Patterns of courtship between cowbird subspecies: evidence for positive assortment

The courtship and mating behaviour of captive individuals from allopatric populations of two cowbird subspecies (Molothrus ater ater and Molothrus ater obscurus) were observed in a large indoor-outdoor aviary, using a multiple-choice design in order to assess whether they would selectively pair with members of their own subspecies. Twenty-nine of the 42 consortships observed were between consubspecifics, and 10 of the 15 copulating pairs were homogamic. Individuals tended to pair with consubspecifics whether or not they had had prior breeding experience with consubspecifics or winter contact with heterosubspecifics. This demonstration of selective courtship between individuals of widely separated populations is one of the first quantified examples, to our knowledge, of the development of assortative pairing in allopatry for a passerine species. These data place in perspective previous findings on the female cowbird's preferential response to playback of her own subspecies' song, thereby suggesting that song may be an important cue for selective mating between passerine populations.