Probability Of Predation Research Articles

It's a dog‐eat‐croc world: dingo predation on the nests of freshwater crocodiles in tropical Australia

AbstractPredation on eggs is an important source of mortality for many long‐lived organisms, but causes of egg mortality from specific predators remain poorly known in most cases. Understanding the identity of predators, and the rates and determinants of their effects on a cohort of recruits, can provide a valuable background for attempts to exploit, control or conserve populations. We used remotely triggered cameras to study predation on the nests of freshwater crocodiles (Crocodylus johnstoni) inhabiting Lake Argyle, in tropical Australia. We also supplemented our work on natural crocodile nests with artificial nests. Overall, 80 of 111 natural nests were opened by predators, and predation occurred throughout the study period (7 weeks). Unlike in other parts of the species’ range, most nest‐robbers were dingoes (Canis lupus dingo, responsible for 98% of all predator visits in the northern sites, and 54% in the Ord River site), with minimal additional predation by reptiles and birds. Contrary to expectation, rates of nest predation were not influenced by spatial clumping of nests: the probability of predation per nest did not change with total numbers of nests laid in an area, and artificially aggregated versus dispersed nests experienced similar levels of predation. Nest vulnerability was linked to abiotic features including slope of surrounding banks, compactness of nesting substrate, and distance from the nearest forest. Abundant aquatic food resources support a large crocodile population, but a lack of suitable nest‐sites forces the crocodiles to concentrate nesting in small areas readily accessible to wide‐ranging nest predators. Collectively, our results suggest that distinctive attributes of the lakeside landscape alter predator guilds and fashion unique predator–prey interactions.

Impacts of dissolved oxygen on the behavior and physiology of bonefish: Implications for live-release angling tournaments

Saltwater tournaments for bonefish ( Albula spp.) often retain fish in livewells for up to 8 h to allow fish to recover from the physiological disturbances associated with angling. During livewell confinement, oxygen concentrations may fall due to elevated biomass of fish, coupled with low exchange of water. Some anglers use oxygen infusion systems, potentially exposing fish to water that is supersaturated with oxygen. Currently, the effects of differing levels of oxygen on bonefish recovery are unknown. Because physiological disturbances related to angling can influence the probability of post-release predation in bonefish, livewell conditions that maximize recovery rates without imparting additional negative consequences need to be defined. The objective of this study was to assess the behavior, physiological response (i.e., blood chemistry), and metabolic rates of bonefish recovered in hypoxic, normoxic, or hyperoxic seawater after exercise (i.e., a simulated angling event). Behavioral experiments consisted of placing bonefish in one of three dissolved oxygen concentrations and monitoring gill ventilation rates. For blood sampling and metabolic rates, bonefish were exercised and then recovered in different dissolved oxygen concentrations, replicating an angling event coupled with different livewell holding conditions. Both hypoxic and hyperoxic conditions caused bonefish to experience behavioral and physiological disturbances, compared to fish in the normoxic treatment. In addition, bonefish used more energy when recovered in hyperoxic seawater and fish in the hypoxic treatment were unable remove lactate compared to fish in the normoxic treatment. These results indicate that anglers and tournament organizers should recover angled bonefish in normoxic seawater. To achieve these conditions, dissolved oxygen concentrations should be monitored with a commercially available meter and maintained between 4–8 mg/L by circulating fresh seawater into livewells.

Cownose Ray (Rhinoptera bonasus) Predation Relative to Bivalve Ontogeny

The purpose of this study was to determine the ability of the cownose ray, Rhinoptera bonasus (Mitchill, 1815), to manipulate oysters and clams, to test for relative prey preference, and to investigate whether susceptibility to cownose ray predation changes with bivalve ontogeny. We investigated patterns of predation for captive adult and young-of-year cownose rays on 4 species of bivalves, including Crassostrea virginica (Gmelin, 1791), Crassostrea ariakensis (Fujita, 1913), Mercenaria mercenaria (Linnaeus, 1758), and Mya arenaria Linnaeus, 1758. In oyster (C. virginica) trials, predation probabilities by adult rays were highest at shell heights of 30–70 mm and shell depths of 8–22 mm. The rates of predation by adult rays in trials in which samesize oysters were used were higher than rates in most comingled trials. Adult rays showed no differences in predation between native oysters (C. virginica) and nonnative oysters (C. ariakensis; P > 0.05). Adult rays selected hard- and soft-shell clams (Manly-Chesson index M. mercenaria, α = 0.736 ± 0.002, electivity = 0.473 ± 0.007; M. arenaria, α = 0.742 ± 0.003, electivity = 0.485 ± 0.013) over oysters (C. virginica, α = 0.263 ± 0.002, electivity = -0.473 ± 0.007; α = 0.257 ± 0.003, electivity = -0.485 ± 0.003). In young-of-year feeding trials, oysters with a shell height of 10–35 mm and a shell depth of 3–12 mm had the highest probability of predation. Native oyster and hard clam peak force or load crush tests resulted in forces of 200–1,500 N and 400–1,400 N across shell depths of 10–35 mm and 21–34 mm, respectively, before valve failure. The results of this study indicate that cownose ray predation on shellfish is limited by shell size and is likely related to ray jaw gape and bite force.

