Erica Nol

INFLUENCE OF FOOD ABUNDANCE, NEST-SITE HABITAT, AND FOREST FRAGMENTATION ON BREEDING OVENBIRDS

A13STRACT.--Between 1994 and 1996, we determined the density and pairing success of territorial male Ovenbirds (Seiurus aurocapillus) in 31 forest fragments in southern Ontario. The density and pairing success of territorial males increased significantly with area of the woodlot core. We tested the hypotheses that area-related changes in food abundance and the availability of suitable nest sites are the causal mechanisms limiting female settlement in small forest fragments. Our results indicated that Ovenbirds chose territories with significantly higher prey biomass than occurred at randomly selected sites in the woodlot. Within Ovenbird territories, prey biomass was 10 to 36 times higher in large woodlots than in small woodlots. Invertebrate biomass at randomly located quadrats in large woodlots was more than twice that found at random sites within small woodlots, and was slightly greater than prey biomass within Ovenbird territories in small fragments. Leaf litter was deeper within Ovenbird territories in large woodlots than at random sites in both large and small forests, and within Ovenbird territories in small woodlots. Ovenbirds in large woodlots selected nest sites that were more than 250 m from the forest edge, distances that were not obtainable in small forest fragments. The lack of potential nest sites, combined with lower food abundance in small fragments, may explain why female Ovenbirds find small fragments unsuitable as breeding sites, and hence why so few males secure mates in small fragments. Understanding the cause of deterioration of breeding sites with decreasing woodlot area should underscore efforts to preserve larger tracts of forest to provide adequate nesting habitat for forest-interior migrants. Received 25 November 1996, accepted 12 June 1997. THE OVENBIRD (Seiurus aurocapillus) is a ground-nesting passerine that typically breeds in the interior of large tracts of forest in northern and northeastern North America (Van Horn and Donovan 1994) and obtains most of its prey from

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.

LANDSCAPE AND FRAGMENT SIZE EFFECTS ON REPRODUCTIVE SUCCESS OF FOREST-BREEDING BIRDS IN ONTARIO

To determine the minimum size requirements and influence of landscape context on reproductive success of forest-breeding songbirds, we monitored nesting success of five species of songbirds on 40 fragments (12–2350 ha in total woodlot size) and two continuous forest sites in south-central Ontario from 1994 through 1997. Woodlot size was the most important variable contributing to differences in reproductive success, with local forest cover (within a 10-km radius) having no significant additional effect on productivity for any species. For all species, except Red-eyed Vireo (Vireo olivaceous), adult female reproductive success was at or above replacement levels in large fragments (mean of 121 ha core area, 849 ha woodlot area) and continuous forest, and below replacement levels in small fragments (mean of 7.8 ha core area, 93 ha woodlot area). Red-eyed Vireo productivity was particularly low, with single-brooded females unable to maintain populations in any woodlots monitored, although populations were clo...

Lower Predation Risk for Migratory Birds at High Latitudes

Predator Avoidance Strategy Selective pressures influencing bird migration can include availability of food, pressure from parasites and pathogens, and predation risk. The importance of the last of these is revealed by McKinnon et al. (p. 326; see the Perspective by Gilg and Yoccoz), who present an experimental analysis of the benefits of long-distance migration for reproduction in arctic-nesting birds. Measurements of a controlled effect of predation risk along a 3350-kilometer north-south gradient across arctic Canada provides evidence that the risk of nest predation decreases with latitude. Thus, birds migrating further north may acquire reproductive benefits in the form of reduced predation risk. Egg predation rates measured at artificial nests along a 3000-kilometer transect decrease northwards. Quantifying the costs and benefits of migration distance is critical to understanding the evolution of long-distance migration. In migratory birds, life history theory predicts that the potential survival costs of migrating longer distances should be balanced by benefits to lifetime reproductive success, yet quantification of these reproductive benefits in a controlled manner along a large geographical gradient is challenging. We measured a controlled effect of predation risk along a 3350-kilometer south-north gradient in the Arctic and found that nest predation risk declined more than twofold along the latitudinal gradient. These results provide evidence that birds migrating farther north may acquire reproductive benefits in the form of lower nest predation risk.

