Parental investment, adult sex ratios, and sexual selection in a socially monogamous seabird

Although most birds are monogamous, theory predicts that greater female par ental investment and female-biased adult sex ratios will lower the polygyny threshold. This should result in polygynous mating, unless obligate bipare ntal care or the spatial and temporal distribution of fertilizable females constrains a male's ability to take advantage of a lowered polygyny thresho ld. Here we present data on the extent of male sexually dimorphic plumage, adult sex ratios and breeding season synchrony in three populations of a so cially monogamous seabird, the brown booby Sula leucogaster. For one of the se populations, San Pedro Martir Island, we also present data on difference s in male and female parental investment, mortality and probability of pair ing. The extent of plumage dimorphism varied among populations. Sex ratios were female biased in all populations. On San Pedro Martir Island, parental investment was female biased, females failed more often than males to find a mate, but there was no polygyny. We suggest that on San Pedro Martir: (1 ) a period of obligate biparental care coupled with a relatively synchronou s breeding season constrained the ability of males to take advantage of a h igh environmental polygamy potential and (2) the resulting socially monogam ous mating system, in combination with the female-biased adult sex ratio, c aused females to be limited by the availability of males despite their grea ter parental investment.