Ecomorphology and disease: cryptic effects of parasitism on host habitat use, thermoregulation, and predator avoidance

Parasites can cause dramatic changes in the phenotypes of their hosts, sometimes leading to a higher probability of predation and parasite transmission. Because an organism's morphology directly affects its locomotion, even subtle changes in key morphological traits may affect survival and behavior. However, despite the ubiquity of parasites in natural communities, few studies have incorporated parasites into ecomorphological research. Here, we evaluated the effects of parasite-induced changes in host phenotype on the habitat use, thermal biology, and simulated predator-escape ability of Pacific chorus frogs (Pseudacris regilla) in natural environments. Frogs with parasite-induced limb malformations were more likely to use ground microhabitats relative to vertical refugia and selected less-angled perches closer to the ground in comparison with normal frogs. Although both groups had similar levels of infection, malformed frogs used warmer microhabitats, which resulted in higher body temperatures. Likely as a result of their morphological abnormalities, malformed frogs allowed a simulated predator to approach closer before escaping and escaped shorter distances relative to normal frogs. These data indicate that parasite-induced morphological changes can significantly alter host behavior and habitat use, highlighting the importance of incorporating the ubiquitous, albeit cryptic, role of parasites into ecomorphological research.

Where to place your eggs: the effects of conspecific eggs and water depth on oviposition decisions in bluefin killifish

In species with external development, egg placement is expected to impact the fitness of females and males via offspring survival. Both environmental and social cues influence the placement of eggs. In nest building fishes with male parental care, females frequently prefer to lay eggs in areas where eggs are already present. Most studies on female oviposition strategies have focused on species where males build nests and care for the eggs. However, few studies have examined oviposition strategies in species lacking parental care. This study tested whether female bluefin killifish Lucania goodei prefer to lay eggs in spawning substrates that already contain eggs from other females. The unique aspect of this study is that L. goodei is a non-nest building species with no parental care and high levels of iteroparity. Females preferred to lay eggs in areas where eggs were already present but these effects decreased with increasing clutch size. We suggest that females prefer to lay small bouts of eggs in areas already containing eggs of other females, but that in nature, they distribute these eggs across multiple males and locations. By doing so, females may increase the probability of offspring survival via either the dilution effect (reduced individual probability of predation due to increased group size) or the selection of ‘good locations’ for offspring development.

Comparative predation rates on larval snappers (Lutjanidae) in oceanic, reef, and nearshore waters

The life cycle of most reef fishes involves pelagic larvae entering the nearshore environment to settle to benthic substrates. Settlement is considered to be highly risky as larvae encounter high rates of predation mortality associated with shallow nearshore habitats. This potential bottleneck may be particularly significant for many tropical snapper (Lutjanidae) species which bypass the reef to settle to very nearshore seagrass areas. To test the hypothesis that predation-related mortality increases as naïve late-stage fish larvae leave the offshore oceanic environment and enter the nearshore to settle, relative nocturnal predation rates on tethered late-stage snapper larvae were measured in oceanic, coral reef, and nearshore surface waters of the lower Florida Keys, USA. Both relative predation rate and probability of predation in oceanic areas seaward of the reef was significantly greater than over reef or nearshore seagrass/hardbottom habitats. This surprising result may be due to differences in the density or spatial distribution of potential predators between deep offshore (near flotsam at the surface) and shallow nearshore environments (demersal). These findings suggest that successful late-stage snapper larvae should avoid surface waters in deep oceanic areas and move upward in the water column as they pass over the reef and other shallow nearshore environments prior to settlement.

