James D. Fraser

Assessing bias in studies of bald eagle food habits

Although studies of bald eagle (Haliaeetus leucocephalus) food habits are numerous, few authors have quantified biases inherent in the techniques used. In our study of food habits of nonbreeding bald eagles on the northern Chesapeake Bay, we examined biases associated with pellet analysis, food remains analysis, and direct observation. We assessed these biases through controlled feedings of 2 captive bald eagles and through observations of free-ranging eagles

Nesting Density and Reproductive Success of Piping Plovers in Response to Storm- and Human-Created Habitat Changes

Abstract The threatened population of Atlantic Coast piping plovers (Charadrius melodus) has increased under intensive management of predation and disturbance. However, the relative importance of habitat quality, nest predation, and chick predation in population dynamics and reproductive success of this species are poorly understood. We examined effects of breeding-habitat alterations, predation, and breeding phenology on population size, habitat use, and reproductive output of piping plovers from 1993 to 2004. We studied piping plovers at a newly colonized site (West Hampton Dunes [WHD]) on a New York, USA, barrier island, and an adjacent reference site (REF) with a long-standing population. We monitored population size and reproductive success; determined chick habitat use and behavior; and monitored changes in habitat availability, prey abundance, and predator presence. Resource agencies managed predation by mammal trapping and by fencing nests with predator exclosures in some years. Following storm- and human-related increases in nesting and foraging habitat, the population at WHD grew from 5 pairs in 1993 to 39 pairs in 2000. The WHD population then declined to 18 pairs by 2004 concurrent with habitat losses to human development. In contrast, the population size at REF was not correlated with nesting habitat area. Population growth rate decreased with density at WHD but not at REF, which was likely close to equilibrium when the study began. Neither reproductive output nor any of its components were correlated with population density, and reproductive output was correlated between the sites despite their different population trajectories, suggesting that the population was primarily regulated by adult survival, emigration, or immigration. The latter 2 factors should be especially sensitive to local habitat quality, and the main differences between our sites was that bayside intertidal flats were available adjacent to nesting habitat at WHD but not at REF, and that a village construction project took place at WHD. Clutch size and renest rate decreased over the breeding season. Predator exclosures improved nest daily survival, and mammal trapping improved chick daily survival. Chick foraging rate was highest in bayside intertidal flats and in ocean- and bayside fresh wrack. Chicks used the bay side more than expected from percentage habitat area, and survived better on the bay side before village construction and the initiation of predator trapping, but not after. At both sites, number of chicks fledged per pair was lowest for pairs that nested late and lost a nest late in the season and increased with the annual number of cats (Felis catus) and foxes (Vulpes vulpes) trapped. Restoring nesting habitat adjacent to bayside intertidal flats may increase the carrying capacity (nesting pairs) at piping plover breeding sites. However, without predation management, restored sites may not contribute many recruits to the regional population.

Piping plover brood foraging ecology on New York barrier islands

Effective management of piping plover (Charadrius melodus) populations requires knowledge of the habitats that foster successful reproduction. We studied piping plover chick foraging ecology and survival on the central barrier islands of Long Island, New York, 1992 and 1993. Within the 90-km study area, all 1-km beach segments with ephemeral pools or bay tidal flats were used for nesting and brood rearing, whereas <50% of beach segments without these habitats were used. On beach segments with ephemeral pools, broods preferred ephemeral pools to ocean intertidal zone, wrack, backshore, open vegetation, and interdune habitat. Indices of terrestrial arthropod abundance and foraging rates were greater in ephemeral pools than in other habitats. In 1992, chick survival was higher on beach segments with ephemeral pools than on segments without ephemeral pools. On beach segments with bay tidal flats, broods preferred bay tidal flats and wrack to ocean intertidal zone, backshore, and open vegetation habitats. Foraging rates in bay tidal flats were similar to those in ephemeral pools and greater than in open vegetation, wrack, and backshore habitats. On beach segments without ephemeral pools and bay tidal flats, broods preferred wrack to all other habitats, and open vegetation was second most preferred. To assist in the recovery of the piping plover, land-use planners should avoid beach management practices (e.g., beach filling, dune building, renourishment) that typically inhibit natural renewal of ephemeral pools, bay tidal flats, and open vegetation habitats.

