Why are There so Many Kinds of Passerine Birds? Because They Are Small. A Reply to Raikow

In a recent note Raikow (1986) addressed the cause of passerine diversity from a cladistic perspective. He noted that 5,274 of the 9,021 extant birds are placed in the Order Passeriformes, whereas the remaining 3,747 species are placed in the remaining nearly two dozen orders. The aim of Raikow's paper was to consider whether the diversity of Passeriformes can be attributed to the existence of "key adaptations," uniquely possessed by the Passeriformes as a monophyletic group. After describing the few synapomorphic characters by which the Passeriformes are distinguished from other birds (the unusual insertion of a forelimb muscle, the spermatozoan morphology, the arrangement of the bellies of a thigh muscle, the structure of the foot, and the aegithognathous condition of the palate), he states that "the synapomorphies of the Passeriformes, whether considered individually or in a broader functional context, fail to provide any convincing key innovations that might explain the evolutionary success of the group" (p. 257). He concludes that the notion of "key adaptation" cannot be used to explain the diversity of the passerines, and that in fact it may be impossible to ever identify key adaptations in a rigorous, scientific fashion. Raikow briefly alluded to the potential significance of body size in passerine evolution by pointing out that their small size makes them well adapted to feeding on arthropods and flowering plants; however, he did not explicitly consider the potential causal relationship between size and diversity. Although the relative sizes of organisms are not traditionally used in cladistic analysis, size is nonetheless an important factor to consider in analyses of taxonomic diversity. Van Valen (1973) found that taxa comprised of small organisms tend to have more species than taxa comprised of large organisms. He documented a negative relationship between mean organism size and species diversity for angiosperms, birds, and mammals. The pattern in mammals is illustrative: of the 4,004 mammal species of the world listed by Corbet and Hill (1980), 1,591 (40%) are in the Order Rodentia and 950 (21%) in the Order Chiroptera; the species in these orders are generally among the smallest of mammals. This negative relationship has been repeatedly observed in many taxonomic groups (Hutchinson and MacArthur, 1959; Lindsey, 1966; Fleming, 1973; Stanley, 1973; May, 1986). Because Van Valen did not statistically evaluate this relationship within the birds, we recorded the coordinates of each observation from Van Valen's bird scatter plot (his fig. 6) for analysis. In Van Valen's data set, the values of mean weight and numbers of species were log transformed. A regression of these data showed a significant negative relationship between the numbers of species in bird families and the mean body weight of the species within these families (n = 102, m = -0.23, r = 0.10, P = 0.001). Such analyses are problematic because of the non-independence of data points drawn from a hierarchically structured system (Felsenstein, 1985). In addition, regression of these data is not entirely valid statistically, because the mean family weights and numbers of species per family are bimodally distributed (n = 103: body weights: D = 0.13, P < 0.01); number of species: D = 0.10, P = 0.014; D = Kolmogorov statistic for normality). This bimodal distribution is due largely to the significantly lower mean weights of passerine families (t = 9.54, P < 0.0001, df = 93.5),