Published online 25 March 2011 | Nature | doi:10.1038/news.2011.187

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Offspring of predator-stressed mothers grow their wings more quickly than chicks from predator-free females.

Predators can affect the physiology as well as the behaviour of their prey. Erica Olsen/FLPA RM

It seems that the constant threat of predation could have a more subtle effect on prey animals than first thought.

Female birds that are exposed to predators while they are ovulating produce smaller offspring than unexposed females, researchers have found. The chicks may be smaller, but surprisingly, their wings grow faster and longer than those of chicks from unexposed mothers — an adaptation that might make them better at avoiding predators in flight.

The mere presence of a predator can change the behaviour of prey animals. Numerous studies show that birds which are frequently presented with predators increase their nest-defence behaviours and usher their youngsters out of the nest faster, presumably to stop them from being sitting ducks. Yet a new study by Swiss ecologists suggests that predator effects could go beyond behaviour, to physiology.

In a previous study in 20051, when ovulating female barn swallows (Hirundo rustica) were presented with models of predators, researchers found that their eggs contained higher than normal levels of corticosterone, a stress hormone. A follow-up examination showed that increased corticosterone reduced egg hatchability and led to fledglings being smaller. However, no one was sure whether this was simply the negative effects of stress or an adaptive response to help offspring cope better with intense predator presence.

Keen to tease apart this problem, evolutionary ecologists Michael Coslovsky and Heinz Richner at the University of Bern in Switzerland studied a natural population of ovulating great tits (Parus major) nesting in Bremgartenwald forest near Bern. The scientists exposed the birds to stuffed models and audio calls of either predatory sparrow hawks (Accipiter nisus) or harmless song thrushes (Turdus philomelos). Two days after these mothers hatched their eggs, the young were collected and placed in the care of foster parents living on unmanipulated forest plots. All offspring were monitored while with their foster parents and tagged so that they could be studied when they left the nest.

Stunted growth

Coslovsky and Richner report in the journal Functional Ecology2 that the offspring of predator-exposed mothers were universally smaller in size than offspring from unexposed mothers just before they fledged — suggesting that either substances such as stress hormones stunted their development, or that stress-related behaviour disrupted incubation. Size at fledging is known to have a close connection with future survival.

However, the duo also found that wing growth rate differed significantly between the two study groups. The offspring of predator-exposed mothers grew their wings faster than the control group, and once matured, their wings were about 1.8 millimetres longer than control birds — a small but significant difference for flight performance.

"While smaller bodies might seem a drawback, when combined with longer wings they probably increase flight performance by decreasing wing loading and improve overall survival," says Coslovsky.

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"Evolutionarily it makes perfect sense to discover that the mothers are doing this, but we would never see it without the careful and insightful analysis that has been done so well here," says evolutionary biologist Timothy Mousseau at the University of South Carolina in Columbia.

Coslovsky and Richner speculate that the deposition of stress hormones in eggs might not be a strictly negative event, but rather could be a signal to the young to adjust their growth such that they develop small bodies and long wings that help them to avoid avian predators.

"The obvious next step is to repeat this experiment over many generations and track the fitness of the offspring over time so we can demonstrate conclusively that this is in fact adaptive," adds Mousseau.