Patterns of mineral lick visitation by spider monkeys and howler monkeys in Amazonia: are licks perceived as risky areas?

Mineral licks--also known as "salados," "saladeros," or "collpas"--are specific sites in tropical and temperate ecosystems where a large diversity of mammals and birds come regularly to feed on soil. Although the reasons for vertebrate geophagy are not completely understood, animals are argued to obtain a variety of nutritional and health benefits from the ingestion of soil at mineral licks. We studied the temporal patterns of mineral lick use by white-bellied spider monkey (Ateles belzebuth) and red howler monkey (Alouatta seniculus) in a lowland rain forest in Amazonian Ecuador. Using camera and video traps at four different mineral licks, combined with behavioral follows of one group of spider monkeys, we documented rates of mineral lick visitation by both primate species and the relative frequency and intensity of mineral lick use by spider monkeys. On the basis of 1,612 days and 888 nights of mineral lick monitoring, we found that A. belzebuth and A. seniculus both visit mineral licks frequently throughout the year (on average ∼14% of days for both species), and mineral lick visitation was influenced by short-term environmental conditions (e.g. sunny and dry weather). For spider monkeys, the area surrounding the lick was also the most frequently and most intensively used region within the group's home range. The fact that spider monkeys spent long periods at the lick area before coming to the ground to obtain soil, and the fact that both species visited the lick preferentially during dry sunny conditions (when predator detectability is presumed to be relatively high) and visited simultaneously more often than expected by chance, together suggest that licks are indeed perceived as risky areas by these primates. We suggest that howler and spider monkeys employ behavioral strategies aimed at minimizing the probability of predation while visiting the forest floor at risky mineral lick sites.

A latitudinal gradient in seed nutrients of the forest herb Anemone nemorosa

The nutrient concentration in seeds determines many aspects of potential success of the sexual reproductive phase of plants, including the seed predation probability, efficiency of seed dispersal and seedling performance. Despite considerable research interest in latitudinal gradients of foliar nutrients, a similar gradient for seeds remains unexplored. We investigated a potential latitudinal gradient in seed nutrient concentrations within the widespread European understorey forest herb Anemone nemorosa L. We sampled seeds of A. nemorosa in 15 populations along a 1900-km long latitudinal gradient at three to seven seed collection dates post-anthesis and investigated the relative effects of growing degree-hours >5 °C, soil characteristics and latitude on seed nutrient concentrations. Seed nitrogen, nitrogen:phosphorus ratio and calcium concentration decreased towards northern latitudes, while carbon:nitrogen ratios increased. When taking differences in growing degree-hours and measured soil characteristics into account and only considering the most mature seeds, the latitudinal decline remained particularly significant for seed nitrogen concentration. We argue that the decline in seed nitrogen concentration can be attributed to northward decreasing seed provisioning due to lower soil nitrogen availability or greater investment in clonal reproduction. This pattern may have large implications for the reproductive performance of this forest herb as the degree of seed provisioning ultimately co-determines seedling survival and reproductive success.

Parental Investment Decisions in Response to Ambient Nest-Predation Risk Versus Actual Predation on the Prior Nest

Abstract. Theory predicts that parents should invest less in dependent offspring with lower reproductive value, such as those with a high risk of predation. Moreover, high predation risk can favor reduced parental activity when such activity attracts nest predators. Yet, the ability of parents to assess ambient nest-predation risk and respond adaptively remains unclear, especially where nest-predator assemblages are diverse and potentially difficult to assess. We tested whether variation in parental investment by a multi-brooded songbird (Brewer's Sparrow, Spizella breweri) in an environment (sagebrush steppe) with diverse predators was predicted by ambient nest-predation risk or direct experience with nest predation. Variation among eight sites in ambient nest-predation risk, assayed by daily probabilities of nest predation, was largely uncorrelated across four years. In this system risk may therefore be unpredictable, and aspects of parental investment (clutch size, egg mass, incubation rhythms, nestlin...

