The early development of forest fragmentation effects on forest organisms is poorly understood partly because most fragmentation studies have been done in agricultural or suburban landscapes, long after the onset of fragmentation. We develop a temporal model of forest fragmentation effects on densities of forest‐breeding birds and provide data from an active industrial forest landscape to test the model. The model and our empirical data indicate that densities of several forest‐dwelling bird species can increase within a forest stand soon after the onset of fragmentation as a result of displaced individuals packing into remaining habitat. Along with higher densities in the newly formed fragments, pairing success in one species, the Ovenbird (Seiurus aurocapillus), was lower in fragments than nonfragments, possibly due to behavioral dysfunction resulting from high densities. Thus, density was inversely related to productivity. The duration and extent of increased densities following onset of fragmentation depends on many factors, including the sensitivity of a species to edge and area effects, the duration and rate of habitat loss and fragmentation, and the proximity of a forest stand to the disturbance. Incipient forest fragmentation may affect populations differently from later stages of fragmentation when the geometry of the landscape has reached a more stable configuration. Our model and data indicate, for reasons unrelated to traditional edge effects, that large tracts of forest can be important because they are relatively free from the variety of plant and animal population dynamics that might take place near new edges, including the encroachment of individuals displaced by habitat loss.

No abstract

No abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. . Wiley and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Ornis Scandinavica. , J. L. 1991. The relationship between latitude and the timing of spring migration in North American landbirds. -Ornis. Scand. 22: 129-136.Under laboratory conditions, two mechanisms have been identified that control the onset of migration. One mechanism inflexibly controls the initiation of migration, while the other requires external input from a changing environment. It has been suggested that the evolution of these mechanisms has an ecological basis related to the wintering latitude of the species. To establish whether the timing of migration was consistent with this ecological explanation, we studied the long-term mean timing of spring migration, and within-year and among-year variance in timing, in 27 free-living Nearctic migrant species. Species that wintered in higher, temperate latitudes migrated significantly earlier than tropical-wintering species. Tropical-wintering species showed significantly less within-year and among-year variation in the timing of migration, suggesting that the mechanism regulating their migration is primarily endogenous, with little influence of external stimuli. In contrast, breeding latitude showed no relationship to the mean date of migration, or the within-year or among-year variation in timing. These results are consistent with the notion that conditions that indicate time-of-year are unreliable or change imperceptibly (photoperiod) in the tropics, making an endogenous circannual clock valuable for controlling the onset and progression of migration. Temperate-wintering species were more likely to show interspecific correlations in timing than tropical species, further suggesting that external cues modulate spring migration in temperate-wintering species. Our results indicate that the migration regulation system employed might be ultimately determined by wintering latitude and associated environmental cues.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. University of California Press and CooperOrnithological Society are collaborating with JSTOR to digitize, preserve and extend access to The Condor.Abstract. Mist netting and point counting have been used equally in the Neotropics for the purpose of surveying bird communities, although their effectiveness is poorly known. We compared mist netting and point counting data collected from the same survey points in a mature subtropical forest in Belize to assess their descriptions of a bird community within a small region and across many survey points. We surveyed each point for three consecutive days using one technique and then for three additional days using the other technique. Mist netting and point counting detected only 25% and 60%, respectively of all forest species. The species lists from mist netting and point counting had similar proportions of species in groupings based on families, abundance, and migratory status, and in diet, habitat use, and foraging substrate guilds. Species lists from mist netting had a greater proportion of understory and small species than the species list from point counting. Species lists from mist netting and point counting had smaller proportions of large-bodied and rare species than a local checklist. Point counting detected more species per point with greater time efficiency and more species per point in 25 of 28 guilds than did mist netting. Point counting detected three common species less frequently than mist netting, whereas mist netting detected 38 common species less frequently than point counting. Point counting detected understory species as frequently as mist netting did at individual points. The two methods had > 50% agreement on the presence of only four species at individual points. Both techniques detected different sets of common species with similar frequency, although point counting detected many more uncommon species. Although using both techniques was more effective than using either alone, point counting alone was significantly more efficient for conducting bird surveys.[955]