LINKED PAPER
Individual differences in foraging site fidelity are not related to time-activity budgets in Herring Gulls. Van Donk, S., Shamoun-Baranes, J., Bouten, W., van der Meer, J. & Camphuysen, K.C.J. IBIS. DOI: 10.1111/ibi.12697. VIEW
Where and how to gather food is a daily question for wild birds. Just as humans repeatedly going to the same shop and picking the same products, birds within one population have their individual habits in looking for food. For instance, individuals can differ consistently in the places they visit (Wakefield et al. 2015). In many seabirds, like Herring Gulls (Larus argentatus), this so-called foraging site fidelity is a common phenomenon. However, why some Herring Gulls are very faithful to a single site which they visit over and over again, while others visit several sites on one day is unclear.
Site fidelity profits
There are several ideas as to why birds might be coming back to the same site. By getting to know a specific environment very well, birds may increase their food intake as they know where to find the best pieces (Hamer et al. 2001). Besides, when more birds also come back to the same site, they get to know most of their immediate competitors (Eason & Hannon 1994). Knowing your neighbours can be beneficial, as the dominance hierarchy is already established.
As a result, site-faithful birds can likely save precious time, as they spent less time searching for food and fighting with neighbours, which may be advantageous. Site-faithful Herring Gulls can for instance rest more or spend more time in the breeding colony protecting their nest or incubating the eggs and thereby increasing their reproductive success. But whether individual Herring Gulls that vary in their level of site fidelity differ in their daily time-activity budgets has never been looked at.
A human-shaped environment
We studied individual Herring Gulls that were incubating their eggs in a breeding colony on the island of Texel, the Netherlands (Figure 1a.). These gulls eat mainly Blue Mussels during incubation which they find along the coast of the Netherlands on hard-substrate structures, called breakwaters (Van Donk et al. 2017). Breakwaters help in protecting the Dutch coast by breaking the waves (Figure 1b.). This human-shaped environment was ideal for our study. As the breakwaters form very clear patches, it was easy for us to distinguish one site from the other and assess site fidelity.
Figure 1 a. Overview of the study area. The breeding colony is indicated with an asterisk. The dashed area indicates where breakwaters are situated. The square on a. indicates the area shown on a larger scale in b., c. and d. b. Map of seven breakwaters along the coast of Noord-Holland. c., d. Examples of area use of respectively an individual with low foraging site fidelity and a very site-faithful individual on a selection of breakwaters (shown in b.). Every point corresponds to one GPS fix
GPS trackers as ‘big brother’
Measuring behaviour in free-ranging birds can be challenging, but is now possible thanks to the technological development of small and lightweight solar-powered GPS trackers (Bouten et al. 2013). These trackers are attached on the back of the Herring Gulls as a small backpack. Trackers can measure the whereabouts of the gulls with a GPS and also behaviour by using an accelerometer, using the same technology applied in smartphones to orient the screen when a phone is tilted. In the GPS trackers, acceleration was measured in three directions and this acceleration can be connected to several behaviours (Figure 2). For instance, when a bird is flying, the body accelerates repeatedly upwards and downwards which is accurately recorded in the data. In fact, GPS trackers act as ‘big brother’ and enable us to follow every detail of the bird’s life; we do not only know where the bird is, but also what it is doing.
Figure 2 Examples of 1 second segment of acceleration data, with the colours representing the three directions; green represents the upwards acceleration, red represents forward acceleration and blue represents lateral acceleration
No short-term benefits
This study shows that individual Herring Gulls within the breeding population on Texel vary a lot in how they make use of the breakwaters. Some spend almost all of their time on one breakwater, while others visit 21 breakwaters. In Figure 1, movements of two individuals are shown with a very site faithful individual in Figure 1d, and a less site faithful bird on the same selection of breakwaters in Figure 1c. We did not find a relationship with how animals behave and spend their time; site faithful birds did not differ in the time they spend resting or the time they spend in the colony, but site-faithful birds were in general bigger than less site-faithful individuals.
Variation in populations
This study suggests that different strategies in how to gather food might be equally good; all roads lead to Rome! Most likely, every bird follows its own optimal individual strategy, partly depending on its body size, maintaining variation in a population. Still, different strategies can have different costs and benefits depending on the environment. Site-faithful animals can have a local benefit in gathering prey, but may get into problems when this local site suddenly disappears. Imagine when your local shop closes; if you would have explored the wider environment already, another shop is quickly found, but otherwise, it would take considerable time searching for another shop and finding your favourite products. Site-faithful individuals might only be in favour in a stable environment. It is therefore needed to also study individual variation and consequences in different situations, as in recent times of rapid environmental change it is important to know how animals with different strategies respond to habitat degradation.
You can follow the movements of the Herring Gulls with GPS trackers here.
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References and further reading
Bouten, W., Baaij, E.W., Shamoun-Baranes, J. & Camphuysen, K.C.J. 2013. A flexible GPS tracking system for studying bird behaviour at multiple scales. J. Ornithol. 154: 571–580. VIEW
Eason, P. & Hannon, S.J. 1994. New birds on the block: new neighbors increase defensive costs for territorial male willow ptarmigan. Behav. Ecol. Sociobiol. 34: 419–426. VIEW
Hamer, K.C., Phillips, R.A., Hill, J.K., Wanless, S. & Wood, A.G. 2001. Contrasting foraging strategies of gannets Morus bassanus at two North Atlantic colonies: Foraging trip duration and foraging area fidelity. Mar. Ecol. Prog. Ser. 224: 283–290. VIEW
Van Donk, S., Camphuysen, C.J., Shamoun-Baranes, J. & van der Meer, J. 2017. The most common diet results in low reproduction in a generalist seabird. Ecol. Evol. 1–10. VIEW
Wakefield, E.D., Cleasby, I.R., Bearhop, S., Bodey, T.W., Davies, R.D., Miller, P.I., Newton, J., Votier, S.C. & Hamer, K.C. 2015. Long-term individual foraging site fidelity-why some gannets don’t change their spots. Ecology 96: 3058–3074. VIEW
Image credits
Featured image: Herring Gull, Larus argentatus © Judith Simon
