Multiple Brooding and Productivity of a Neotropical Migrant, the Black-Throated Blue Warbler (Dendroica caerulescens), in an Unfragmented Temperate Forest

We studied a population of Black-throated Blue Warblers (Dendroica caerules- cens) in the White Mountains of central New Hampshire to determine the reproductive performance of this Neotropical migrant species in an unfragmented north-temperate forest. Over a four-year period (1986-1989), female Black-throated Blue Warblers laid an average of 6.6 eggs per season of which 5.2 hatched and 4.3 fledged. This high annual production of young was due to a combination of overall high nesting success (63%), low nest depredation rates (22%), no brood parasitism, and, most importantly, frequent multiple brooding (48%). Over the four years, 35% (range = 14-50%) of the individually-marked females in this pop- ulation successfully fledged two or more broods; one female fledged three successive broods in one season. Multiple brooding accounted for most of the annual variation in numbers of young fledged per female, and its frequency was related to annual differences in food abun- dance and, potentially, to time constraints imposed by the intensity and timing of nest depredation. Multiple brooding was significantly and positively related to age of the male parent, and possibly the female parent. Older males were also more likely to be polygynous, although such matings were infrequent (4-16% of mated males each year). Higher repro- ductive output by older individuals implies that demographic structure of Neotropical mi- grant populations may have important consequences for maintenance of population size. Recruitment of yearling male Black-throated Blue Warblers into the population was positively related to the number of young fledged per female in the preceeding summer. Although not statistically significant, this finding in combination with the above results and other published information suggests that reproductive success and the factors affecting it may be particularly important in influencing the abundance and population dynamics of this Neotropical migrant passerine, and may overide the impact of events occurring in winter or on migration. Received 6 May 1991, accepted 2 December 1991. RECENT evidence suggests that many North American bird populations are declining, es- pecially those that breed in temperate forests and winter in the Neotropics (Robbins et al.