Reproductive consequences for males of paternal vs territorial strategies in the polygynous spotless starling under variable ecological

Abstract

AbstractThe males' trade-off between caring for the offspring or investing in attracting additional mates is well established in the theory of mating systems. The reproductive consequences for males of alternative strategies adopted by them in response to these conflicting demands should depend on several ecological and social factors that may strongly vary among years. This variation, however, has been rarely addressed in field studies despite it being essential to understand the evolution of parental care in facultatively polygynous species. In the present paper, we examine the reproductive consequences of paternal vs. territorial strategies in the polygynous spotless starling (Sturnus unicolor) during four consecutive breeding seasons (1996-1999) in which the number of male defending nest boxes and several population variables such as nestling starvation rates, nest predation rates and fledgling production varied markedly. In 1996 we manipulated the propensity of males to feed their offspring by means of androgens and antiadrogens, and examined the consequences of male behaviour on reproductive success during the subsequent four breeding seasons. Males implanted in1996 with the antiandrogen cyproterone acetate (Cy-males) fed more frequently than males implanted with testosterone (T-males) or control males (C-males) both during the season in which they were implanted and in the next season. The number of nest boxes defended influenced total breeding success mainly in the years when male feeding frequency was less important for breeding success per nest. A lower proportion of Cy-males than T- or C-males still held at least a nest box in 1999, three years after they were implanted. As a result, the number of fledglings produced over the four years studied was smaller for Cy-males than for T- or C-males. These results suggest that the oscillating selective forces acting on breeding male starlings seem to operate more frequently against the development of parental care strategies, while the polygynous strategy seems to be favoured under a wide array of ecological conditions. Although increased density of potential breeders may constrain territorial expansion and polygyny, the high cost of losing a minimum breeding site may select for the maintenance of aggressive behaviour and low parental investment.