Factors affecting piping plover productivity on Assateague Island

We studied piping plovers (Charadrius melodus) on Assateague Island (Md., Va.) in 1986-87 to estimate population size and to identify factors affecting productivity. Fledging rates (0.19-1.11 chicks/pair) appeared to be lower than the level necessary to maintain a stable population. Fifty-four percent of the nests were unsuccessful. Predators accounted for most (91%) of the known causes of nest losses. Only 1 nest (2.2% of losses with known cause) was lost due to direct human destruction, and we found no evidence that suggested recreational disturbance was a factor affecting productivity. Mean chick fledging success was 69% for broods foraging at bay flats or tidal pools and 19% for broods foraging on ocean beach (P < 0.05). J. WILDL. MANAGE. 55(3):525-531 Piping plovers were listed as threatened throughout their Atlantic coastal breeding range (Newf. to S.C.) in 1986 due to concern about population declines. Declines have been attributed to loss of breeding habitat and poor productivity (U.S. Fish Wildl. Serv. [USFWS] 1988). Factors thought to be contributing to habitat loss include beach development, dune reclamation and beach stabilization, and recreational use (Wilcox 1959, Cairns and McLaren 1980). Low productivity has been attributed to recreational disturbance and high nest predation (Cairns and McLaren 1980, Cairns 1982, Flemming et al. 1988, MacIvor et al. 1990). Nest predation has been linked to nesting habitat characteristics including nesting substrate, amount of vegetative cover, and beach width (Burger 1987, Gaines and Ryan 1988). We studied piping plovers on Assateague Island in Virginia and Maryland. Our objectives were to (1) estimate population size, (2) estimate nest success, (3) examine the influence of habitat characteristics on nest predation, (4) estimate chick survival, and (5) identify factors influencing chick survival. We thank the National Park Service, the Virginia Department of Game and Inland Fisheries, and the Virginia Society of Ornithology for funding. Thanks also go to P. A. Buckley, J. F. Karish, R. B. Rodgers, and K. A. Terwilliger for support. D. F. Stauffer and J. D. Wellman provided criticism.

Effects of human activity on bald eagle distribution on the northern Chesapeake Bay

We determined the relationship between bald eagle (Haliaeetus leucocephalus) distribution and human activity on the northern Chesapeake Bay shoreline during 1985-89. Only 55 of 1,117 locations of radio-tagged eagles (4.9%) occurred in the developed land-cover type (≥4 buildings/4 ha), although 18.2% of potential eagle habitat was developed (χ 2 =428.9, 4 df, P<0.001). Eagle use of the shoreline was inversely related to building density (χ 2 =22.1, P<0.001) and directly related to the development set-back distance (χ 2 =5.3, P=0.02). Few eagles used shoreline segments with boats or pedestrians nearby (P<0.001). Only 360 of 2,532 segments (14.2%) had neither human activity nor shoreline development

Survival Rates and Population Dynamics of Bald Eagles on Chesapeake Bay

Survival of 39 radio-tagged bald eagles (Haliaeetus leucocephalus) in the Chesapeake Bay region was 100% in the first year of life. Mean minimum survival per year of all eagles was 91% (95% CI=86-96%); mean maximum survival was 98% (95% CI=96-100%). A deterministic life-table model predicted a finite growth rate of 5.8% per year, whereas the growth rate based on the maximum survival estimates was 16.6% per year. The breeding population actually increased 12.6% per year from 1986 to 1990. We estimated the intrinsic growth rate at 6.9% based on natality and minimum survival data and 19.2% based on maximum survival data. Because eagle habitat is being converted to human developments at a rapid rate on the Chesapeake, models incorporating these habitat losses are needed to accurately predict future population trends

Horseshoe Crab Eggs Determine Red Knot Distribution in Delaware Bay

Abstract A decline in red knots (Calidris canutus rufa) has been attributed to horseshoe crab (Limulus polyphemus) egg shortages on the Delaware Bay, an important foraging area for migrating knots. We studied the movements and distribution of 65 radiotagged red knots on Delaware Bay from May to June 2004 and related movements to the distribution and abundance of horseshoe crab eggs and other prey and to other habitat characteristics. The number of horseshoe crab eggs was the most important factor determining the use of Delaware Bay beaches by red knots (logistic regression cumulative Akaikes Information Criterion adjusted for small sample size [AICc] w = 0.99). The knots shifted from emergent marsh and peat-beaches to sandy Delaware Bay beach when crab eggs became abundant, which also suggested the importance of crab eggs. While red knots used sandy beach zones more than expected, given their availability, 44% of red knot low tide locations were in bay and coastal emergent marsh. The abundance of Donax variabilis (AICc w = 0.95) and Mytilus edulis (AICc w = 0.94) spat, both food for red knots, had a relationship with red knot use of sandy beaches. Levels of disturbance and the abundance of laughing gulls (Larus atricilla) also were important factors in red knot sandy beach use, although secondary to prey resources (AICc w < 0.4). These results are consistent with the hypothesis that the abundance of horseshoe crab eggs on sandy beaches is driving movement and distribution of red knots and that there is little alternative food during the migratory stopover in Delaware Bay. Our findings that red knots disproportionately use Delaware Bay sites with abundant eggs and that there is a lack of surplus eggs at areas used and unused by red knots support the continuation of management for sustained yield of horseshoe crabs and other food resources at this stopover.

