Junco

Abstract: Changes in species composition and abundance of birds breeding in an unfragmented temperate deciduous forest in New Hampshire, USA, were studied intensively during 16 consecutive breeding seasons, 1969-1984. The number of species breeding in the 10-ha study area in any one year varied from 17 to 28, and averaged 24. Total numbers of individuals breeding on the 10-ha plot ranged from 214 to 89, with many species (70%) declining during the 16-yr period. Overall, there was significant positive covariation among population trends of all species, suggesting a major, perhaps single, factor affecting population levels, such as weather or food supply. No two species, however, had identical patterns of change across all 16 yr. Thus, populations in this forest fluctuated largely independently of one another, suggesting a different combination of regulatory factors for each species. The major factors pulsing or stressing bird populations in this forest included: (1) changes in food abundance due largely to irruptions of defoliating Lepidoptera (most bird species, but especially vireos and warblers), (2) harsh late spring and summer weather (Scarlet Tanager, American Redstart), (3) changes in habitat structure related to forest succession (Least Flycatcher, Philadelphia Vireo), (4) interspecific interactions, particularly interference competition (Least Flycatcher-American Redstart, Red-eyed Vireo-Philadelphia Vireo), and (5) mortality during winter for both resident and migrant populations (Hermit Thrush, Dark-eyed Junco, permanent resident species such as woodpeckers and nuthatches. Some, but not all, irruptions of defoliating Lepidoptera significantly influenced many bird popu- lations in this forest and contributed to the observed positive covariation in abundances. These irruptions produced pulses of food for breeding birds, but occurred at long and variable time intervals. Between outbreaks, food may regularly limit reproductive output of these forest birds, and low food abundance, along with the effects of predators, weather, and other mortality factors, contributes to high variability in bird reproductive success, which ultimately influences population size. Competition may also beome important during these periods of food scarcity. We therefore propose that birds in these temperate deciduous forests experience periods of prolonged food limitation, interrupted by relatively brief periods of superabundant food, a pattern contrasting with that described for other temperate systems (e.g., by Wiens 1977). From this examination of bird community dynamics on one site over many years, it is apparent that each species responds to its environment in a unique way, as determined by a variety of influences on its populations. Some of these operate on a local scale (e.g., vegetation structure, food abundance, interspecific competitors), while others function at regional (e.g., some weather effects) and global (e.g., winter events) geographic scales. These influences also differ as to their temporal persistence and predictability, and thus are difficult to detect in short-term studies. This pluralistic view of community structure for birds in temperate forests represents a compromise between nonequilibrial and com- petitionist models, and argues against the existence of a tightly organized community at any one spatial or temporal scale.

Abstract: We measured the allocation of time and energy in a population of adult Yellow—eyed Juncos (Junco phaeonotus) and their young (nestlings, fledglings, independent juveniles) throughout the breeding season using concurrent time—activity budgets and doubly labeled water. We constructed energy budgets by extrapolating laboratory measurements of metabolic heat production to field conditions using a linear heat—transfer model and the operative temperature and wind speed experienced by the free—living bird. From our data we calculated daily energy expenditure (HD), the proportion of HD allocated to physical activity vs. maintenance metabolism (basal + thermostatic costs), and foraging efficiency. We examined diet selectivity among parents and their young, and we calculated prey capture rates based on the measured energy content of insect prey. We found that adult juncos feeding young are neither food limited nor working maximally. Adults fulfilled their own energy demands, and those of their four dependent young, while foraging for 75% or less of the daylight hours. In contrast, recently independent young, 4—7 wk after fledging, must forage for >90% of the daylight hours to meet their own energy demands. Juncos younger than this cannot attain energy balance in a 15—h day without supplemental feeding by their parents, even if they forage continuously. The HD of adult juncos (mean mass 19.5 g) remained fairly constant throughout the breeding season, averaging 73.8 kJ/d (n=51), which is 2.1 times their measured nighttime basal metabolic rate. In contrast, HD of fledging juncos increased steadily with age, from 59.7 kJ/d during the 1 st wk out of the nest to 73.8 kJ/d during week 3. This increase resulted partly from an increase in the proportion of time spent foraging. Once their parents quit supplying them with food, the young juncos' HD increased dramatically, peaking at 100 kJ/d in 10—12 wk—old juveniles. The increase in HD of independent young resulted initially from a dramatic increase in time spent foraging, which gradually declined only to be supplanted by an increase in aggressive interactions, especially flights within the juvenile flocks, and a marked increase in energy demand associated with the postjuvenile molt. For adult and young juncos, thermostatic costs represented respectively 20 and 23% of total daily energy expenditure, an amount of energy approaching that devoted to all physical activities. Thermostatic costs would have been even greater than this except that juncos are much better insulated than typical 19.5—g passerine birds (their coefficient of heat transfer is only 76% of that predicted from their mass). For these small birds, adaptations that affect body insulation can have more effect on daily energy budgets than changes in the time allocated to various physical activities. Our data reveal that energy constraints are a major selective force in Yellow—eyed Juncos, operating not through food limitation among adults but rather through the inefficient foraging of young juncos. The critical stage when selection operates most strongly on the efficient use of time and energy occurs during the first 3 mo of a junco's life. The lack of proficient foraging by young juncos explains observed patterns of parental time and energy expenditure and breeding season phenology.

