THE EFFECT OF MANIPULATED BROOD SIZE ON PARENTAL DEFENSE IN A PRECOCIAL BIRD, THE WILLOW PTARMIGAN

Offspring number was manipulated in Willow Ptarmigan Lagopus lagopus t o determine the effect of brood size on parental effort in defence of young. Three chicks were added (n = 8), exchanged (n = 10) or removed (n = 12) among broods of females that had laid 7-9 eggs. If parents ba sed effort on the value of the brood, it was predicted that defence sh ould increase with offspring number because ptarmigan parents receive greater returns from large broods. Parental effort in defence was unaf fected by brood size, probably because a predator cannot threaten the entire brood simultaneously when ptarmigan young are scattered. This e vidence supports theoretical models which predict that if the young ar e able to disperse, patterns of defence will be different in precocial birds from that in fish or altricial birds.