Predation risk affects offspring growth via maternal effects

1. Predation risk is known to influence behaviour and development, especially in breeding animals. Mothers may be selected to transfer information about the intensity of such risk to their offspring through maternal effects and thus influence their development. 2. Here, we test for this maternal effect via manipulation of perceived predation risk by exposing great tit females before and during ovulation to stuffed models and sounds of either a predator bird (sparrowhawk - Accipiter nisus) or of a non-predatory control (song thrush - Turdus philomelos) in their environment. Offspring of exposed mothers were then raised by foster parents subjected to no treatment in order to separate maternal effects from effects during post-hatching parental care. 3. Nestlings of mothers under increased predator density were smaller than those of control mothers yet showed higher growth rates of the wings. Additionally, first-year recruits from the predator treatment had longer wings at maturity. 4. This maternal effect may be a passive consequence of higher circulating stress hormone levels in mothers. The accelerated wing growth during the nestling stage may be a result of compensatory growth. 5. Alternatively, the accelerated wing growth, coupled with the longer wings at maturity, suggest that the maternal response to the environmental risk may be adaptive since lower weight and bigger wings are a selective advantage for predator evasion. In this case the maternal effect probably influences the distribution of resources to different growth functions in offspring. 6. We show for the first time through an environmental rather than a direct hormonal manipulation, that predation risk may elicit adaptive maternal effects in birds.