USGS Science⌕ Search

SEARCH · USGS Science

Results for “Journal of Animal Ecology”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5Linked to original sources
USGS2023

Revealing the extent of sea otter impacts on bivalve prey through multi-trophic monitoring and mechanistic models

Sea otters are apex predators that can exert considerable influence over the nearshore communities they occupy. Since facing near extinction in the early 1900s, sea otters are making a remarkable recovery in Southeast Alaska, particularly in Glacier Bay, the largest protected tidewater glacier fjord in the world. The expansion of sea otters across Glacier Bay offers both a challenge to monitoring and stewardship and an unprecedented opportunity to study the top-down effect of a novel apex predator across a diverse and productive ecosystem. Our goal was to integrate monitoring data across trophic levels, space, and time to quantify and map the predator–prey interaction between sea otters and butter clams Saxidomus gigantea , one of the dominant large bivalves in Glacier Bay and a favoured prey of sea otters. We developed a spatially-referenced mechanistic differential equation model of butter clam dynamics that combined both environmental drivers of local population growth and estimates of otter abundance from aerial survey data. We embedded this model in a Bayesian statistical framework and fit it to clam survey data from 43 intertidal and subtidal sites across Glacier Bay. Prior to substantial sea otter expansion, we found that butter clam density was structured by an environmental gradient driven by distance from glacier (represented by latitude) and a quadratic effect of current speed. Estimates of sea otter attack rate revealed spatial heterogeneity in sea otter impacts and a negative relationship with local shoreline complexity. Sea otter exploitation of productive butter clam habitat substantially reduced the abundance and altered the distribution of butter clams across Glacier Bay, with potential cascading consequences for nearshore community structure and function. Spatial variation in estimated sea otter predation processes further suggests that community context and local environmental conditions mediate the top-down influence of sea otters on a given prey. Overall, our framework provides high-resolution insights about the interaction among components of this food web and could be applied to a variety of other systems involving invasive species, epidemiology or migration.

Journal of Animal Ecology↗
USGS2023

Stream hydrology and a pulse subsidy shape patterns of fish foraging

Pulsed subsidy events create ephemeral fluxes of hyper-abundant resources that can shape annual patterns of consumption and growth for recipient consumers. However, environmental conditions strongly affect local resource availability for much of the year, and can heavily impact consumer foraging and growth patterns prior to pulsed subsidy events. Thus, a consumer's capacity to exploit pulse subsidy resources may be influenced by antecedent environmental conditions, but this has rarely been shown in nature and is unknown in aquatic ecosystems. Here, we sought to understand the importance of hydrologic variation and a salmon pulse subsidy on the foraging and growth patterns of two stream salmonids in a coastal southeast Alaska drainage. To do this, we sampled fish stomach contents at a high temporal frequency (daily–weekly measurements) and analyzed fish consumption rates in relation to streamflow and pulse subsidy resource availability. We then explored the influence of interannual hydrologic variation on access to pulse subsidy resources (i.e. whether fish exceeded an egg consumption gape limit) in a bioenergetic simulation. Prior to Pink Salmon spawning, Dolly Varden and Coho Salmon displayed distinct and nonlinear flow-foraging relationships, where forage for both species consisted primarily of macroinvertebrates. During this time period, consumption maxima coincided with baseflow and the highest observed flow conditions, and consumption minima were observed at severe low-water and intermediate flow values. After salmon spawning began, forage was not significantly related to flow and consisted primarily of salmon eggs. Further, consumption rates increased overall, and foraging patterns did not appear to be affected by flow in either species. Bioenergetic simulations revealed that patterns of interannual hydrologic variation may shift Coho Salmon growth trajectories among years. Together, our results suggest that access to marine pulse subsidy resources may depend on whether antecedent hydrologic conditions are suitable for juvenile salmonids to grow large enough to consume salmon eggs by the onset of spawning.

