Looking at the extent of maculation, or spotting, of Herring Gulls eggs, we found more maculated eggs in colonies where females consumed more marine resources. Some studies have found that greater egg maculation reflects higher female or egg quality, which is what we might expect if consuming marine resources benefits gulls. However, this is not a straightforward relationship to explain. Results in the published literature are not conclusive on what egg pigmentation reflects about female quality or the local environment, especially in species where egg camouflage is also an important factor.
We did not find any significant relationships between background eggshell colour or shape with resource use. However, we did find more pointed, and smaller eggs, in the larger gull colonies, which suggests food shortages around these larger colonies, which not only affects the size, but also the shape of their eggs.
Figure 3 Clutch of three Herring Gull eggs © Nina O’Hanlon
Herring Gull colonies with different habitat compositions in their landscape resulted in difference in resource use during egg formation and measurable differences in egg traits at the colony level. Our results indicate changes in local resource availability, for example due anthropogenic changes or habitat loss, can affect even opportunistic and generalist foragers. Herring Gulls specialising on certain abundant or profitable local resources may be more efficient in obtaining nutrients and energy than if feeding on a broad range or changing mix of resources. And this varying efficiency of extracting key resources is reflected in the appearance of eggs. Taking digital images of eggs in the field using a specially designed tray is a quick and efficient way to measure multiple egg traits with minimal disturbance to breeding individuals and may provide a useful monitoring tool for environmental changes affecting birds during the early stages of the breeding cycle.
O’Hanlon N.J., McGill R.A.R. & Nager R.G. 2017. Increased use of intertidal resources benefits breeding success in a generalist gull species. Marine Ecology Progress Series 574: 193–210. VIEW
Stoddard, M. C., Kilner, R. M. & Town, C. 2014. Pattern recognition algorithm reveals how birds evolve individual egg pattern signatures. Nature Communications 5: 1–10. VIEW
Featured image: Herring Gull Larus argentatus nest with two chicks and an unhatched egg © Nina O’Hanlon