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Several species of birds and mammals overcome periods of scarcity by caching food, but for the vast majority of species, it is virtually unknown whether they are food limited during these periods. The Gray Jay (Perisoreus canadensis) is a boreal-resident, food-caching passerine that breeds in late winter when fresh food is scarce. Using a two-year experiment and 43 years of monitoring data, we examined the food limitation hypothesis in a population of Gray Jays in Algonquin Park, Ontario, Canada, that has declined by over 50% in the last three decades. Breeding pairs that were experimentally food supplemented during the pre-breeding period laid eggs earlier in the season and had larger brood sizes than non-supplemented controls. From the long-term data, we found strong evidence that pairs that were regularly supplemented by the public (park visitors) tended to lay eggs earlier and have larger clutches and brood sizes compared to pairs that were not supplemented. Nestling body condition (mass controlled for body size) was not influenced by either experimental or public food supplementation. Our results support the hypothesis that Gray Jays are food limited during their late-winter breeding period and suggest that warmer fall temperatures, which have been hypothesized to lead to cache spoilage, may have a significant impact on reproductive success in this declining population. Moreover, our results contribute to understanding how public feeding can influence the fitness of wild animals. [ABSTRACT FROM AUTHOR]

According to conceptual models, the distribution of resources plays a critical role in determining how organisms distribute themselves near habitat edges. These models are frequently used to achieve a mechanistic understanding of edge effects, but because they are based predominantly on correlative studies, there is need for a demonstration of causality, which is best done through experimentation. Using artificial seagrass habitat as an experimental system, we determined a likely mechanism underpinning edge effects in a seagrass fish. To test for edge effects, we measured fish abundance at edges (0-0.5 m) and interiors (0.5-1 m) of two patch configurations: continuous (single, continuous 9-m2 patches) and patchy (four discrete 1-m2 patches within a 9-m2 area). In continuous configurations, pipefish (Stigmatopora argus) were three times more abundant at edges than interiors (positive edge effect), but in patchy configurations there was no difference. The lack of edge effect in patchy configurations might be because patchy seagrass consisted entirely of edge habitat. We then used two approaches to test whether observed edge effects in continuous configurations were caused by increased availability of food at edges. First, we estimated the abundance of the major prey of pipefish, small crustaceans, across continuous seagrass configurations. Crustacean abundances were highest at seagrass edges, where they were 16% greater than in patch interiors. Second, we supplemented interiors of continuous treatment patches with live crustaceans, while control patches were supplemented with seawater. After five hours of supplementation, numbers of pipefish were similar between edges and interiors of treatment patches, while the strong edge effects were maintained in controls. This indicated that fish were moving from patch edges to interiors in response to food supplementation. These approaches strongly suggest that a numerically dominant fish species is more abundant at seagrass edges due to greater food availability, and provide experimental support for the resource distribution model as an explanation for edge effects. [ABSTRACT FROM AUTHOR]

Although the possibility that food and predators may interact in limiting avian populations has long been recognized, there have been few attempts to test this experimentally in the field. We conducted a manipulative food addition experiment on the demography of Song Sparrows (Melospiza melodia) across sites that varied in predator abundance, near Victoria, British Columbia, Canada, over three consecutive breeding seasons. We previously showed that food and predators had interactive effects on annual reproductive success (young fledged per female). Here, we report the effects on egg production. Our results show that food limits the total number of eggs laid over the breeding season ("total egg production") and that interactive food and predator effects, including food effects on nest predation, determine how those eggs are "parceled out" into different nests. Food addition alone significantly affected total egg production, and there was no significant interannual variability in this result. At the same time, both food and predators affected the two determinants of total egg production: "clutch number" (total number of clutches laid) and average clutch size. Both clutch number and size were affected by a food × predator × year interaction. Clutch number was lower at low-predator locations because there was less nest predation and thus less renesting. Food addition also significantly reduced nest predation, but there was significant interannual variation in this effect. This interannual variation was responsible for the food × predator × year interactions because the larger the effect of food on nest predation in a given year, the smaller was the effect of food on clutch number; and the smaller the effect of food on clutch number, the larger was the effect of food on clutch size. Potential predator and year effects on total egg production were thus cancelled out by an inverse relationship between clutch number and clutch size. We suggest that combined food and predator effects on demography could be the norm in both birds and mammals. [ABSTRACT FROM AUTHOR]

We investigated the relative importance and interaction of ecological processes affecting annual fecundity in birds by simultaneously manipulating food availability and nest predation risk in a small songbird, the Wrentit (Chamaea fasciata). From 2000 to 2002 we provided supplemental food to individual Wrentit territories, find during 2002 we altered nest predation risk by providing supplemental food to their principal predators, Western Scrub-Jays (Aphelocoma californica). These experiments were conducted during a period of high interannual variation in rainfall, with 2002 being one of the driest years on record. Food-supplemented Wrentits in a normal predation environment produced an average of 0.54 more fledglings per year than control pairs over the three breeding seasons. During the feeding plus predation manipulation experiment, Wrentit food supplementation and lowered nest predation risk each independently increased the probability that a Wrentit pair would fledge young; however, the interaction between food supplementation and altered nest predation risk was not significant. Thus, even in an extreme drought year, both food and nest .predation had equal but independent effects on reproductive success and annual fecundity. Combining supplemental food with reduced nest predation did not result in a synergistic increase in annual fecundity, primarily because Wrentits did not produce multiple broods. Our results suggest that whether food and predation have additive or synergistic effects on reproductive success depends on the life .history of the species and the environment in which they live. [ABSTRACT FROM AUTHOR]

In short-lived species, fecundity strongly influences population size. For those species with multiple breeding attempts per breeding season, variance in fecundity is best explained by the number of breeding attempts. For birds, multiple brooding may he influenced by food availability. Here, we report results of a food supplementation experiment that tests the role of food as a mechanism driving variation among individuals in the frequency of multiple brooding in a Neotropical migrant songbird. the Black-throated Blue Warbler (Dendroica caerulescens). Supplementally fed females produced more second broods, spent less time foraging and more time loafing, and stayed closer to their nests than did control females. Fed and control females did not differ in the number or mass of young fledged from the first nesting attempt. Supplemental food increased the probability that females would initiate second broods in both a low and an average food year, suggesting that this population is food limited during the breeding season in most years. Our results thus demonstrate that food availability can strongly influence annual fecundity in migratory bird species breeding in temperate forests, which, in turn, affects annual recruitment rates arid population size. [ABSTRACT FROM AUTHOR]