Parental Care and Sexual Selection in the Bluethroat,Luscinia s. svecica: A Field‐experimental Test of the Differential Allocation Hypothesis

Abstract

One of the processes by which secondary sexual characters in monogamous species with biparental care may evolve is described by the differential allocation hypothesis. It predicts that mates adjust their parental effort to the attractiveness of their partner. The more attractive sex is expected to withhold parental effort while the less attractive sex is expected to compensate for the partner’s decreased effort thus preventing a reduction in breeding success. Asymmetry in attractiveness then imposes costs on the less attractive sex which are thought to be exceeded by the indirect (i.e. genetic) benefits which result from sharing offspring with a highly attractive partner. This idea was tested with bluethroats,Luscinia s. svecica, a bird species in which the male is characterized by a complex multiple‐coloured plumage ornament on the throat and chest. We manipulated male attractiveness by attaching conspicuous coloured leg bands which matched the blue and chestnut components of the male ornament. Control males were banded with green and yellow bands. The results from another field experiment on mate‐guarding behaviour demonstrate the effectiveness of this treatment. As measures of parental effort we determined: 1. clutch size; 2. feeding rates by video recording feeding activity of both parents; and 3. brood mass up to day 9. The results do not support the differential allocation hypothesis. The experimental and control groups differed in none of the above variables. On the basis of these results we discuss the ecological factors which may influence the differential allocation process in monogamous birds.