Jonathan B. Cohen

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.

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.

Substrate and Vegetation Selection by Nesting Piping Plovers

Abstract We studied substrate composition and vegetation cover at Piping Plover (Charadrius melodus) nests and paired random plots on New York beaches that had been widened by renourishment (deposition of dredged sand). Most nests (59.4%, n = 32) were in unvegetated plots, mean ± SE vegetative cover around nests was 7.5 ± 1.7%, and all plovers nested in <47% cover. Most nests (59.4%) were on pure sand and mean coarse grain cover (pebble and cobble-sized objects) on nest plots was 9.1 ± 2.6%. Nest plots were more likely to be vegetated than paired random plots. Coarse substrate also was of high relative importance in distinguishing nests and random plots. Beach management projects can reduce sparse vegetation and coarse substrate, which may affect Piping Plover nest site selection.

Residence Probability and Population Size of Red Knots During Spring Stopover in the Mid-Atlantic Region of the United States

Abstract Weekly counts of western Atlantic red knots (Calidris canutus rufa) at their Delaware Bay migration stopover site have suggested a major decline since the 1980s. We estimated red knot spring passage population size in the New Jersey Coast–Delaware Bay region (DENJ; 2004 and 2006) and Virginia (VA; 2006 and 2007), USA, by correcting weekly aerial counts for mean daily residence probability between counts in a Monte-Carlo simulation. We used daily telemetry relocations in mark–resight models to estimate mean daily residence probability. Average daily residence probability was approximately 1.0 in mid-May, 0.96–0.97 in the week of 22 May, and 0.64–0.77 after May 28 in DENJ in 2004 and 2006 and in VA in 2006. Average daily residency was approximately 0.88 in VA in 2007 from 22 May to 5 June. No birds moved from VA to DENJ in 2006 and only 2 birds (5.5%) moved in 2007. Stopover population sizes (±SE) in DENJ were 17,108 ± 1,322 in 2004 and 19,555 ± 831 in 2006, and in VA were 7,224 ± 389 in 2006 and 8,332 ± 718 in 2007, significantly greater than peak aerial counts. Years with similar peak counts had different residence probabilities; hence, adjustments for turnover should be used in the future to assess annual population changes. Our results suggest that VA can support a significant portion of this red knot subspecies during migration in at least some years. Managing red knots for recovery should entail improving our understanding of the use of other Atlantic Coast sites and protecting key coastal habitat from disturbance and development.

Winter Ecology of Piping Plovers at Oregon Inlet, North Carolina

Abstract Humans may modify winter habitat of the imperiled Piping Plover (Charadrius melodus), yet published accounts of the species’ winter ecology are rare. We studied Piping Plovers at Oregon Inlet, North Carolina from December 2005 to March 2006. Plovers used a 20.1 km2 area (100% minimum convex polygon home range) containing narrow barrier islands with ocean and sound-side beaches, and small shoals, dredged-material islands, and marsh islands in shallow-water sounds. Plover activity was concentrated in twelve areas totaling 2.2 km2 (95% fixed kernel home range). When plovers were on ocean beaches, they spent less time foraging (18%) than when on Sound Island beaches (88%) and islands (83%, P = 0.003). Sound island use increased and beach use decreased as the tide dropped (Logistic regression, P < 0.001). Plover use of dredged-material islands implied that habitat managers can create or restore attractive foraging sites where habitat may be declining or limiting. Wintering habitat management should aim to provide foraging opportunities during most of the day and across a range of tide conditions and ensure that foraging habitat is close to roost sites.

The effect of benthic prey abundance and size on red knot (Calidris canutus) distribution at an alternative migratory stopover site on the US Atlantic Coast

A population decline of the western Atlantic red knot (Calidris canutus rufa) has been linked to food limitation during the spring migratory stopover in Delaware Bay, USA. The stopover ecology at potential alternative sites has received little attention. We studied factors affecting red knot habitat selection and flock size at a coastal stopover site in Virginia in 2006–2007. The most common potential prey items were coquina clams (Donax variabilis) and crustaceans. Red knot foraging sites had more clams and crustaceans than unused sites in 2006. Prey abundance increased during the 2007 stopover period and remained high after the red knot peak. Red knot flock size in 2007 increased with mean clam shell length, and probability of flock presence decreased with increasing distance from night use locations. Our results suggest that red knots preferred coquina clams and that these clams were not depleted during the stopover period in 2007. Thus prey abundance did not appear to be a population-limiting factor at this coastal stopover site in Virginia in that year. Protection of coastal sites outside of Delaware Bay, many of which have been altered by human development, would likely benefit red knot population recovery, as they can apparently provide abundant food resources during at least some years.

