Why does a predatory bird avoid its prey?

Blog written by Karl Busdieker and  Dr. Samantha Patrick. Read the full paper here. Photo above of a South polar skua, Catharacta maccormicki, by Alice Trevail

If you were to ask the average person where they thought an animal might prefer to feed, they would probably reply along the lines of “wherever the most food is”. On a purely logical level, this seems obvious, as high food density provides easy pickings in many cases. However, ecology is rarely so simple. This study investigates a fairly unusual predator-prey system where the opposite case is true, and predators actively avoid high densities of prey despite the seemingly obvious benefits of such a natural buffet.

Our study concerns the effect of prey density on the habitat selection behaviour of a predatory species in Svarthamaren, Antarctica. Svarthamaren has just one predator – the south polar skua, Catharacta maccormicki – and effectively one prey species, the Antarctic petrel,  Thalassoica antarctica. It is also 200km from the nearest coast, meaning that skuas are unable to feed at sea as they might in other habitats, and emphasising their reliance on this single prey species.

An important point to note here is that in Svarthamaren, prey are so abundant that skuas do not need to maintain feeding territories, and instead just feed at will throughout the whole petrel colony. This is particularly unusual for a species that is normally so protective of individual feeding grounds (Trillmich 1978). It is therefore important to understand how and why they choose where to feed within this colony, when food is essentially everywhere (at varying densities).

Antarctic petrel chicks (Thalassoica antarctica) in their nests. Image Credit: Alice Trevail

This study made use of a comprehensive dataset, which used regularly spaced plots to quantify the density of petrel nests during their breeding season, in addition to detailed tracking data for 47 individual skuas (36 of which were sexed). We wanted to investigate the effects of prey density on habitat selection in skuas, as well as the interactions of prey density with skua sex, and the breeding stage of both predator and prey (whether the birds are incubating an egg at a given time, or are rearing a chick in their nest). To do so, we used resource selection models to compare areas which the skuas chose to use (used points) with points they could have used but did not (available points, represented by pseudo-absences).

Our main finding was that as petrel density increases, the likelihood of a skua feeding in the area decreases – which is seemingly counter-intuitive as such areas are full of potential food! However, Antarctic petrel chicks are not helpless, and to defend themselves against attackers they are able to spit up a foul oil, coating skua feathers and severely inconveniencing avian predators on future flights. It therefore seems that skuas prefer to go for “easy prey” – they would rather prey on relatively lonely petrel chicks rather than battle dense groups of them.

Interestingly, this pattern was only visible after petrel hatching – prior to this, skuas feed indiscriminately on petrel eggs, which are indeed helpless when unguarded. Also, when skuas are incubating eggs of their own, they are more likely to select areas of lower petrel density – perhaps being less willing to take risks while their own offspring are so vulnerable. Later on, once their chicks have hatched, they are slightly less likely to avoid areas of high petrel density. Finally, female skuas show a slightly stronger preference for lower prey densities than males, but post-hoc testing suggests that this difference is minimal.

An Antarctic petrel chick (Thalassoica antarctica) is taken by a south polar skua (Catharacta maccormicki). Image Credit: Sébastien Descamps

Readers may wonder why this matters at all, especially in such a remote area. However, we believe our results have some quite significant implications for these species. The Antarctic petrel colony at Svarthamaren is one of the largest in the world (Mehlum et al. 1988), and due to its remoteness and lack of other predatory species could be a “safe haven” for these petrels should their numbers decline elsewhere. However, our findings could mean that if petrel populations do decline, targeted predation on areas of lowered petrel density could exacerbate this effect and speed up population losses.  

Our findings provide an interesting basis from which future studies could begin to look at the effects of prey density on predator behaviour in other populations, perhaps in more typical skua populations with alternative sources of prey available, or in multi-predator ecosystems where competition between predators could play a role. This study also has important implications for skua behavioural studies, as some of the behaviours witnessed at Svarthamaren, such as a lack of individual feeding territories, and attacking prey solely from above are very different to populations elsewhere in the world.

This research was funded by the NFR-NARE program (http://www.forskningsradet.no/), and also by a summer internship for recent graduates, provided by the School of Environmental Sciences at the University of Liverpool. We would like to thank Arnaud Tarroux for his help with the skua capture work and for the useful comments provided at the early stage of this project. We also thank Eva Soininen and Johan Nils Swärd for their help in collecting the data. Thanks to Dr Jonathan Green for valuable comments on the manuscript.

References:

Mehlum, F., Y. Gjessing, S. Haftorn, and C. Bech. 1988. Census of breeding Antarctic Petrels Thalassoica antarctica and physical features of the breeding colony at Svarthamaren, Dronning Maud Land, with notes on breeding Snow Petrels Pagodroma nivea and South Polar Skuas Catharacta maccormicki. Polar Research 6:1–9.

Trillmich, F. 1978. Feeding territories and breeding success of south polar skuas. The Auk 95:23–33.

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