We all know that diet can have a big impact on our health, and the same is true for the wildlife around us. Some animals adapt to new food resources, often bringing them into our everyday, where we see them scavenging for morsels. Recent Institute graduate Dr Nina O’Hanlon (@Nina_OHanlon) talks about her research into gull food choices and how it is affecting the colonies.
But what is best for the gulls?
Food and resources in a changed environment
Take a trip to a coastal town and you will likely encounter some scavenging gulls. Here in the UK, these will most probably be herring or lesser black-backed gulls, both of which are in decline. Each summer much is made in the press about these birds stealing chips and ice creams. While it is pretty clear that they can readily adapt to these new resources, it is not really understood whether foraging in human-modified areas, rather than in their more conventional marine habitats, has an impact on the gulls’ breeding success.
Being opportunistic in what they eat, gulls feed in a wide range of habitats. Historically, gulls foraged largely on marine fish and intertidal prey, such as crabs and mussels found along the shoreline. Many species are increasingly foraging on terrestrial sites including farmland, landfills, and in towns and cities, but it’s unclear from previous research whether this switch is good or bad for the birds. Some reports have concluded that gulls with access to landfill sites have benefited, resulting in higher breeding success. However, in other studies gulls have fared better when foraging on marine or intertidal prey. This conflicting evidence may partly be due to different colonies having different resources available to them within their foraging range. Certain habitats may provide food items at a higher abundance or of higher quality, in terms of energy or nutrients, than others.
Due to the complications of visiting multiple colonies within a single field season, most previous studies have looked at the diet of gulls from just one or two colonies. This is useful as the gulls can be studied quite intensively at these locations, often over multiple years. Unfortunately, we don’t always know whether these are representative of all colonies in an area. For my own research, I collected herring gull pellets from several colonies to determine whether gulls from different locations used similar resources. The different locations will be surrounded by different habitats. Because they are opportunistic, it’s expected that the gulls will forage on the most abundant or profitable food resource available within their foraging range. By examining the pellets, I could see what the gulls were eating, and trace it back to the habitats available to them around their breeding colony. This could then help determine whether what the gulls consumed had any influence on their breeding success!
I selected eight herring gull colonies around the coast of south-west Scotland (Islay, Oronsay, Jura, Pladda, Lady Isle and Portpatrick) and Northern Ireland (Copeland and Strangford Lough), choosing sites along a gradient from low to high human population density and with varying distances to potential foraging habitats. These foraging habitats were built-up areas (urban and suburban locations, including landfill), farmland, rocky shore (intertidal) and offshore areas (where the gulls largely scavenge from fishing boats).
Over two field seasons, from 2013 to 2014, my team and I collected 781 pellets from the eight colonies! These came back to the lab, where the items in each pellet were categorised as coming from terrestrial, intertidal or offshore habitats. There was a lot of variation in what the gulls from each colony were feeding on, though herring gulls in all colonies mainly fed on terrestrial items, especially grain. We did find that some colonies (Jura, Oronsay, Strangford and Pladda) foraged on more marine, particularly intertidal, prey than other colonies (Islay, Lady Isle, Copeland and Portpatrick).
This actually matched with the local habitat availability: colonies located in areas with good intertidal habitat did consume more intertidal invertebrates, whilst colonies located nearer to built-up areas fed more on terrestrial items. As expected, the habitats within the vicinity of each colony did influence what the gulls foraged on!
This connection really makes sense, as during the energetically demanding breeding period, the parents will not want to travel too far to find food and will use the resources most available to them nearer to the breeding colony. However, food items from different habitats will vary in their quality, which will influence how well a chick grows and, ultimately, survives. If the gulls from different colonies are foraging on different resources, does this subsequently affect the number of chicks they successfully rear?
One problem of having so many colonies to visit during a relatively short breeding season is that we were not able to spend a great deal of time in each colony. Because of this, and particularly in larger colonies, it would have been difficult to use traditional measures of breeding success: the total number of chicks fledged from all nesting attempts. Instead, our measure of breeding success was the average brood size (number of chicks in a nest) that were at least three weeks old (and therefore likely to fledge) from successful nests.
This showed that in colonies where herring gulls foraged on more intertidal resources, broods of chicks were larger. Colonies where gulls fed primarily on terrestrial items raised smaller broods.
Compared to scavenging on fish or food from landfill sites, intertidal invertebrate prey may not always be the most beneficial to gulls in terms of energy, protein or fat. However, in this region we did find that feeding on intertidal food benefits herring gulls during the breeding season. It’s likely that this is because these meals provide necessary nutrients, such as calcium, to the growing chicks. In comparison, we know that grain, the most common food item in the colonies largely feeding on terrestrial food, is very low in calcium. Chicks from these colonies could therefore be disadvantaged for nutrients, resulting in lower growth and survival.
Herring gulls are on the Red List of Birds of Conservation Concern as they have experienced large declines in breeding numbers over recent decades. The cause of these declines is largely unknown, but is likely due to a number of reasons. To understand what’s happening, it’s important to identify what affects the gulls’ breeding success, as this can influence their population growth. Our results indicate that the foraging habitats available to herring gulls during the breeding season influences their diet with subsequent consequences on breeding success. Therefore, where humans alter landscapes in which species forage, opportunistic species such as gulls may switch to alternative resources that not only bring them in to closer contact to humans but also have significant impacts on their breeding success and population growth.
This research was recently published in Marine Ecology Progress Series and can be found at https://doi.org/10.3354/meps12189.