Piping Plover Chick Foraging, Growth, and Survival in the Great Plains

Abstract We tested the hypothesis that piping plover (Charadrius melodus) habitat quality and chick survival on the Missouri River, USA, were lower on a cold-water reservoir and downstream from a hypolimnetic (cold-water) release dam with diel water fluctuations (Garrison Dam) than downstream from an epilimnetic dam (Gavins Point Dam). Plovers in adjacent alkali wetlands provided an index to the maximum reproductive potential in the region. Chicks gained weight more rapidly in the alkali wetlands than on epilimnetic and hypolimnetic river reaches. Invertebrate numbers and biomass were higher in the wetlands and epilimnetic reach, but chick survival was lower on the epilimnetic reach. Thus, piping plovers adapted to a variety of prey densities, and other factors, likely predation, reduced survival rates in the epilimnetic reach. Temporal and spatial variability in site quality indices suggests the need for a regional management strategy with different strategies at each site. Managers can minimize effects of local fluctuations in resource abundance and predators by ensuring protection of or creating geographically dispersed habitat.

Perch trees and shoreline development as predictors of bald eagle distribution on Chesapeake Bay

We studied the influence of shoreline perch trees and human development on bald eagle (Haliaeetus leucocephalus) distribution on the northern Chesapeake Bay. Bald eagle distributions may be determined by available suitable shoreline perch areas. Models based on human development and shoreline habitat variables may alleviate problems associated with classifying bald eagle habitat by identifying characteristics predictive of eagle presence. We observed 2,962 eagles during 36 shoreline surveys and relocated 110 radiomarked eagles 1,350 times during 1985-92. We counted 5,928 suitable (height ≥ 6.1 m, diam at breast height [dbh] ≥ 20.0 cm, and shoreline accessibility ≥ 30°) perch trees in 229, 250- x 50-m segments along shoreline during 1990-91. Shoreline segments used by eagles had more suitable perch trees (x = 30.3 vs. 22.0; P < 0.001) and a larger percentage of forest cover (x = 54.9 vs. 39.4; P < 0.001) than unused segments. Suitable trees on segments with eagle use were closer to water than suitable trees on segments without eagle use (x = 8.4 vs. 17.0 m; P = 0.009). Most segments classified as marsh (66.7%) were unused. Marsh segments had fewer suitable perch trees, less forest cover, and a greater mean distance from water to the nearest suitable perch tree than did other land types (P < 0.001). Developed segments had fewer suitable perch trees, less forest cover, and a shorter distance from water to the nearest suitable perch tree than undeveloped forested segments (P ≤ 0.01). Logistic regression models based on various measures of perch tree abundance and shoreline development correctly predicted eagle use for 65.9-71.0% of segments

Range-Wide Piping Plover Survival: Correlated Patterns and Temporal Declines

Abstract Geographically isolated breeding populations of migratory shorebirds may be demographically connected through shared nonbreeding habitats. We used long-term (1998–2008) mark–recapture data on piping plovers (Charadrius melodus) collected from 7 separate studies located throughout North America to conduct a range-wide analysis of after hatch year apparent survival (ΦAHY). Our objectives were to compare concurrent survival estimates from disparate breeding sites and determine whether estimates followed similar trends or were correlated among breeding populations with shared wintering grounds. Average survival estimates were higher for Great Plains populations (range  =  0.69–0.81) than for Great Lakes and Atlantic Coast populations (range  =  0.56–0.71). Linear trend models indicated that apparent survival declined in 4 out of 7 populations, was unchanged in 3, and was generally highest among Great Plains populations. Based on a post hoc analysis, we found evidence of correlated year-to-year fluctuations in annual survival among populations wintering primarily along the southeastern United States Atlantic Coast and Gulf Coast. Our results indicate shared overwintering or stopover sites may influence annual variation in survival among geographically disparate breeding populations. Declines in piping plover survival are a cause for concern, and our results highlight the need for conservation efforts to include habitat used during the migratory and wintering periods.