Within-reach spatial variability of snails and molluscivory by brown trout

Linking habitat distributions of prey to the probability of predation is important to understanding consumptive effects of predators on prey populations. This study reports how within-reach spatial variability of two snails, the hydrobiid Potamopyrgus antipodarum and the physid Physella acuta, was linked to habitat-based predation risk by young brown trout (Salmo trutta) of different age classes. Potamopyrgus is endemic to New Zealand streams and lakes, where it commonly co-exists with the invader P. acuta, but both snails are worldwide invaders to many freshwater systems. Examination of egested snails revealed Potamopyrgus and Physella were consumed in similar numbers within age classes. However, 10-month-old trout consumed, on average, fewer snails than 20-month-old trout, and 8-month-old trout ate essentially no snails, suggesting snails were a more important prey item for larger age-1 fish than smaller age-0 fish. No Physella were egested alive by any trout age class. However, 38% and 16% of the Potamopyrgus consumed were egested alive by 10- and 20-month-old trout, respectively, with some passing live after ~70 h in digestive tracts. Physella and the spiny-shell form of Potamopyrgus were significantly denser on macrophytes than on stony sediments in midchannel, and these habitat distributions affected their odds of consumption. Risk of consumption by trout was ~10 times greater for Physella than Potamopyrgus on stones, but their risk was similar in protective macrophytes. Odds of consumption were similar for spiny and smooth shell forms of Potamopyrgus on stones, suggesting spines do not provide protection from large predators like trout. My results suggest that brown trout can potentially exert stronger population regulatory effects on Physella than on Potamopyrgus and that these effects are partly mediated by macrophyte cover.

Indirect predator effects on clutch size and the cost of egg production

Predator-induced changes in physiology and behaviour may negatively affect a prey's birth rate. Evidence of such indirect predator effects on prey demography remains scarce in birds and mammals despite invertebrate and aquatic studies that suggest ignoring such effects risks profoundly underestimating the total impact of predators. We report the first experimental demonstration of indirect predator effects on the annual 'birth' rate resulting from negative effects on the size of subsequent clutches laid by birds. We manipulated the probability of nest predation and measured the size of subsequent clutches and multiple indices of the mother's physiological condition, while controlling for food availability, date and stage of breeding. Females subject to frequent experimental nest predation laid smaller subsequent clutches and were in poorer physiological condition, particularly regarding non-resource-based indices (e.g. oxidative stress and glucocorticoid mobilization) consistent with both a response to the threat of predation and an increased cost of egg production.

Facultative nest patch shifts in response to nest predation risk in the Brewer’s sparrow: a “win-stay, lose-switch” strategy?

Facultative shifts in nesting habitat selection in response to perceived predation risk may allow animals to increase the survival probability of sessile offspring. Previous studies on this behavioral strategy have primarily focused on single attributes, such as the distance moved or changes in nesting substrate. However, nest site choice often encompasses multiple habitat elements at both the nest site and nest patch scales. We studied the within-season re-nesting strategy of a multi-brooded songbird, the Brewer's sparrow (Spizella breweri), to determine whether pairs utilized a "win-stay, lose-switch" decision rule with respect to inter-nest distance, nest substrate and/or nest patch characteristics in response to previous nest fate. Pairs moved sequential nest sites slightly farther following nest predation versus success. When inter-nest distance was controlled, however, pairs changed nest patch attributes (shrub height, potential nest shrub density) associated with probability of nest predation to a greater extent following nest predation than success. The strategy appeared to be adaptive; daily nest survival probability for previously depredated pairs increased with greater Euclidian habitat distances between attempts, whereas previously successful pairs were more likely to fledge second attempts when nest sites were similar to those of previous attempts. Our results suggest that nesting birds can use prior information and within-season plasticity in response to nest predation to increase re-nesting success, which may be a critical behavioral strategy within complex nest predator environments. Re-nesting site selection strategies also appeared to integrate multiple habitat components and inter-nest distances. The consideration of such proximate, facultative responses to predation risk may clarify often unexplained variation in habitat preferences and requirements.

Impacts of Anthropogenic Structures on Predation of Painted Turtle (Chrysemys picta) Nests

Anthropogenic factors can negatively impact wildlife populations, but deleterious effects may not be universal. We investigated the relationship between nest predation and spatial proximity to anthropogenic structures (campsites, trash bins, etc.) for 1375 painted turtle (Chrysemys picta) nests over 6 years for a population on the Mississippi River, Illinois. Although varying among years, the probability of nest predation increased with greater distance from anthropogenic structures over all years combined and did not differ between supplemental food attractant structures (e.g., fish cleaning table) and nonsupplemental food attractant structures (e.g., horseshoe pits).

