MIGRANT STOPOVER AND POSTFLEDGING DISPERSAL AT A MONTANE FOREST SITE IN VERMONT

Abstract

To investigate the use of high elevation fir forests by fall passage migrants, we conducted standardized mist-netting and banding at a 1150-1175 m elevation site on Mt. Mansfield in north-central Vermont during the autumns of 1995-1997. Overall, we captured 3024 individuals of 62 species in 10,048 cumulative net hours (30.1 birds/100 net hr). We divided species into 3 classes: (1) those breeding regularly on Mt. Mansfield above 916 m elevation (16 species), (2) those breeding only sporadically or at very low densities above 916 m (6 species), and (3) those occurring only as transients (40 species). Breeding species accounted for 68% of new captures, followed by transients (22%), and sporadic/low density breeders (10%). Of the ten most abundant species captured, only two, Black-throated Blue Warbler (Dendroica caerulescens)and Ovenbird (Seiurus au- rocapillus), were transients, accounting for 50% and 9%, respectively, of all transient captures. Hatching-year birds accounted for 94% of known-age transients and 81% of known-age individuals among locally breeding species. Recapture rates of transients were extremely low (0.3%), while recapture rates among species known to breed locally (including presumed transient individuals) were higher (2.2%). Nearly 75% of all birds captured were very lean at first capture, and only 26% of recaptured individuals increased their fat scores between first and final captures. Weight changes of recaptured birds varied: 48% lost weight, 44% gained weight, and 8% maintained the same weight between first and final captures. Our data suggest that conditions on the Mt. Mans- field ridgeline are not conducive to prolonged migratory stopovers and that most migrants may be unable to meet their energetic requirements for continued migration. However, we believe that montane forest habitats may be preferentially selected by those migrants that use them for breeding. We further believe that montane fir forests may be an important postfledging dispersal habitat for Black-throated Blue Warblers and other low or mid-elevation breeding species, and that conservation planning for montane forest sites should carefully consider the needs of migrants outside the breeding season. Received 18 March 1999, accepted 25 Oct. 1999. Of the numerous demands that migrant birds face during their annual migrations, se- lection of enroute stopover habitat is para- mount. Choice of stopover habitat may play a pivotal role in determining a migrant's surviv- al and is dependent both on intrinsic suitabil- ity of the habitat and on extrinsic factors such as weather, time constraints, energetic condi- tion, and availability of the habitat (Hutto 1985, Moore and Simons 1992). Stopover mi- grants must resolve potentially conflicting needs to meet their energy demands, avoid predation, adjust to unfamiliar habitats, com- pete with other migrants and residents, and deal with unfavorable weather conditions (Moore et al. 1995). Loss or degradation of