Dirty dinner.
Parasites may help put songbirds on this European sparrowhawk's dinner plate.

John Hawkins / CORBIS

To Kill a Songbird

In natural selection's dance of predator and prey, predators have long been thought to target the weak and sick. But this idea has seldom been tested because it's not easy to know the health of the prey, and how this correlates with the possibility it will be killed by a predator. However, two researchers have now zeroed in on what may make certain songbirds more likely to end up in a raptor's talons: parasites. Danish songbirds infected with either of two parasites were more likely to end up a meal. The study also sheds light on how predation may keep such parasites in check.

To determine the health of the raptors' prey, Jan Tottrup Nielsen, an ornithologist in Sindal, Denmark, spent 27 years collecting and identifying the remains of nearly 46,000 birds from some 3200 nests of European sparrowhawks and Eurasian goshawks in the forests of northern Denmark. The sparrowhawks targeted small songbirds from 64 species including tits, thrushes, finches, sparrows, and buntings. In contrast, the goshawks went after larger prey, such as gulls, pheasants, partridges, pigeons, and starlings, killing 76 different species.

To find out whether parasites played a role in the deaths of these birds, Nielsen's colleague, Anders Pape Møller, an evolutionary biologist at the University of Pierre and Marie Curie in Paris, calculated how prevalent such parasites are in European birds overall by compiling several published and unpublished data sets. After taking into account such factors as the abundance and size of the prey, Møller's analysis showed that the risk of predation for prey targeted by goshawks and sparrowhawks increased by a factor of 25 if the prey birds were infected with the protozoan blood parasite Leucocytozoon. Further, birds infected with the avian malaria parasite were 16 times more likely to find themselves in the talons of a sparrowhawk.

Besides bolstering the theory that weak and sick prey are more vulnerable to being eaten, the study indicates that predation may ultimately keep the malaria parasite in check, the authors note. The severity of a malaria infection correlates with the parasite's replication rate: The higher the rate, the more virulent the infection. In essence, Møller and Nielsen argue in their study published in this month's issue of Ecology, the sparrowhawks are helping to keep the malaria parasite relatively benign by killing songbirds with the most virulent form of the disease. The organism is spread via mosquito bites, and so once the host is dead, so is the parasite.

The team examined "an impressive sample size," says Dennis Murray, a population geneticist at Trent University in Peterborough, Canada. But he cautions that the duo's reliance on the databases of other researchers to calculate the prevalence of parasites in these species is risky. "It would be better to find a partially eaten prey and then test it to see if it actually has malaria," he notes. Still, Murray acknowledges, Møller and Nielsen have shown for a great variety of birds that a heavy load of parasites is likely to put you in natural selection's cross-hairs.

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