Nest Predation of Greater Sage‐Grouse in Relation to Microhabitat Factors and Predators

ABSTRACT Nest predation is a natural component of greater sage‐grouse (Centrocercus urophasianus) reproduction, but changes in nesting habitat and predator communities may adversely affect grouse populations. We used a 2‐part approach to investigate sage‐grouse nest predation. First, we used information criteria to compare nest survival models that included indices of common raven (Corvus corax) abundance with other survival models that consisted of day of incubation, grouse age, and nest microhabitat covariates using measurements from 77 of 87 sage‐grouse nests. Second, we used video monitoring at a subsample of 55 of 87 nests to identify predators of depredated nests (n = 16) and evaluated the influence of microhabitat factors on the probability of predation by each predator species. The most parsimonious model for nest survival consisted of an interaction between day of incubation and abundance of common ravens (wravenXincubation day = 0.67). An estimated increase in one raven per 10‐km transect survey was associated with a 7.4% increase in the odds of nest failure. Nest survival was relatively lower in early stages of incubation, and this effect was strengthened with increased raven numbers. Using video monitoring, we found the probability of raven predation increased with reduced shrub canopy cover. Also, we found differences in shrub canopy cover and understory visual obstruction between nests depredated by ravens and nests depredated by American badgers (Taxidea taxus). Increased raven numbers have negative effects on sage‐grouse nest survival, especially in areas with relatively low shrub canopy cover. We encourage wildlife managers to reduce interactions between ravens and nesting sage‐grouse by managing raven populations and restoring and maintaining shrub canopy cover in sage‐grouse nesting areas.

Local outbreaks of Operophtera brumata and Operophtera fagata cannot be explained by low vulnerability to pupal predation

One of the unresolved questions in studies on population dynamics of forest Lepidoptera is why some populations at times reach outbreak densities, whereas others never do. Resolving this question is especially challenging if populations of the same species in different areas or of closely‐related species in the same area are considered. The present study focused on three closely‐related geometrid moth species, autumnal Epirrita autumnata, winter Operophtera brumata and northern winter moths Operophtera fagata, in southern Finland. There, winter and northern winter moth populations can reach outbreak densities, whereas autumnal moth densities stay relatively low. We tested the hypothesis that a lower vulnerability to pupal predation may explain the observed differences in population dynamics. The results obtained do not support this hypothesis because pupal predation probabilities were not significantly different between the two genera within or without the Operophtera outbreak area or in years with or without a current Operophtera outbreak. Overall, pupal predation was even higher in winter and northern winter moths than in autumnal moths. Differences in larval predation and parasitism, as well as in the reproductive capacities of the species, might be other candidates.

Are There Optimal Densities for Prairie Birds?

Abstract. The major forces of food and predation shape fitness-enhancing decisions of birds at all stages of their life cycles. During the breeding season, birds can minimize nest loss due to predation by selecting sites with a lower probability of predation. To understand the environmental and social aspects and consequences of breedingsite selection in prairie birds, we explored variation in nest-survival patterns of the Lark Bunting (Calamospiza melanocorys) in the shortgrass prairie region of North America. Over four breeding seasons, we documented the survival of 405 nests, conducted 60 surveys to estimate bird densities, and measured several vegetative features to describe habitat structure in 24 randomly selected study plots. Nest survival varied with the buntings' density as described by a quadratic polynomial, increasing with density below 1.5 birds ha-1 and decreasing with density between 1.5 and 3 birds ha-1, suggesting that an optimal range of densities favors reproductive success of the Lark ...

Responses of generalist invertebrate predators to pupal densities of autumnal and winter moths under field conditions

Abstract. Generalist natural enemies are usually not considered as being capable of causing population cycles in forest insects, but they may influence the population dynamics of their prey in the low density cycle phase when specialist enemies are largely absent. In the present field study, the total response of the generalist invertebrate predator community to experimentally established pupal densities of the closely related autumnal (Epirrita autumnata) and winter moths (Operophtera brumata) was analysed. Due to the high amount of variation in the dataset, the exact shape of the response curve could not be convincingly estimated. Nevertheless, two important conclusions can be drawn from the analyses. Firstly, the natural invertebrate predator community seems to become saturated at rather low densities of both autumnal and winter moth pupae. Secondly, the predator community seems to become saturated at much lower densities of autumnal than of winter moth pupae. Furthermore, pupal mass was significantly negatively correlated with invertebrate predation probability in autumnal moth pupae. These results indicate that differences in the predator assemblage being able to consume pupae of the two moth species, as well as different handling times, could be responsible for the substantially higher predation rates in winter than in autumnal moth pupae. As a consequence, the population dynamics of autumnal moths might be less affected by generalist invertebrate predators than those of winter moths, as autumnal moths seem able to escape from the regulating influence of generalist predators at much lower population densities than winter moths.

