Spacing and Breeding Density of Willow Ptarmigan in Response to an Experimental Alteration of Sex Ratio

Abstract

Willow ptarmigan usually pair monogamously and each member of the pair defends the territory against intruders of the same sex during the breeding season. The effect of reducing numbers of 1 sex on the density of the same and opposite sex was determined to test the hypothesis that spacing behavior limits breeding density of both sexes independently. When removed, territorial females were largely replaced by yearlings, of unknown origin, able to breed. A large reduction in the density of females did not result in a similar reduction in the density of males. When removed, territorial males were largely replaced by yearlings of unknown origin, able to breed. Remaining resident males expanded their territories. When density of males was reduced, females settled at high density, mated polygynously, and defended territories against each other within the larger territories of the males. The density of each sex is determined independently through within-sex spacing behavior, but settlement patterns of males and choice of territories by females may influence final breeding density of the opposite sex.