Alaska↗
USGS2023

Springing forward: Migrating songbirds catch up with the start of spring in North America

In temperate regions, the annual pattern of spring onset can be envisioned as a ‘green wave’ of emerging vegetation that moves across continents from low to high latitudes, signifying increasing food availability for consumers. Many herbivorous migrants ‘surf’ such resource waves, timing their movements to exploit peak vegetation resources in early spring. Although less well studied at the individual level, secondary consumers such as insectivorous songbirds can track vegetation phenology during migration as well. We hypothesized that four species of ground-foraging songbirds in eastern North America—two warblers and two thrushes—time their spring migrations to coincide with later phases of vegetation phenology, corresponding to increased arthropod prey, and predicted they would match their migration rate to the green wave but trail behind it rather than surfing its leading edge. We further hypothesized that the rate at which spring onset progresses across the continent influences bird migration rates, such that individuals adjust migration timing within North America to phenological conditions they experience en route . To test our hypotheses, we used a continent-wide automated radio telemetry network to track individual songbirds on spring migration between the U.S. Gulf Coast region and northern locations closer to their breeding grounds. We measured vegetation phenology using two metrics of spring onset, the spring index first leaf date and the normalized difference vegetation index (NDVI), then calculated the rate and timing of spring onset relative to bird detections. All individuals arrived in the southeastern United States well after local spring onset. Counter to our expectations, we found that songbirds exhibited a ‘catching up’ pattern: Individuals migrated faster than the green wave of spring onset, effectively closing in on the start of spring as they approached breeding areas. While surfing of resource waves is a well-documented migration strategy for herbivorous waterfowl and ungulates, individual songbirds in our study migrated faster than the green wave and increasingly caught up to its leading edge en route . Consequently, songbirds experience a range of vegetation phenophases while migrating through North America, suggesting flexibility in their capacity to exploit variable resources in spring.

Journal of Animal Ecology↗
USGS2024

Quantifying intraspecific variation in host resistance ad tolerance to a lethal pathogen

Testing for intraspecific variation for host tolerance or resistance in wild populations is important for informing conservation decisions about captive breeding, translocation, and disease treatment. Here, we test the importance of tolerance and resistance in multiple populations of boreal toads ( Anaxyrus boreas boreas ) against Batrachochytrium dendrobatidis (Bd), the amphibian fungal pathogen responsible for the greatest host biodiversity loss due to disease. Boreal toads have severely declined in Colorado (CO) due to Bd, but toad populations challenged with Bd in western Wyoming (WY) appear to be less affected. We used a common garden infection experiment to expose post-metamorphic toads sourced from four populations (2 in CO and 2 in WY) to Bd and monitored changes in mass, pathogen burden and survival for 8 weeks. We used a multi-state modelling approach to estimate weekly survival and transition probabilities between infected and cleared states, reflecting a dynamic infection process that traditional approaches fail to capture. We found that WY boreal toads are more tolerant to Bd infection with higher survival probabilities than those in CO when infected with identical pathogen burdens. WY toads also appeared more resistant to Bd with a higher probability of infection clearance and an average of 5 days longer to reach peak infection burdens. Our results demonstrate strong intraspecific differences in tolerance and resistance that likely contribute to why population declines vary regionally across this species. Our multi-state framework allowed us to gain inference on typically hidden disease processes when testing for host tolerance or resistance. Our findings demonstrate that describing an entire host species as ‘tolerant’ or ‘resistant’ (or lack thereof) is unwise without testing for intraspecific variation.

