The effect of manipulated brood size on parental defence in a precocial bird, the Willow Ptarmigan

Offspring number was manipulated in Willow Ptarmigan Lrzgopus lagopus to 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 based effort on the value of the brood, it was predicted that defence should increase with offspring number because ptarmigan parents receive greater returns from large broods. Parental effort in defence was unaffected by brood size, probably because a predator cannot threaten the entire brood sin~ultaneously when ptarmigan young are scattered. This evidence supports theoretical models which predict that if the young are able to disperse, patterns of defence will be different in precocial birds from that in fish or altricial birds.