NESTING SYNCHRONY BY FEMALE RED-WINGED BLACKBIRDS - EFFECTS ON PREDATION AND BREEDING SUCCESS

I analyzed some of the factors affecting female breeding success in an eastern population of Red-winged Blackbirds (Agelaius phoeniceus). Over three seasons, 37% of all nests produced at least one fledgling. Over 77% of nests that failed were destroyed by predators. Nests in all 3 yr were clumped in time, with 50% of the nests being started in just 16% of the season. Temporal proximity of other nests (number of other nests started within +/- 2 d) was positively associated with the number of chicks fledged. The probability of avoiding predation increased with number of temporal neighbors even with date of first egg and female age controlled in a multivariate analysis. Clutch size was associated only with settlement date and not number of temporal neighbors nor date of first egg. Breeding earlier by females resulted in higher total success for some measures in some seasons, but early nests failed sooner and more frequently. Returning females bred earlier than new females, and even though their first nests were more likely to fail, they typically renested within the peak of nesting or, if their first nest succeeded, sometimes started a second brood before the end of the season. New females to the study area started nests late, but tended to do so within the peak of nesting thereby improving their chances of success. These results highlight the importance of considering demography in studies of the ecological causes of synchronous breeding and the timing of avian breeding seasons.