Colorado, Wyoming↗
USGS2024

A ‘how-to’ guide for estimating animal diel activity using hierarchical models

Animal diel activity patterns can aid understanding of (a) how species behaviourally adapt to anthropogenic and natural disturbances, (b) mechanisms of species co-existence through temporal partitioning, and (c) community or ecosystem effects of diel activity shifts. Activity patterns often vary spatially, a feature ignored by the kernel density estimators (KDEs) currently used for estimating diel activity. Ignoring this source of heterogeneity may lead to biased estimates of uncertainty and misleading conclusions regarding the drivers of diel activity. Thus, there is a need for more flexible statistical approaches for estimating activity patterns and testing hypotheses regarding their biotic and abiotic drivers. We illustrate how trigonometric terms and cyclic cubic splines combined with hierarchical models can provide a valuable alternative to KDEs. Like KDEs, these models accommodate circular data, but they can also account for site-to-site and other sources of variability, correlation amongst repeated measures, and variable sampling effort. They can also more readily quantify and test hypotheses related to the effects of covariates on activity patterns. Through empirical case studies, we illustrate how hierarchical models can quantify changes in activity levels due to seasonality and in response to biotic and abiotic factors (e.g. anthropogenic stressors and co-occurrence). We also describe frequentist and Bayesian approaches for quantifying site-specific (conditional) and population-averaged (marginal) activity patterns. We provide guidelines and tutorials with detailed step-by-step instructions for fitting and interpreting hierarchical models applied to time-stamped data, such as those recorded by camera traps and audio recorders. We conclude that this approach offers a viable, flexible, and effective alternative to KDEs when modelling animal activity patterns.

Journal of Animal Ecology↗
USGS2025

Navigating new threats: Prey naivete in native mammals

1. Invasive predators pose a substantial threat to global biodiversity. Native prey species frequently exhibit naïveté to the cues of invasive predators, and this phenomenon may contribute to the disproportionate impact of invasive predators on prey populations. However, not all species exhibit naïveté, which has led to the generation of many hypotheses to explain patterns in prey responses. These hypotheses primarily fall into two categories: system-centric hypotheses related to biogeographic isolation (BIH) and species-centric hypotheses, like the arche type similarity hypothesis (ASH). 2. We tested the predictions of these hypotheses by assessing the response of the common raccoon ( Procyon lotor ) and hispid cotton rat ( Sigmodon hispidus ), two native mammal species with divergent snake predation histories, to the cues of the invasive Burmese python ( Python bivittatus ) in the Florida Everglades (USA). Using giving-up densities (GUDs), we assessed the responses of both cotton rats and raccoons to life-size replicas of Burmese pythons and two North American predators eastern diamondback rattlesnakes ( Crotalus adamanteus ) and coyotes ( Canis latrans ). 3. Although cotton rats increased their GUD in the presence of all three predators relative to the novel-object control, raccoons only increased their GUD in coyote treatments. 4. These results align with the predictions of the ASH but not the BIH, and mirror observed patterns of population declines in invaded areas of the Florida Everglades. 5. More broadly, our findings suggest that naïveté may contribute to the vulnerability of some species to invasive predators even in large continental systems

Florida↗
USGS2025

Demographic mechanisms of snowshoe hare population cycles in Yukon, Canada

One hundred years have elapsed since Charles Elton (1924) described the periodic fluctuations in North American snowshoe hare abundance, yet mechanisms underlying 9–11-year population cycles in snowshoe hares continue to be debated. We applied multistate capture–mark–recapture models to long-term field data (1977–2020) based on >20,000 captures of >7000 unique snowshoe hares ( Lepus americanus ) from Kluane Lake, Yukon, Canada, to estimate and model state-specific demographic parameters. Juveniles had the lowest and reproductive adult females the highest apparent survival. Apparent survival of all sex-age classes was highest during the mid- and late-breeding seasons and was generally better during the increase phase. Conditional probability of females transitioning from non-reproductive to reproductive state, and reproductive females remaining in the reproductive state, increased substantially as the population transitioned from low to increase phase throughout the breeding season. Analysis of stage-structured matrix population models revealed that population-dynamic characteristics were strongly phase-specific, and also varied across seasons, with the increase phases being characterized by high monthly asymptotic population growth rate. Snowshoe hares experienced short stage-specific generation time during the early breeding season across all phases; they experienced relatively long generation time during the increase and low phase of the mid-breeding season, and the increase and peak phase of the late breeding season. Elasticity analyses showed that asymptotic population growth rate was proportionately most sensitive to changes in survival of adult females across all phases and seasons. However, retrospective life table response experiment analysis showed that rapid growth of the snowshoe hare populations during the increase phase was due to improvements in reproductive transitions and pre-weaning survival, whereas population declines are caused primarily by reduced survival (primarily, pre-weaning survival), with reduced reproductive transitions and smaller litter sizes playing a secondary role. Our results suggest that cyclic populations of snowshoe hares are characterized by complex demographic and population-dynamic patterns, depending on phase of the cycle and reproductive season, and that different demographic mechanisms underlie rapid population growth during the increase phase, and swift population declines as the population transitions from the peak to the decline phase. Because our study represents the first comprehensive demographic and population-dynamic study of a cyclic population, similar studies would be needed to test the generalities of our conclusions. Whereas density-dependent predation has been shown to be the primary cause of phase-related changes in survival, future research should focus on identifying mechanisms underlying phase-related changes in reproductive parameters.

