POPULATION DYNAMICS OF INDIGO BUNTINGS AND THE EVOLUTION OF AVIAN POLYGYNY.

Avian mating systems and their evolution continue to be the subject of attention in behavioral and ecological literature (e.g., Wiley, 1974; Wilson, 1975; Wittenberger, 1976; Emlen and Oring, 1977). We define polygyny, the subject of this paper, as the simultaneous possession of more than one mate by a male; systems in which pair bonds are successive or in which no male-female bond is formed and the sexes meet only for copulation are excluded from our definition. Lack (1968) concluded that polygyny is the mating system of only 2% of the world's bird species; Verner and Willson (1969) found that 14 of 317 North American passerines are polygynous, although recent investigations have increased that number slightly (see below). A model for the evolution of polygyny in birds was suggested by Verner (1964) and Verner and Willson (1966) and was developed into a general theory for the evolution of avian and mammalian mating systems by Orians (1969). This model has been widely accepted (e.g., Emlen, 1973; Brown, 1975; Wilson, 1975), elaborated on (Wittenberger, 1976; Altmann et al., 1977), and incorporated into more comprehensive theories about mating systems (Emlen and Oring, 1977). However, acceptance of it has not been universal (Wiley, 1974). The purpose of this study was to investigate the Verner-Wilson-Orians (hereafter VWO) model in two ways. Primarily, the objective was to test the model's ability to predict polygyny in a population of an altricial bird species whose mating system was unknown. Reasons for selecting the indigo bunting (Passerina cyanea) as the subject are presented below. Secondly, data were collected on the breeding biology and population dynamics of the indigo bunting in order to learn whether these would conform to predictions of the model. The study was conducted from 1973 through 1976. Preliminary results were reported earlier by Carey and Nolan (1975). The rarity of avian polygyny is usually attributed to the lowered fitness to be anticipated for a female that chooses an already-mated male. A male having two or more altricial broods simultaneously dependent on him will reduce, of necessity, the parental attention given to each. Other disadvantages may arise from density effects, such as attraction of predators to, or depletion of food resources on, a populous territory. However, according to the VWO hypothesis, all such disadvantages may be compensated for if critical resources are distributed sufficiently unequally among male territories and therefore controlled in disparate amounts by males (see Fig. 1). Given a wide enough range in territory quality (i.e., a range exceeding the "polygyny threshold" of Fig. 1), then a female's expected productivity may be greater if she chooses a mated male on a high-quality territory rather than a bachelor on a poor territory. Orians (1969) emphasized two circumstances that may affect the probability of occurrence of polygyny: 1) range in quality of territories and 2) extent to which female productivity is dependent on male aid in parental care. (Other conducive circumstances have been pointed out by various authors-e. g., asynchronous formation of sexual bonds [Emlen and Oring, 1977] and dissolution and reformation of bonds in midseason [Nolan, 1978].) To consider Orians' points, under two conditions a