Polygyny in the bobolink : habitat quality and the adaptive complex Public Deposited



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  • The adaptiveness of polygyny in passerines poses a particularly intriguing problem. The altricial young of this group require extended parental care, but young of polygynous pairings generally lack the undivided attention of the male which young of monogamous pairings receive. Polygynous offspring may thus be undernourished and experience greater mortality. Hence, the selection for polygyny in populations where unmated males are available suggests that an uneven distribution of ecological factors affecting breeding success permits females settling in excellent habitat to raise more offspring unassisted by a polygynous male than can be produced with full male cooperation in a less suitable area. This study's central objective was to determine if male Bobolink (Dolichonyx oryzivorus) pairing success was correlated with specific ecological factors within their territories, and if territory to territory differences in quality were sufficient to account for the selection of this mating pattern. In addition, I examined those adaptive modifications of breeding biology and parental behavior which served to minimize reproductive failure at nests not attended by males. Measures of various structural features of the vegetation were made at 1921 randomly-located sample points within an 11.3 ha sedge-grass meadow in south central Wisconsin. The data from sample points falling within bachelor-occupied territories were grouped and statistically compared with values from territories of other mating classes, and with values from 388 sample points which surrounded nest cups. In addition, the breeding biology of a large number of color-banded individuals was intensively studied over five breeding seasons. These observations included carefully standardized recordings of adult time-budgetings. Morphological and behavioral differences between males and inter-territorial differences in food abundance were of minor importance in female mate choice compared to the enormous habitat disparities between different territory classes. Areas defended by bachelors, monogamists, bigamists and trigamists differed significantly in the density of large forbs which provided the only satisfactory nest concealment at the time nesting commenced. Furthermore, significant (P < 0.01) discrepancies existed between these territories in the distribution and abundance of almost all habitat features characterizing nesting ares (per cent coverage of grasses, sedges and forbs, shading properties of the vegetation, and the vegetative foliage height profile). Bachelor territories were deficient in features important in female nest site selection and were thus unsatisfactory for breeding. The quality of monogamous and bigamous territories was improved over that in bachelor holdings, and trigamists occupied the most satisfactory habitat. Thus, when some males are forced into habitats too marginal for successful breeding, this polygynous system grants each female an opportunity to nest in a satisfactory location. Bobolink males generally assisted only their first (primary) female in feeding and brooding nestlings. A number of interrelated adaptations (the adaptive complex) functioned to circumvent heavy reproductive failure at secondary nests, where males did not aid. Secondary clutch sizes were significantly smaller than primary egg sets. This, combined with asynchronous hatching which promoted rapid brood reduction, brought brood size into correspondence with food transfer capabilities of secondary females. Moreover, male flexibility in feeding unusually large secondary broods and in caring for secondary nestlings late in the season increased secondary nesting success. Two of a series of other adaptive patterns were: (1) an optimization of the food delivery tempo of primary pairs, which probably extended breeding longevity; (2) a postponement of energy expenditure by yearling males in territorial and sexual advertisement behavior until special circumstances permitted opportunistic establishment in good habitat. This decreased unnecessary competition with older, experienced males.
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