Yukon↗
USGS2026

Predicting disease spread from host movement data: Chronic wasting disease in North America as a case study

1. Rare long-distance movements can increase the spatial spread of invasive species and shifts in species ranges. However, in the context of wildlife disease spread, seasonal migrations only matter if they lead to transmission beyond an individual’s initial set of contacts, and the importance of juvenile dispersal depends on whether those juveniles become infected before they disperse. 2. High resolution animal movement data are increasing, but it is unclear how to leverage these data to improve predictions future disease spread particularly in regions without pre-existing movement data. 3. We predicted the speed of chronic wasting disease (CWD) spread based on movement metrics of mule deer ( Odocoileus hemionus ) and white-tailed deer ( Odocoileus virginianus ) and compared our predictions to observed spreading rates in North America from 2005 to 2022. We hypothesized that CWD should spread faster in regions with long-distance migratory mule deer, but this hypothesis was only partially supported. 4. Observed rates of CWD spread were faster in western epicenters with more mobile mule deer than in epicenters with only white-tailed deer (14-18 km/yr compared to 5-7 km/yr, respectively). However, the observed speed of CWD spread did not align well with migration distances. Deer movements in the Canadian Prairie Provinces (CPP) were far shorter than those in Colorado and Wyoming, yet had faster CWD spread much of which was open prairies where deer movements tend to be along riverine corridors rather than the longer elevational migrations in more montane regions. 5. Generally, our predicted rates of CWD spread from three different models were far slower than the observed rates regardless of whether the model included dispersal or migration. Understanding of these discrepancies is needed before high-resolution animal movement data can be used to predict broad-scale patterns of disease invasion. 6. Anthropogenic movements likely explain longer-distance movements of CWD across North America, but it remains unclear whether human-mediated movements or alternative analytical methods and deer movement metrics can explain the faster than predicted local CWD spread.

Journal of Animal Ecology↗
USGS2004

Investigating species co-occurrence patterns when species are detected imperfectly

1. Over the last 30 years there has been a great deal of interest in investigating patterns of species co-occurrence across a number of locations, which has led to the development of numerous methods to determine whether there is evidence that a particular pattern may not have occurred by random chance. 2. A key aspect that seems to have been largely overlooked is the possibility that species may not always be detected at a location when present, which leads to 'false absences' in a species presence/absence matrix that may cause incorrect inferences to be made about co-occurrence patterns. Furthermore, many of the published methods for investigating patterns of species co-occurrence do not account for potential differences in the site characteristics that may partially (at least) explain non-random patterns (e.g. due to species having similar/different habitat preferences). 3. Here we present a statistical method for modelling co-occurrence patterns between species while accounting for imperfect detection and site characteristics. This method requires that multiple presence/absence surveys for the species be conducted over a reasonably short period of time at most sites. The method yields unbiased estimates of probabilities of occurrence, and is practical when the number of species is small (< 4). 4. To illustrate the method we consider data collected on two terrestrial salamander species, Plethodon jordani and members of the Plethodon glutinosus complex, collected in the Great Smoky Mountains National Park, USA. We find no evidence that the species do not occur independently at sites once site elevation has been allowed for, although we find some evidence of a statistical interaction between species in terms of detectability that we suggest may be due to changes in relative abundances.

