Although avian nesting success is much studied, little is known about the relative importance of the factors that contribute to annual reproductive success and population limitation, especially for long-distance migratory songbird species. We combined a fi eld experiment limiting access to nests by mammalian predators with modeling of long-term fi eld data of American redstarts (Parulidae: Setophaga ruticilla ) to assess the eff ects of multiple environmental variables on breeding success and population limitation. Experimental treatment (baffl es placed around tree boles beneath active nests; n ϭ 71) increased nesting success of this single-brooded species signifi cantly (77 vs 50% in controls; n ϭ 343), demonstrating that scansorial mammals, primarily red squirrels Tamiasciurus hudsonicus and eastern chipmunks Tamias striatus , reduced reproductive success. Based on unbaffl ed nests (n ϭ 466), daily nest survival varied annually, and was positively infl uenced by May temperature and negatively by sciurid nest predator abundance. Daily nest survival was also infl uenced positively by June rainfall, and declined with nest age but not with calendar date. Since nest failure was overwhelmingly caused by nest predation, these signifi cant climate and nest-age eff ects in our models are indirect, likely infl uencing nest predator and/or nesting bird behaviors that in turn infl uenced nest predation. Redstart population density had no eff ect on nesting success, after accounting for other factors. Annual reproductive success accounted for 34% of the variability in annual population change in redstarts in our study area. Our fi ndings document 1) breeding season population limitation in this species, 2) a link between tree masting and bird population dynamics via mammal population fl uctuations, 3) the independent contributions of summer versus winter population processes in a migratory species, and 4) the potential complexity of climate-biotic interactions.

Approximately 90,000 shoots of eelgrass (Zostera marina) were planted over 3 years (2003-2005) at Piney Point (PP) in the lower Potomac River estuary in the Chesapeake Bay (mid-Atlantic coast of North America) following 3 years of habitat evaluation using a Preliminary Transplant Suitability Index (PTSI) and test plantings. Initial survival was high for the 2003 and 2004 plantings; however, most of the eelgrass died during the summer following the fall planting. Habitat quality and restoration success were monitored for the 2005 plantings and compared to a nearby restoration site (St. George Island [SGI]). Eelgrass planted at PP in the fall of 2005 declined through the summer of 2006 with some recovery in the spring of 2007, but was gone by the end of the summer of 2007. The summer decline from late July to mid-August of 2006 coincided with water temperatures greater than 30 • C, hypoxic oxygen (0-3 mg/L) concentrations, and low percent light at leaf level (PLL < 15%).Epiphyte loads were much heavier at PP than at SGI, despite similar water quality. We suggest that this was the result of higher wave exposure at PP. All of these factors are likely to have contributed to the mortality of the 2005 plantings. Submerged aquatic vegetation habitat quality based on the PTSI, median PLL during the growing season, and test plantings did not explain the decline of the plantings. Restoration site selection criteria should be expanded to include the effects of wave exposure on self-shading and epiphyte loads, and the potential for both short-term exposures to stressful conditions and long-term changes in habitat quality.

New World wood warblers (Parulidae) represent one of the most dramatic adaptive radiations in North America. However, the ecological bases for the morphological differences among these species remain poorly understood, especially considering how many foraging and habitat studies the family has inspired. We hypothesized the existence of relationships between parulid morphology and diet. We combined a principal component analysis (PCA) of 18 external morphological traits of 11 species of warblers with stomach-content data from coexisting species in one breeding community in Louisiana and three wintering communities in Jamaica. The primary morphological differences, corresponding with the first three PCA axes, were body size, morphological adaptations for aerial foraging versus gleaning, and arboreal versus groundforaging adaptations. Our analysis revealed little morphological overlap among the 11 species. Differences in diet among the warblers showed a significant relationship to the first two PCA axes of morphological traits. For five coexisting, foliage-gleaning species wintering in Jamaican wet limestone forest, larger warblers ate larger beetles and Orthopterans but not larger ants. In analyses including all four communities, species of warblers with aerial foraging morphologies consumed a greater proportion of winged insects than other warbler species. These findings document prey selection relevant to multiple subtle morphological differences among coexisting species. Overall, our results indicate that food and foraging have likely played an integral role in the morphological diversification and coexistence of species in the family Parulidae.

Although both interspecific competition and coexistence mechanisms are central to ecological and evolutionary theory, past empirical studies have generally focused on simple (two-species) communities over short time periods. Experimental tests of these species interactions are challenging in complex study systems. Moreover, several studies of 'imperfect generalists', consistent with Liem's Paradox, raise questions about the ability of evolved species differences to partition niche space effectively when resources vary considerably across the annual cycle. Here we used a recently developed theoretical framework to combine past research on population-level processes with observational data on resource use to test for ongoing interspecific competition and understand the nature of resource overlap. We compared species diet overlaps and differences in several distinctive communities centred on a focal species, the American Redstart Setophaga ruticilla replicated both spatially and seasonally, in combination with documentation of population regulation to assess the ability of similar species to partition dietary niche space and limit interspecific competition. Our results document high dietary overlap in most of the communities studied, with only subtle differentiation consistent with known species differences in foraging behaviour and morphology. These findings are largely consistent with species foraging as imperfect generalists. However, in contrast to past studies, the high diet overlaps observed here during times of inferred resource scarcity were driven by low-value prey taxa (e.g. small ants) and did not involve truly 'private' resources. All of these factors increase the potential negative impacts of interspecific competition, and limit the ability of these birds to avoid competition if food availability deteriorates further than observed in our study, either seasonally or at longer intervals.