Broodedness in Bobolinks

Extensive quantitative information on the number of broods raised per female bird per season is scarce (Cody 1971).This knowledge is necessary for an accurate analysis (or modelling) of population dynamics or reproductive "strategies" of individuals.In particular, a comparison of the reproductive fitness of monogamous males with that of polygynous males depends on knowing the reproductive success of paired femalesß which is pertinent also to questions dealing with sexual selection, female choice, and polygyny threshold models.To obtain complete data on reproduction, it is necessary that individual females be marked and monitored throughout the breeding season.Replacement of lost clutches of eggs is common in birds (Lack 1968), but Lack (1968: 302) generalized that "most species of birds raise only one brood in a year, because the time required for courtship, nestbuildingß incubation and raising the young to independence is so long that a second brood could not normally be completed before the ecological conditions which permit breeding have ended for the year."Great Tits (Parus major), however, were found to raise two broods per year more frequently in habitat where food was more abundant (Perrins 1965), and older females were more likely to be double-brooded than younger females in this species (Kluijver 1951).The latitude at which a bird breeds apparently is related to broodedness in two opposing ways.Lack (1968: 196) stated that "... most passerine birds of high latitudes raise more than one brood each year .... "presumably because the longer daylength reduces the time to fledging relative to that at lower latitudes.Ecological conditions, however, may be suitable for nesting for a longer period of time at lower latitudes, and, therefore, more broods per female may be raised in a year than at higher latitudes (e.g.doves, many passerines).The Bobolink (Dolichonyx oryzivorus) is a polygynous, ground-nesting icterid that winters from November to March in South America between latitudes 8øS and 32øS (Engels 1969) and breeds in North American hayfields and meadows from May to July between latitudes 40øN and 50øN.Recent studies of populations of marked birds in Wisconsin (Martin 1971), Oregon (Wittenberger 1978), and New York (R. L. Kalinoski pers.comm.)reaffirmed the earlier conclusion of Bent (1958) that Bobolinks can renest after nest failure but do not attempt a second nesting after fledging young from the first nest.An un-