Some problems and approaches in avian mate choice

-Most studies of mate choice in birds consider questions such as the existence of mate choice, which traits are preferred, and how the preferences evolved. In order to answer these questions, it is necessary to distinguish between pairs of key processes such as dominance and choice, assortative mating and type preference, or male choice and female choice. We review these and other difficulties in the study of avian mate choice. As an illustration of methodological issues, we discuss our work on Pinyon Jays (Gymnorhinus cyanocephalus). Males that were successful in mate-choice experiments were more brightly colored and had large testes, but were not necessarily larger or dominant. Preferred females were large, dominant, and had thicker bills. In free-living populations, smaller than average males and larger than average females lived longer. Pairs composed of small males and large females had sons with higher fecundities and greater survivorship. We suggest that many ornithologists could combine their long-term field observations with controlled experiments to profitably study mate choice. Received 27 January 1989, accepted 6 November 1989. THE AREA of sexual selection is currently one of the most controversial and rapidly developing in biology (Andersson and Bradbury 1987). There is a large body of theory on sexual selection; and an increasing number of empirical studies, particularly of mate choice, focus on birds (e.g. Komers and Dhindsa 1989, Zuk et al. in press a, b). Empirical studies often fail because they do not address important theoretical questions, their methodology does not allow discrimination of key processes, or both. Our discussion of the first problem is brief, as this topic is reviewed in detail elsewhere (Arnold 1985, Bradbury and Andersson 1987). We focus here on commonly encountered methodological difficulties in the empirical study of avian mate choice. We suggest approaches to these problems and provide an example from work with captive and free-living Pinyon Jays (Gymnorhinus cyanocephalus) that illustrates some of these methodological issues. DIFFICULTIES IN THE STUDY OF MATE CHOICE A central theoretical question is how female choice can affect the evolution of secondary sex characteristics. Closely associated is the question of how the mating preferences themselves 3Current address: Department of Ecology, Ethology, and Evolution, University of Illinois, 606 East Healey, Champaign, Illinois 61820 USA. 4Current address: Star Route Box 2905, Dryden, Maine 04225 USA. evolved. The considerable theory in this area offers numerous questions for empiricists to address (Heisler et al. 1987). Empirical studies of female choice usually begin with the question of whether female mating preferences for particular male traits exist. Identifying traits.-The first difficulty in investigating mate choice is to decide which traits might be important. If there are field data for the species of interest, one can identify traits used in courtship displays, search for correlations between specific traits and mating or reproductive success, or look for assortative mating with respect to specific traits. These traits suggest a starting point for experimental study. If there are no relevant field data, theory and common sense can suggest traits for experimentation. Ideally, one should then go to the field to determine the fitness consequences for individuals with the preferred traits. Is mating nonrandom?-Once appropriate traits have been identified, it is necessary to document nonrandom mating for the traits of interest. Standard statistical procedures can be applied to the results of laboratory experiments. In the field, however, it can be unclear whether an observed pattern is the result of preferences for a particular trait (type preference; Burley 1983) or prevalence of that trait in the pool of prospective mates at the time mating occurs (Cooke and Davies 1983). For example, in several bird species, the ages of partners are correlated (Reid 1988). Reid (1988) has shown that, with stable pair bonds, such correlations can 296 The Auk 107: 296-304. April 1990 This content downloaded from 157.55.39.132 on Thu, 15 Sep 2016 05:06:37 UTC All use subject to http://about.jstor.org/terms April 1990] Avian Mate Choice 297 occur in the absence of mate choice, through random mating alone. To investigate apparent nonrandom mating patterns, it is not appropriate to apply standard statistical procedures (such as Chi-square approximations or correlation analyses) to the traits of a free-living population of existing pairs, because these tests require that different pair bonds form independently. Mate pools are finite, and once two individuals in a finite mate pool pair, the remaining individuals have reduced choices that depend on previous choices. In monogamous species, this continues until the last two individuals have no choice but to pair with one another. Such clearly dependent events cannot be analyzed with parametric or nonparametric tests of association. Randomization procedures can avoid this problem by comparing all possible (random) pairings at a given time with the pairings actually observed (Marzluff and Balda 1988b, Johnston and Johnson 1989). For this approach, it is necessary to know which individuals were available in the mate pool at the beginning of the pairing period and calculate the N! possible patterns that can be formed between N males and N females. Nonrandomness of the observed pairing pattern is calculated by determining how rare that pattern is relative to all possible patterns. Factors confounding choice. -To decide whether nonrandom mating patterns are the result of mate choice, it is necessary to rule out the process of intrasexual competition as a complicating factor. Consider female choice of males in a typical nonterritorial species in which males usually show behavioral dominance to females. If nonrandom mating occurs, it can be unclear whether the mating pattern is a result of female preference for certain male types or whether intra-male competition allows dominant males to monopolize access to females. Even when females appear to choose males, male-male competition can mediate female choice. For example, female American Wigeons (Anas americana) mate more often with larger, heavier adult males in full alternate plumage, but these males also tend to be dominant (Wishart 1983; see also West et al. 1981, for Brown-headed Cowbirds,