Thomas W. Sherry

Tropical Winter Habitat Limits Reproductive Success on the Temperate Breeding Grounds in a Migratory Bird

Identifying the factors that control population dynamics in migratory animals has been constrained by our inability to track individuals throughout the annual cycle. Using stable carbon isotopes, we show that the reproductive success of a long‐distance migratory bird is influenced by the quality of habitat located thousands of kilometres away on tropical wintering grounds. For male American redstarts (Setophaga ruticilla), winter habitat quality influenced arrival date on the breeding grounds, which in turn affected key variables associated with reproduction, including the number of young fledged. Based on a winter‐habitat model, females occupying high‐quality winter habitat were predicted to produce more than two additional young and to fledge offspring up to a month earlier compared with females wintering in poor‐quality habitat. Differences of this magnitude are highly important considering redstarts are single brooded, lay clutches of only three to five eggs and spend only two‐and‐a‐half months on the breeding grounds. Results from this study indicate the importance of understanding how periods of the annual cycle interact for migratory animals. Continued loss of tropical wintering habitat could have negative effects on migratory populations in the following breeding season, minimizing density‐dependent effects on the breeding grounds and leading to further population declines. If conservation efforts are to be successful, strategies must incorporate measures to protect all the habitats used during the entire annual cycle of migratory animals.

The use of isotope tracers for identifying populations of migratory birds

Abstract To determine whether stable isotopes can be used for identifying the geographic origins of migratory bird populations, we examined the isotopic composition of hydrogen (deuterium, δD), carbon (δ13C), and strontium (δ87Sr) in tissues of a migratory passerine, the black-throated blue warbler (Dendroica caerulescens), throughout its breeding range in eastern North America. δD and δ13C values in feathers, which are grown in the breeding area, varied systematically along a latitudinal gradient, being highest in samples from the southern end of the species’ breeding range in Georgia and lowest in southern Canada. In addition, δD decreased from east to west across the northern part of the breeding range, from New Brunswick to Michigan. δ87Sr ratios were highest in the Appalachian Mountains, and decreased towards the west. These patterns are consistent with geographical variation in the isotopic composition of the natural environment, i.e., with that of precipitation, plants, and soils for δD, δ13C, and δ87Sr, respectively. Preliminary analyses of the δD and δ13C composition of feathers collected from warblers in their Caribbean winter grounds indicate that these individuals were mostly from northern breeding populations. Furthermore, variances in isotope ratios in samples from local areas in winter tended to be larger than those in summer, suggesting that individuals from different breeding localities may mix in winter habitats. These isotope markers, therefore, have the potential for locating the breeding origins of migratory species on their winter areas, for quantifying the degree of mixing of breeding populations on migratory and wintering sites, and for documenting other aspects of the population structure migratory animals – information needed for studies of year-round ecology of these species as well as for their conservation. Combining information from several stable isotopes will help to increase the resolution for determining the geographic origins of individuals in such highly vagile populations.

SITE‐DEPENDENT REGULATION OF POPULATION SIZE:A NEW SYNTHESIS

The nature and extent of population regulation remains a principal unanswered question for many types of organisms, despite extensive research. In this paper, we provide a new synthesis of theoretical and empirical evidence that elucidates and extends a mechanism of population regulation for species whose individuals preemptively use sites that differ in suitability. The sites may be territories, refuges from predation, oviposition sites, etc. The mechanism, which we call site dependence, is not an alternative to density dependence; rather, site dependence is one of several mechanisms that potentially generate the negative feedback required for regulation. Site dependence has two major features: (1) environmentally caused heterogeneity among sites in suitability for reproduction and/or survival; and (2) preemptive site occupancy, with the tendency for individuals to move to sites of higher quality as they become available. Simulation modeling shows that these two features, acting in concert, generate nega...

Habitat-specific demography of breeding black-throated blue warblers (Dendroica caerulescens) : implications for populations dynamics

The distribution of individuals among habitats and their relative success in those habitats can have important consequences for population dynamics. To examine these processes for a long-distance migratory bird species, we studied the population structure, age-specific reproductive output, and local survival of black-throated blue warblers (Dendroica caerulescens, Gmelin) in two breeding habitats differing in shrub density within northern hardwoods forests in New Hampshire, USA. On forest plots with dense shrubs, warblers occurred at higher densities, and fledged significantly more young per capita per season than those occupying areas with lower shrub density. This differential productivity was due to higher reproductive output, mainly through double-brooding, of older (≥ 2 years of age) individuals, which were disproportionately more abundant in high shrub density sites. Clutch initiation dates, clutch sizes, and predation rates at individual nests did not differ significantly between habitats. Mean body mass of nestlings on day 6 following hatching were higher on average on plots with high shrub density, but differences were not significant. Annual return rates, as indices of local survival, did not differ between habitats for older males or for females. Yearling males, however, returned in subsequent years at a significantly lower rate to low shrub density plots, a result of either lower survival or, more likely, dispersal to more suitable habitat in their second year of breeding. Parental age and habitat suitability interact in that older individuals, through their experience and/or dominance, acquire sites of higher quality, which results in higher reproductive output and probably higher survival. These differences between habitats in density, reproductive performance and local survival are consistent with an ideal-despotic/preemptive distribution of individuals, and suggest that this population could be regulated by the availability, distribution, and extent of high and low quality breeding habitats.