North Carolina, Tennessee↗
USGS2004

Energy density and variability in abundance of pigeon guillemot prey: Support for the quality-variability trade-off hypothesis

1. The quality-variability trade-off hypothesis predicts that (i) energy density (kJ g-1) and spatial-temporal variability in abundance are positively correlated in nearshore marine fishes; and (ii) prey selection by a nearshore piscivore, the pigeon guillemot (Cepphus columba Pallas), is negatively affected by variability in abundance. 2. We tested these predictions with data from a 4-year study that measured fish abundance with beach seines and pigeon guillemot prey utilization with visual identification of chick meals. 3. The first prediction was supported. Pearson's correlation showed that fishes with higher energy density were more variable on seasonal (r = 0.71) and annual (r = 0.66) time scales. Higher energy density fishes were also more abundant overall (r = 0.85) and more patchy at a scale of 10s of km (r = 0.77). 4. Prey utilization by pigeon guillemots was strongly non-random. Relative preference, defined as the difference between log-ratio transformed proportions of individual prey taxa in chick diets and beach seine catches, was significantly different from zero for seven of the eight main prey categories. 5. The second prediction was also supported. We used principal component analysis (PCA) to summarize variability in correlated prey characteristics (energy density, availability and variability in abundance). Two PCA scores explained 32% of observed variability in pigeon guillemot prey utilization. Seasonal variability in abundance was negatively weighted by these PCA scores, providing evidence of risk-averse selection. Prey availability, energy density and km-scale variability in abundance were positively weighted. 6. Trophic interactions are known to create variability in resource distribution in other systems. We propose that links between resource quality and the strength of trophic interactions may produce resource quality-variability trade-offs.

Journal of Animal Ecology↗
USGS2008

Effects of food limitation and emigration on self-thinning in experimental minnow cohorts

1. &ensp;The theory of food-regulated self-thinning (FST) for mobile animals predicts population density ( N ) to be an inverse function of mean body mass ( W ) scaled to an exponent ( b ), such that N = k W &minus;b , where k is a constant. FST also predicts energy requirements (or energy flow) to remain constant over time (termed energetic equivalence) as losses to cohorts (e.g. emigration and mortality) are balanced by increased growth of surviving individuals. 2. &ensp;To test these predictions, we analysed the dynamics of six experimental minnow cohorts. Replicate populations of fish were held under identical conditions with a constant and limited supply of food over a 126-day period. Half of the cohorts were open to emigration, and half were closed so that fish could only be lost through starvation mortality. 3. &ensp;Patterns of self-thinning indicated non-linear changes in population density and energy flow in relation to changes in mean body mass and time, respectively. Non-linear patterns of self-thinning were probably due to a delayed growth response to changes in population density effected through mortality and/or emigration. Contrary to results of similar experiments on other fish, emigration did not have a significant influence on the pattern of self-thinning. 4. &ensp;These results may be attributed to trophic interactions within cohorts and the importance of social behaviour to cohort dynamics. Both population density and energy flow in our experimental populations appeared to cycle, with episodes of starvation and mortality alternating with food recovery and weight gain, as predicted by recent models of stepwise die-off and stunted growth in animal cohorts. 5. &ensp;Most of the support for FST in mobile animals comes from observational data on mean body mass and population density. Potentially important mechanisms, including the manner in which individuals are lost or retained in populations, are usually not investigated directly. Such tests of FST can only provide equivocal support. Detailed observational study and controlled experiments are needed to understand casual mechanisms.

