Ovarian and oviductal morphology of a brood parasitic bird, Molothrus bonariensis (Passeriformes, Icteridae)

Brood parasitic birds such as cowbirds (Molothrus spp., Icteridae) lay their eggs in the nests of other species, abandoning them to be incubated and raised by the hosts. Lack of investment in parental care results in high annual fecundities of female parasitic birds. Brood parasites lay eggs in series or clutches, separated by gaps or non-laying intervals of a few days, but they may continuously lay for prolonged periods. In tropical latitudes, the fecundity of female shiny cowbirds (Molothrus bonariensis) has been estimated to be 120 eggs per year in a six-month breeding season, a fecundity that is paralleled only by domestic fowl. However, no detailed morphological descriptions of the reproductive tracts of brood parasites are available. We studied the reproductive tract of female shiny cowbirds in northeastern Colombia, searching for morphological features that might be related to this high fecundity. The reproductive tracts of female shiny cowbird showed no departures from the standard morphology or follicular dynamics known for birds with different postural patterns and fecundities, suggesting that high fecundity in cowbirds does not require a specialized reproductive tract. Instead, explanations must be sought at physiological levels.