Timing of breeding and the seasonal importance of passerine prey to northern harriers (Circus cyaneus)

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Abstract
Despite a numerical and functional dependence on microtine mammals, breeding northern harriers (Circus cyaneus) in New Brunswick preyed upon large numbers of young passerine birds following the hatch of their own nestlings. An independent index of juvenile passerine availability showed that harrier pairs switched to young passerines as soon as they became available, and not simply because their own young had hatched. Many nests in which young hatched during the period of juvenile passerine availability had high fledging success, but the mean was significantly lower than that of early nests. A seasonal decline in success was highly significant, so if an adaptive temporal breeding strategy exists, it may simply be to breed early if possible and, if not, to coincide with the flush of passerines. We conclude that the coincidence of harrier and passerine prey nesting seasons is as likely to be fortuitous as strategic.
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