EFFECTS OF SELECTION CUTTING ON BIRD COMMUNITIES IN CONTIGUOUS EASTERN HARDWOOD FORESTS

Abstract We used point counts to sample bird communities in hardwood forest stands following single-tree selection harvest to determine the impacts on birds of this harvesting system. We sampled at 1–5 years post-harvest (n = 24), 15–20 years post-harvest (n = 23), and in reference stands subjected only to natural disturbances for >30 years (n = 24). White-throated sparrow (Zonotrichia albicollis), chestnut-sided warbler (Dendroica pensylvanica), and mourning warbler (Oporornis philadelphia) abundances were significantly higher in recently logged stands than in other treatments. Ovenbird (Seiurus aurocapillus) abundance was about 50% lower in recently logged stands and in stands logged 15–20 years previous than in reference stands. Black-throated blue warbler (Dendroica caerulescens) and yellow-bellied sapsucker (Sphyrapicus varius) abundances were similar in reference and recently logged stands but significantly lower in stands harvested 15–20 years previously. Redundancy Analysis (RDA) identified 6 habitat variables associated with changes in avian abundances. Percent shrub and slash cover were higher in recently logged stands than in older logged and reference stands. Deciduous canopy cover and basal area of living deciduous trees were greater in reference stands than in both logged treatments. Although the abundance of some bird species were statistically lower in selection cut stands, the implications to population persistence will require data on reproductive success in combination with population modeling with varying proportions of the forested landscape committed to selection cutting.

Philopatry. nest-site tenacity, and mate fidelity of Semipalmated Plovers

Philopatry, nest-site tenacity, and mate fidelity were examined in a population of Semipalmated Plovers Charadrius semipalmatus breeding near Churchill, Manitoba, Canada from 1992 to 1997. Natal philopatry was low (7 of 445 (1.57%) hatchlings returned to the study area to breed) and no difference in sex of recruits was detected. Adult males returned at a significantly higher rate (58.9%) than adult females (41.2%). For both sexes, but more strongly for females, success in the previous year resulted in higher return rates in the subsequent year. Of pairs that returned, 41.6% divorced. Females that divorced or changed mates through death or disappearance of a partner between breeding seasons were significantly less site tenacious than females that reunited and also less so than males of any pairing status. Females that experienced breeding failure and changed mates in the subsequent breeding season moved farther than successful females that changed mates. In only one of four years did reunited pairs nest earlier and have greater hatching success than novel pairs. After divorce new mates of females did not differ significantly in morphological or plumage characteristics from old mates, nor did they possess territories with greater visibility, or territories nearer to water. After divorce new mates of males were significantly smaller than old mates. Our return rates underestimate survivorship in females because both the probability of return and site tenacity of females are strongly influenced by their previous success.

IMPACTS OF HOUSING DEVELOPMENTS ON WOOD THRUSH NESTING SUCCESS IN HARDWOOD FOREST FRAGMENTS

Abstract We studied the impacts of low density, exurban housing developments on Wood Thrushes (Hylocichla mustelina) breeding in small forest fragments in two regions of rural southern Ontario. In both regions, Wood Thrushes breeding in woodlots with embedded houses (housing penetrating the forest border) experienced significantly higher rates of parasitism by Brown-headed Cowbirds (Molothrus ater) than Wood Thrushes breeding in woodlots with adjacent houses (houses within 100 m of the forest edge), or undeveloped woodlots (no houses within 100 m of the forest edge). Wood Thrushes breeding in Peterborough area woodlots with embedded or adjacent houses experienced significantly increased rates of nest predation compared to Wood Thrushes breeding in undeveloped woodlots. This increased nest predation resulted in significant reductions in seasonal productivity in developed woodlots. No increase in nest predation was experienced by Wood Thrushes nesting in developed woodlots in the Ottawa region. The effects of housing developments appear to be region-specific and may depend on other factors influencing the overall abundance of cowbirds. Impactos de la Construcción de Viviendas en el Éxito de Nidificación de Hylocichla mustelina en Fragmentos de Bosque Resumen. Estudiamos los impactos de la construcción en baja densidad de viviendas peri-urbanas sobre individuos de Hylocichla mustelina que se encontraron criando en fragmentos pequeños de bosque en dos regiones rurales del sur de Ontario. En ambas regiones, los individuos de H. mustelina que se reprodujeron en bosques donde había casas inmersas (que penetraban el borde del bosque) experimentaron tasas de parasitismo por Molothrus ater significativamente mayores que los individuos criando en bosques con casas adyacentes (dispuestas a menos de 100 m del borde del bosque), o en bosques no alterados por la presencia de casas (a más de 100 m del borde del bosque). Los individuos de H. mustelina que se encontraron criando en áreas boscosas de Peterborough, donde las casas estaban adentro o adyacentes al bosque, experimentaron incrementos significativos en las tasas de depredación de nidos comparados con individuos que criaron en bosques no alterados. Este incremento en la depredación de nidos llevó a reducciones significativas en la productividad estacional en los bosques con viviendas. No registramos un incremento en la depredación de nidos de H. mustelina en bosques con viviendas en la región de Ottawa. Los efectos de la construcción de viviendas parecen estar relacionados de modo específico con la región y podrían depender de otros factores que influencian la abundancia de M. ater.

