Effect of Prey and Land Use on Mating Systems of Harriers

Abstract

Of the mating systems used by Harriers (Circus c. hudsonius) monogamy was the commonest, with polygyny accounting for about one quarter of the nests. Vole (Microtus pennsylvanicus) abundance triggered both polygyny and breeding by subadult females. Polygyny occurred most often when the sex ratio was essentially equal. Successful breeders, rather than the unsuccessful tended to return to nest. Pair fidelity was not the rule. Females nested twice as far from their earlier nests, on the average (1,092 m), than males (546 m). Constriction of available habitat during the 20-yr period has altered the pattern of nest distribution and mating systems. Loss of habitat (1963-1979) was 71%, but there were more nests in 1979 than in any other year. There is a strong suggestion here that crowding, even in the presence of abundant voles, caused a decline in production that was most likely mediated through changes in breeding behavior.