Swainson's Warbler Nest-Site Selection in Eastern Arkansas

Abstract. By affecting the probability of nest predation and brood parasitism, avian nest-site selection has significant implications for reproduction and fitness. Therefore, understanding factors associated with habitat use at the nest-site scale is imperative, especially for species of conservation concern. One such species, the Swainson's Warbler (Limnothlypis swainsonii), is a rare neotropical migrant that breeds mainly in bottomland hardwood forests throughout the southeastern United States, but relatively little is known about its nesting habitat. From 2004 through 2007, we studied nest-site selection of Swainson's Warblers at two study areas in eastern Arkansas. We found that, relative to paired random plots, nest sites were characterized by dense understory vegetation, high total canopy cover, abundant leaf litter, and high density of woody stems, especially of giant cane (Arundinaria gigantea), a once-abundant bamboo native to the southeastern U.S. Indeed, most nests (90%) were placed in giant ca...

Marbled Murrelet nest predation risk in managed forest landscapes: dynamic fragmentation effects at multiple scales

The effects of forest fragmentation on bird populations have been studied primarily as static phenomena. Yet when forests are allowed to regenerate, local edge contrast and landscape matrix composition change with time, and we would expect fragmentation effects to change accordingly. Describing this process is critical for the conservation of avian species sensitive to forest fragmentation, including the Marbled Murrelet (Brachyramphus marmoratus), a seabird threatened by ongoing harvest of old-growth forest nesting habitat. We experimentally assessed potential murrelet nest predation probability in four regions of southwestern British Columbia, Canada. We compared the fates of 448 simulated murrelet nests at paired edge and interior treatments, at sites with "hard" edges (recent clearcuts), "soft" edges (regenerating forest), and natural edges (i.e., riparian areas). Motion-sensitive digital nest cameras enabled us to focus on known predators of real nests, and patterns of nest fates did not differ between real and simulated nests. Using information-theoretic model selection (AIC) with the combined data set (116 sites), we assessed effects at patch (approximately 13 ha), landscape (approximately 1700 ha), and regional (approximately 96000 ha) scales. Nest disturbance probability at hard edges was 2.5 times that of interior sites, but soft edges had less than half the disturbance probability of interiors. There was no edge effect at natural edges. At the landscape scale, overall avian disturbance risk declined by as much as 50% with increasing amounts of regenerating forest in the surrounding matrix. These results indicate that initially negative fragmentation effects decrease as forests regenerate, at both patch and landscape scales. There was no evidence that these patterns differed between regions. Predator surveys suggested that Steller's Jays drive patterns of nest predation risk at the regional scale. Assuming that corvids are the most important predators, larger reserves of habitat will lessen negative hard-edge effects. Smaller reserves should be embedded in a protective matrix of regenerating forest to reduce predation risk at both patch and landscape scales. Our results suggest that dynamic fragmentation effects are generalizable across widespread regions and can be broadly applied to both murrelet management and the conservation of old-growth forest-breeding birds in general.

Linking edge effects and patch size effects: Importance of matrix nest predators

The edge effect is usually considered to be the proximate cause of area sensitivity in forest birds. We tested if birds nesting in large patches are less vulnerable to the edge effect using a simple model that assumes an increase in patch size reduces the probability of a matrix predator moving to the core areas of forest and that larger perimeter/area ratios result in a higher number of matrix predators per unit of area. The probability of a nest being successful decreased asymptotically with an increase in either the patch penetration distance of predators or predator density, but those effects were reduced when patch size was increased. Large patches have a lower probability of being affected by an Allee effect and they can function as sink habitats only if penetration distance and predator density are largely increased. However, the transition from an Allee effect to a sink condition occurs with a small increase in penetration distance and predator density. Since birds nesting in large patches are less vulnerable to an increase in matrix predator populations, persistence of bird populations may be possible by increasing the size of habitat patches that can act as source populations.