21 December 2008

Shrikes take their cues from the competition

ResearchBlogging.orgOver evolutionary time, the easiest way to deal with a competitor is to do something different - if your competitor eats big seeds, say, it may be easier to start eating small seeds than to fight for the big ones. This idea goes all back to the Origin, wherein Darwin proposed that competition drives evolutionary diversification, with living things dividing up available resources into ever-finer slices as they scramble for shares:
Lighten any check, mitigate the destruction [of offspring] ever so little, and the number of the species will almost instantaneously increase to any amount. The face of Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.
But what if competition can sometimes make competitors more like each other? A new study, published through PLoS ONE this week, shows that red-backed shrikes prefer to set up hunting territories in places where their competitors have already been hunting.

Photo by phenolog.
Shrikes are cute but vicious predators - they capture small prey and spear them on thorns or twigs for storage, or to indicate to a prospective mate what great hunters they are. Red-backed shrikes migrate from Africa to Eastern Europe for the summer mating season. When they arrive, male red-backed shrikes must establish a hunting territory with a nesting site, but they have to contend with the established territories of great gray shrikes, which live in the same area year-round, and eat the same kind of prey.

You might expect, then, that red-backed shrikes would establish nest sites well away from the impaled victims of great gray shrikes. In fact, as the paper's authors show, red-backed shrikes are more likely to nest near great gray shrike caches. They don't raid the competitors' larders, but, the authors argue, understand the presence of a great gray shrike's cache to mean there is plenty of prey nearby.

This could mean a number of things: perhaps great gray shrikes and red-backed shrikes prey on critters that are so abundant, it's arguable that they're not really competing. If that's the case, it makes plenty of sense for red-backed shrikes to use great gray shrike caches as cues to find particularly good hunting grounds. Alternatively, red-backed shrikes settling near great gray shrike caches might shift their prey preferences to avoid competition - the presence of one type of prey may very well correlate with the abundance of many other types, so that the great gray shrike caches are only indirect indicators of prey abundance. Unfortunately, the current paper has no data comparing prey preferences of red-backed shrikes nesting nearby and away from great gray shrike caches, so there's no way to test this hypothesis.

Still, this observation has significant implications for the way we think about species interactions across evolutionary time. If competitors can be drawn together as well as driven apart, maybe competition doesn't contribute to diversification as much as we think it does.


M. Hromada, M. Antczak, T.J. Valone, P. Tryjanowski (2008). Settling decisions and heterospecific social information use in shrikes PLoS ONE, 3 (12) DOI: 10.1371/journal.pone.0003930

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