589 The Auk 118(3):589–609, 2001 THIRTY-YEAR BIRD POPULATION TRENDS IN AN UNFRAGMENTED TEMPERATE DECIDUOUS FOREST: IMPORTANCE OF HABITAT CHANGE RICHARD T. HOLMES 1,3 AND THOMAS W. SHERRY 2 1 Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA; and 2 Department of Ecology and Evolutionary Biology, Tulane University, New Orleans, Louisiana 70118, USA ABSTRACT.—Abundances of forest birds in an unfragmented, undisturbed, and relatively mature temperate deciduous forest at the Hubbard Brook Experimental Forest, New Hamp- shire, changed markedly between 1969 and 1998. Total numbers of birds (all species com- bined) declined from 210–220 individuals/10 ha in the early 1970s to 70–90/10 ha in the 1990s. Of the 24 regularly occurring species, 12 decreased significantly (four to local extinc- tion), three increased significantly, and nine remained relatively constant in abundance. Nine of the 12 declining species were Neotropical migrants. Most species exhibited similar trends on Breeding Bird Survey (BBS) routes in New Hampshire during the same 30 year period and on three replicate study sites in nearby sections of the White Mountains from 1986–1998. Probable causes of trends were diverse and differed among species. Most could be accounted for by individual species’ responses to events occurring primarily in the local breeding area. The most important local factor affecting bird abundance was temporal change in forest veg- etation structure, resulting from natural forest succession and local disturbances. Four spe- cies that declined markedly and in some cases disappeared completely from the study plot (Least Flycatcher, Empidonax minimus; Wood Thrush, Hylocichla mustelina; Philadelphia Vir- eo, Vireo philadelphicus; and American Redstart, Setophaga ruticilla) appear to attain peak abundance in early or mid successional forests. Species preferring more mature forests, such as Black-throated Green Warbler (Dendroica virens) and Ovenbird (Seiurus aurocapillus), in- creased significantly in abundance over the 30 year study. Other important factors influ- encing bird abundances were food availability and events in the migratory and winter pe- riods. Nest-predation rates, although varying among years, showed no long-term pattern that would account for population declines, and brood parasites were absent from this forest. Findings from this study demonstrate that major changes in bird abundances occur over time even in undisturbed and relatively mature forests, and illustrate the need for considering habitat requirements of individual species and how habitat suitability changes over time when trying to assess the causes of their long-term population trends. The results also imply that any conclusions about the effects of other factors affecting forest bird abundances, such as increased nest predation or brood parasitism associated with habitat fragmentation, must also account for successional changes that may be affecting habitat suitability. Received 28 July 2000, accepted 28 February 2001. MANY SPECIES AND POPULATIONS of North American birds have undergone declines in abundance in recent decades, especially migra- tory songbirds. Some species have declined throughout their breeding range (Robbins et al. 1989, Peterjohn et al. 1995), whereas others have declined in some regions but remained stable or even increased in others (James et al. 1996). The extent and causes of such changes in abundance are much debated (Askins et al. 1990, Hagan and Johnston 1992, Rappole and McDonald 1994, 1998; Sherry and Holmes 1995, 1996; Martin and Finch 1995, Latta and Baltz 3 E-mail: richard.t.holmes@dartmouth.edu 1997, Askins 2000). Some explanations have fo- cused on the effects of deforestation and other changes in the tropical habitats where many of these species winter, whereas others empha- size habitat degradation and events in the north temperate breeding grounds. For the latter, ex- tensive fragmentation of large tracts of forest land, with associated increases in nest preda- tion and in brood parasitism by Brown-headed Cowbirds (Molothrus ater), is perhaps the most frequently cited factor affecting the abundanc- es of breeding songbirds in North America (Brittingham and Temple 1983, Wilcove and Robinson 1990, Askins et al. 1990, Bo ¨hning- Downloaded from https://academic.oup.com/auk/article/118/3/589/5562119 by guest on 19 November 2023