NEST-BUILDING, SEXUAL SELECTION AND PARENTAL INVESTMENT

Avian nest building has traditionally been viewed as resulting in natu ral selection advantages, but it is also been associated with courtshi p and pair formation. We hypothesize that nest-building activity could be used as a sexually selected display, allowing each sex to obtain r eliable information on the condition of the other. In this paper, we t est the `good parent' process in a scenario where nest size is a sexua lly selected trait. Thus, individuals with more extreme displays (larg er nests) might obtain benefits in terms of either parental investment or differential parental investment by the partner. We predicted that : (1) species in which both sexes contribute to nest building have lar ger nests than those in which the nest is built only by one sex, becau se both sexes are using the nest-building process as a signal of their quality; (2) species in which both sexes work together in the nest-bu ilding process invest more in reproduction, because each can assess th e other more reliably than in species where only one sex participates in nest building; and (3) in light of the two preceding predictions, n est size should be positively related to investment in parental care. A comparative analysis of 76 passerine species confirmed that nest siz e, relative to the species' body size, is larger when both sexes build the nest and that species with a larger nest relative to their body s ize invest more in reproduction.