Winter Habitat Quality, Population Limitation, and Conservation of Neotropical-Nearctic Migrant Birds

Recent declines in Neotropical-Nearctic migrant songbird populations are often attributed to events during the nonbreeding season, such as tropical habitat conversion and drought. Support for this hypothesis in most species, however, is largely anecdotal or conjectural. There is a dearth of demographic information about migrants on their Neotropical winter grounds. Such data are needed to identify specific ecological factors influencing survival, dispersal, and, ultimately, population abundances aggregated over multiple habitats at regional spatial scales. In this paper, we review several lines of evidence, emphasizing results of our research on paruline warblers in Jamaica, which indicate that migrant passerines often compete intraspecifically in winter for preferred quality habitats and that their populations may be limited at least in part by ecological conditions in winter. The demographic and ecological evidence supporting this hypothesis for migrant passerines includes : (1) differing densities among habitats, suggesting variation in habitat suitability ; (2) strong territoriality, site attachment, and site fidelity ; (3) experimental demonstrations of habitat saturation ; (4) nonrandom distributions of sex and age classes among habitats ; (5) overwinter decline of body mass by individuals occupying the most drought-stressed habitats ; and (6) different residence times among habitats, suggesting differences in survival or dispersal. We review ecological and behavioral explanations for these demographic patterns, and make conservation recommendations based on our understanding of how local demographic circumstances affect broader scale population processes.

Bird Community Dynamics in a Temperate Deciduous Forest: Long‐Term Trends at Hubbard Brook

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.

Conserving migratory land birds in the New World: Do we know enough?

Migratory bird needs must be met during four phases of the year: breeding season, fall migration, wintering, and spring migration; thus, management may be needed during all four phases. The bulk of research and management has focused on the breeding season, although several issues remain unsettled, including the spatial extent of habitat influences on fitness and the importance of habitat on the breeding grounds used after breeding. Although detailed investigations have shed light on the ecology and population dynamics of a few avian species, knowledge is sketchy for most species. Replication of comprehensive studies is needed for multiple species across a range of areas, Information deficiencies are even greater during the wintering season, when birds require sites that provide security and food resources needed for survival and developing nutrient reserves for spring migration and, possibly, reproduction. Research is needed on many species simply to identify geographic distributions, wintering sites, habitat use, and basic ecology. Studies are complicated, however, by the mobility of birds and by sexual segregation during winter. Stable-isotope methodology has offered an opportunity to identify linkages between breeding and wintering sites, which facilitates understanding the complete annual cycle of birds. The twice-annual migrations are the poorest-understood events in a birds life. Migration has always been a risky undertaking, with such anthropogenic features as tall buildings, towers, and wind generators adding to the risk. Species such as woodland specialists migrating through eastern North America have numerous options for pausing during migration to replenish nutrients, but some species depend on limited stopover locations. Research needs for migration include identifying pathways and timetables of migration, quality and distribution of habitats, threats posed by towers and other tall structures, and any bottlenecks for migration. Issues such as human population growth, acid deposition, climate change, and exotic diseases are global concerns with uncertain consequences to migratory birds and even less-certain remedies. Despite enormous gaps in our understanding of these birds, research, much of it occurring in the past 30 years, has provided sufficient information to make intelligent conservation efforts but needs to expand to handle future challenges.

Effects of removal of red squirrels, Tamiasciurus hudsonicus, and eastern chipmunks, Tamias striatus, on nest predation in a northern hardwood forest: an artificial nest experiment

An importance of eastern chipmunks (Tamias striatus) and red squirrels (Tamiasciurus hudsonicus) as avian nest predators was examined by monitoring predation on quail-egg-baited artificial nests on six experimental plots, from which the two predators were removed, paired with six control plots. Nests were placed 50 m apart at densities of 16 per 2.25 ha. The number of nests depredated on each plot differed significantly for each treatment, but did not differ significantly between removal and control plots. We attribute the latter finding to the combined effects of compensatory mortality due to a larger-than-expected variety of nest predators and to patchy nest predation resulting from variable predator distribution. Photographs of seven species of nest predators were obtained from cameras with trip devices placed in the field indicating that several taxa, in addition to red squirrels and chipmunks, may be important nest predators. The use of artificial nests for field experiments, the impact of a diverse set of predators, and the spatially variable pattern of nest predation are discussed.

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.

TERRITORIAL EXCLUSION BY A LONG-DISTANCE MIGRANT WARBLER IN JAMAICA: A REMOVAL EXPERIMENT WITH AMERICAN REDSTARTS (SETOPHAGA RUTICILLA)

To test whether male American Redstarts (Setophaga ruticilla) excluded conspe- cifics from selected habitats on their wintering grounds, we removed territorial individuals from four replicate sites in two different habitats in Jamaica, West Indies, and recorded territory shifts and new colonizations of the vacated areas. In total, 12 American Redstarts (nine after-hatch-year (AHY) or older males, two yearling (HY) males, and one female) were removed, and their vacated territories were re-occupied by 18 neighboring or newly-ap- pearing redstarts (five AHY males, four HY males and nine females). The re-occupation of the vacated areas supports the hypothesis that territoriality in this species acts to exclude conspecifics from certain winter habitat, and shows that American Redstarts compete with conspecifics for habitat in their winter grounds. Moreover, a statistically significant shift in sex composition following removal indicates that AHY males excluded females from mutually acceptable habitats. Such behavioral dominance, if confirmed, could help account for sexual habitat segregation during winter, and could result in differential winter survival rates between the sexes, thereby influencing population structure and regulation in this long- distance migrant. Received 10 August 1992, accepted 15 December 1992.