ENVIRONMENTAL CORRELATES OF BREEDING IN THE CRESTED CARACARA (CARACARA CHERIWAY)

ABSTRACT. We evaluated the influence of weather on reproduction of the Crested Caracara (Caracara cheriway) in an agricultural landscape in south-central Florida. We used a mixed logistic-regression modeling approach within an information-theoretic framework to examine the influence of total rainfall, rainfall frequency, and temperature on the number of breeding pairs, timing of breeding, nest success, and productivity of Crested Caracaras during 1994–2000. The best models indicated an influence of rainfall frequency and laying period on reproduction. More individuals nested and more pairs nested earlier during years with more frequent rainfall in late summer and early fall. Pairs that nested later in each breeding season had smaller clutches, lower nest success and productivity, and higher probability of nest failure. More frequent rainfall during early spring months that are usually characterized by water deficit (March–May), more frequent rainfall during the fall drawdown period (September–November), and a shorter winter dry period showed some association with higher probability of brood reduction and lower nest success. The proportion of nests that failed was higher in “wet” years, when total rainfall during the breeding season (September–April) was >10% above the 20-year average. Rainfall may influence reproduction in Crested Caracaras indirectly through food resources. As total rainfall increased during February–April, when most pairs are feeding nestlings or dependent fledglings, the proportion of drawdown-dependent species (those that become available as rainfall decreases and wetlands become isolated and shallow) in the diet of Crested Caracaras declined, which may indicate reduced availability of foraging habitat for this primarily terrestrial raptor.

ENVIRONMENTAL CORRELATES OF NESTING SUCCESS IN RED-SHOULDERED HAWKS

Abstract Abstract. We evaluated the influence of weather on reproduction of the Red-shouldered Hawk (Buteo lineatus) in an agricultural landscape in south-central Florida where we found relatively high densities of successfully nesting hawks. We used a generalized linear modeling approach within an information-theoretic framework to examine the influence of total rainfall, rainfall frequency, and temperature on the timing of nesting, nesting success, and productivity of hawks during 1995–2000. The best models indicated an influence of rainfall frequency and laying period on hawk reproduction. During years with less frequent rainfall in the summer and fall months prior to the beginning of the breeding season, fewer pairs attempted to nest, and hawks nested later and had smaller clutch sizes and lower productivity. Hawks that nested later in the breeding season had lower hatching success and lower overall nest success. Although Red-shouldered Hawks are generally reported to inhabit forested landscapes throughout their range, a common feature seems to be a dependence on wetlands and riparian habitat for foraging. We propose that the proportion of wetlands throughout the landscape is a unique aspect of south-central Florida that may allow for persistence of unusually high numbers of hawks.

Shorebirds Forage Disproportionately in Horseshoe Crab Nest Depressions

Abstract. Horseshoe Crab (Limulus polyphemus) eggs are an important shorebird food during the Delaware Bay spring stopover, and shorebird management plans aim to increase and monitor this resource. If shorebirds focus their foraging on Horseshoe Crab nesting depressions they may find richer food supplies than if they forage randomly on the beach. The amount of shorebird sign in quadrats centered on Horseshoe Crab nest depressions was compared with the amount of sign in paired beach areas with no Horseshoe Crab nests. Horseshoe Crab nest depressions had more pecks, probes, digit marks, Ruddy Turnstone excavations, Horseshoe Crab eggs and a greater coverage by shorebird sign than paired beach plots with no Horseshoe Crab nests. Foraging disproportionately within Horseshoe Crab nesting depressions may facilitate the rapid mass gain needed to prepare shorebirds for their flight to the breeding grounds. Horseshoe crab egg monitoring currently estimates mean egg abundance per beach. However, birds are able to find patches with high egg densities even when foraging on beaches with lower average densities.

Use of Retrospective Data to Assess Ecotoxicological Monitoring Needs for Terrestrial Vertebrates Residing in Atlantic Coast Estuaries

The “Contaminant Exposure and Effects–Terrestrial Vertebrates” (CEE-TV) database contains 4,336 records of ecotoxicological information for free-ranging amphibians, reptiles, birds, and mammals residing in Atlantic and Florida Gulf coast estuaries and their drainages. To identify spatial data gaps, those CEE-TV records for which the specific study location were known (n=2,740) were combined with watershed and wildlife management unit boundaries using Geographic Information Systems software. The US Environmental Protection Agencys Index of Watershed Indicators (IWI), which classifies watersheds based on water quality and their vulnerability to pollution, was used to prioritize these data gaps. Of 136 watersheds in the study area, 15 that are classified by the IWI as having water quality problems or high vulnerability to pollution lacked terrestrial vertebrate ecotoxicological monitoring or research in the past decade. Older studies within some of these watersheds documented high levels of contaminants in wildlife tissues. Of 90 National Wildlife Refuge units, 42 without current data fall within watersheds of concern. Of 40 National Park units larger than 1 km2, 17 without current data fall within watersheds of concern. Issues encountered in this analysis highlighted the need for spatially and temporally replicated field monitoring programs that utilize random sampling. Without data from such studies, it will be difficult to perform unbiased assessments of regional trends in contaminant exposure and effects in terrestrial vertebrates.