Researchers have found that female birds exposed to predators while ovulating produce smaller offspring than unexposed females.
Their study also showed that the chicks may be smaller, but 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 the latest study by Swiss ecologists suggests that predator effects could go beyond behaviour, to physiology.
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.
Coslovsky and Richner found 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," Nature quoted Coslovsky as saying.
The study has been reported in the journal Functional Ecology. (ANI)