Effects of local and landscape-scale habitat variables on abundance and reproductive success of wetland birds

Both local and landscape-scale habitat variables influence the abundance of wetland breeding birds. Few studies, however, simultaneously assess the effects of habitat variables at multiple spatial scales or consider effects on reproductive success. Therefore, we examined the effects of wetland and landscape-scale habitat variables on the abundance of nine breeding bird species and the effects of nest, wetland, or landscape-scale habitat variables on the nest success, clutch size, or number of fledglings of four species at 15 cattail (Typha sp.)-dominated wetlands in an agricultural region around Peterborough, Ontario, Canada. The abundance of Least Bittern (Ixobrychus exilis), Common Moorhen (Gallinula chloropus), and Marsh Wren (Cistothorus palustris) increased as wetland water depth increased; the abundance of Common Moorhen and Marsh Wren increased as wetland size increased; and the abundance of Marsh Wren increased as the amount of wetland in the surrounding landscape increased. Red-winged Blackbird (Agelaius phoeniceus) nest success decreased as nest cover increased. Clutch sizes were uninfluenced by the habitat variables that we considered. The number of Red-winged Blackbird fledglings per successful nest increased as wetland size increased and as the amount of wetland in the surrounding landscape increased. We speculate that food limitation in small wetlands may be responsible for the pattern in Red-winged Blackbird fledging success. The abundance and nest success of Virginia Rail (Rallus limicola) and Sora (Porzana carolina) were uninfluenced by the habitat variables we considered. Future research should consider mate attraction and productivity in relation to local and landscape-scale habitat variables for these and other secretive species. Our study suggests that wetland conservation will be most effective if it considers habitat variables at multiple spatial scales.

Unexpected diversity in socially synchronized rhythms of shorebirds

The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment. Such behavioural rhythms are well studied in isolated individuals under laboratory conditions, but free-living individuals have to temporally synchronize their activities with those of others, including potential mates, competitors, prey and predators. Individuals can temporally segregate their daily activities (for example, prey avoiding predators, subordinates avoiding dominants) or synchronize their activities (for example, group foraging, communal defence, pairs reproducing or caring for offspring). The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood. Here we investigate these rhythms in the context of biparental care, a particularly sensitive phase of social synchronization where pair members potentially compromise their individual rhythms. Using data from 729 nests of 91 populations of 32 biparentally incubating shorebird species, where parents synchronize to achieve continuous coverage of developing eggs, we report remarkable within- and between-species diversity in incubation rhythms. Between species, the median length of one parent’s incubation bout varied from 1–19 h, whereas period length—the time in which a parent’s probability to incubate cycles once between its highest and lowest value—varied from 6–43 h. The length of incubation bouts was unrelated to variables reflecting energetic demands, but species relying on crypsis (the ability to avoid detection by other animals) had longer incubation bouts than those that are readily visible or who actively protect their nest against predators. Rhythms entrainable to the 24-h light–dark cycle were less prevalent at high latitudes and absent in 18 species. Our results indicate that even under similar environmental conditions and despite 24-h environmental cues, social synchronization can generate far more diverse behavioural rhythms than expected from studies of individuals in captivity. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms.

Predation by Bears on Woodpecker Nests: Are Nestling Begging and Habitat Choice Risky Business?

ABSTRACT. We evaluated hypotheses explaining risk of predation by American Black Bear (Ursus americanus) at 418 Yellow-bellied Sapsucker (Sphyrapicus varius) and Hairy Woodpecker (Picoides villosus) nests, on the basis of nestling begging and nest-site habitat features in Algonquin Provincial Park, Ontario. Ninety-three percent of Yellow-bellied Sapsuckers in stands dominated by Sugar Maple (Acer saccharum) nested in Sugar Maple or American Beech (Fagus grandifolia) trees that were dead or in declining health, whereas 86% of Yellow-bellied Sapsuckers in stands dominated by aspen (Populus spp.) nested in Quaking Aspen (P. tremuloides) that were in declining health. Black Bears depredated 17% of 315 nests of Yellow-bellied Sapsuckers in Sugar Maple stands, which accounts for 71% of all Yellow-bellied Sapsucker nest failures. Only 1 (2%) of 46 Hairy Woodpecker nests in the same Sugar Maple stands was depredated by a bear. None of 51 Yellow-bellied Sapsucker nests in aspen stands was depredated. In Sugar Maple stands, daily nest survival of Yellow-bellied Sapsucker nests was lowest when nestling begging calls were loudest and carried the farthest, in more recently harvested stands, and in trees other than American Beech (mostly Sugar Maple). Nest substrates were hardest at Hairy Woodpecker nests, followed by successful Yellow-bellied Sapsucker nests in American Beech and Quaking Aspen; Yellow-bellied Sapsucker nests were softest in stands that had been harvested within the past 30 years. Our study suggests that the risk of predation by American Black Bears at woodpecker nests is a combined function of nestling begging calls, which attract bears to the nest, and nest habitat characteristics, which influence accessibility to the interior of the cavity.