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Life Span, Competition, and Delayed Plumage Maturation in Male Passerines: The Breeding Threshold Hypothesis

Potential life span has been largely ignored in previous hypotheses for the evolution of delayed plumage maturation in male passerines. Furthermore, its interaction with other factors has not been considered fully. We develop a multiple-factor breeding threshold model based on the costs for a subadult male competing with other males for reproductive opportunities, and on the potential number of subsequent breeding seasons available. In our model, we assume that adult males are more capable of acquiring and defending territories and mates under conditions of habitat or female limitation than are subadults. Subadult males should delay breeding only if there is a large enough difference in the probability of mortality between those subadults who attempt to breed and those who do not. The size of the necessary mortality difference increases as potential life span decreases. The model also predicts that subadult males should make some competitive effort to facultatively exploit available reproductive opportunities, so long as the additional probability of mortality does not exceed the threshold value. Subadult plumage would evolve in those species to further reduce mortality from direct competition with older males, probably by signaling subordinance or mimicry of females. Therefore, we view delayed plumage maturation as a component of facultatively delayed breeding. Although reproductive maturation is not delayed, full reproductive effort is. Comparison of the group of strongly dichromatic passerine species that have subadult plumage with the group of strongly dichromatic species that lack this plumage revealed that species with subadult plumage tend to be larger. This result indicates that life span is an important factor because large species tend to live longer. This supports our model for the evolution of subadult plumage. Since the model predicts that competition would have to be very intense to favor the evolution of subadult plumages in short-lived species, and since close resemblance to an adult female could minimize costly aggressive interactions with adult males when competition is intense, there also should be a correlation between degree of resemblance and life span. As predicted, subadult males in the smaller, short-lived species that have subadult plumage more closely resemble adult females than those in the larger, longer-lived species.