Abstract: Pleistocene glacial cycles are thought to have played a major role in the diversification of temperate and boreal species of North American birds. Given that coalescence times between sister taxa typically range from 0.1 to 2.0 Myr, it has been assumed that diversification occurred as populations were isolated in refugia over long periods of time, probably spanning one to several full glacial cycles. In contrast, the rapid postglacial range expansions and recolonization of northern latitudes following glacial maxima have received less attention as potential promoters of speciation. Here we report a case of extremely rapid diversification in the songbird genus Junco as a result of a single continent-wide range expansion within the last 10 000 years. Molecular data from 264 juncos sampled throughout their range reveal that as the yellow-eyed junco ( Junco phaeonotus ) of Mesoamerica expanded northward following the last glacial maximum, it speciated into the dark-eyed junco ( Junco hyemalis ), which subsequently diversified itself into at least five markedly distinct and geographically structured morphotypes in the USA and Canada. Patterns of low genetic structure and diversity in mitochondrial DNA and amplified fragment length polymorphism loci found in dark-eyed juncos relative to Mesoamerican yellow-eyed juncos provide support for the hypothesis of an expansion from the south, followed by rapid diversification in the north. These results underscore the role of postglacial expansions in promoting diversification and speciation through a mechanism that represents an alternative to traditional modes of Pleistocene speciation.

Abstract: Resource pulses can have cascading effects on the dynamics of multiple trophic levels. Acorn mast is a pulsed resource in oak-dominated forests that has significant direct effects on acorn predators and indirect effects on their predators, prey, and pathogens. We evaluated changes in acorn mast, rodent abundance, raptor abundance, and reproductive success of a ground-nesting songbird over a 24-year period (1980–2004) in the southern Appalachian Mountains in an effort to determine the relationships among the four trophic levels. In particular, we examined the following: acorn mast from red oaks (Quercus rubra) and white oaks (Q. alba), abundance of white-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus), population estimates of seven raptor species from three feeding guilds, and nest failure and number of juveniles of dark-eyed juncos (Junco hyemalis). Finally, we recorded seasonal temperature and precipitation to determine the effects of weather on each trophic level. We found that weather patterns had delayed effects of up to 3 years on these trophic interactions. Variation in acorn mast, the keystone resource in this community, was explained by weather conditions as far back as 2 years before the mast event. Acorn mast, in turn, was a strongly positive predictor of rodent abundance the following year, whereas spring and summer temperature and raptor abundance negatively affected rodent abundance. Dark-eyed junco nests were more likely to fail in years in which there were more rodents and raptors. Nest failure rate was a strong predictor of the number of juvenile juncos caught at the end of the summer. Our results improve our understanding of the complex ecological interactions in oak-dominated forests by illustrating the importance of abiotic and biotic factors at different trophic levels.