Journal of Animal Ecology↗
USGS2006

Multiscale patterns of movement in fragmented landscapes and consequences on demography of the snail kite in Florida

1. Habitat loss and fragmentation are major factors affecting vertebrate populations. A major effect of these habitat alterations is that they reduce movement of organisms. Despite the accepted importance of movement in driving the dynamics of many natural populations, movement of vertebrates in fragmented landscapes have seldom been estimated with robust statistical methods. 2. We estimated movement probabilities of snail kites Rosthramus sociabilis within the remaining wetlands in Florida. Using both radio-telemetry and banding information, we used a multistate modelling approach to estimate transition probabilities at two temporal scales (month; year) and multiple spatial scales. We examined kite movement among wetlands altered by three different levels of fragmentation: among wetlands separated by small physical barriers (e.g. road); among wetlands separated by moderate amount of matrix (< 5 km); and among wetlands separated by extensive matrix areas (> 15 km). 3. Kites moved extensively among contiguous wetlands (movement probability 0?29 per month), but significantly less among isolated wetlands (movement probability 0?10 per month). 4. Kites showed high levels of annual site fidelity to most isolated wetlands (probability ranged from 0?72 to 0?95 per year). 5. We tested the effects of patch size and interpatch distance on movement. Our modelling indicated an effect of both distance and patch size on juveniles' movement (but not adult) when examining movements among fragments. 6. Only a small proportion of kites escaped a regional drought by moving to refugia (wetlands less affected by drought). Many individuals died after the drought. During drought adult survival dropped by 16% while juvenile survival dropped by 86% (possibly because juveniles were less likely to reach refugia). 7. We hypothesize that fragmentation may decrease kite's resistance to drought by restricting exploratory behaviour.

Florida↗
USGS2006

Patterns of growth and body condition in sea otters from the Aleutian archipelago before and after the recent population decline

1. Growth models for body mass and length were fitted to data collected from 1842 sea otters Enhydra lutris shot or live-captured throughout south-west Alaska between 1967 and 2004. Growth curves were constructed for each of two main year groups: 1967–71 when the population was at or near carrying capacity and 1992–97 when the population was in steep decline. Analyses of data collected from animals caught during 2004, when the population density was very low, were precluded by a small sample size and consequently only examined incidentally to the main growth curves. 2. Growth curves demonstrated a significant increase in body mass and body length at age in the 1990s. Asymptotic values of body mass were 12–18% higher in the 1990s than in the 1960s/70s, and asymptotic values for body length were 10–11% higher between the same periods. Data collected in 2004 suggest a continued increase in body size, with nearly all data points for mass and length falling significantly above the 1990s growth curves. 3. In addition to larger asymptotic values for mass and length, the rate of growth towards asymptotic values was more rapid in the 1990s than in the 1960s/70s: sea otters reached 95% of asymptotic body mass and body length 1–2 years earlier in the 1990s. 4. Body condition (as measured by the log mass/log length ratio) was significantly greater in males than in females. There was also an increasing trend from the 1960s/70s through 2004 despite much year-to-year variation. 5. Population age structures differed significantly between the 1960s/70s and the 1990s with the latter distribution skewed toward younger age classes (indicating an altered l x function) suggesting almost complete relaxation of age-dependent mortality patterns (i.e. those typical of food-limited populations). 6. This study spanned a period of time over which the population status of sea otters in the Aleutian archipelago declined precipitously from levels at or near equilibrium densities at some islands in the 1960s/70s to < 5% of estimated carrying capacity by the late 1990s. The results of this study indicate an improved overall health of sea otters over the period of decline and suggest that limited nutritional resources were not the cause of the observed reduced population abundance. Our findings are consistent with the hypothesis that the decline was caused by increased killer whale predation.

Journal of Animal Ecology↗
USGS2007

Climatic variation and the distribution of an amphibian polyploid complex

1. The establishment of polyploid populations involves the persistence and growth of the polyploid in the presence of the progenitor species. Although there have been a number of animal polyploid species documented, relatively few inquiries have been made into the large-scale mechanisms of polyploid establishment in animal groups. Herein we investigate the influence of regional climatic conditions on the distributional patterns of a diploid-tetraploid species pair of gray treefrogs, Hyla chrysoscelis and H. versicolor (Anura: Hylidae) in the mid-Atlantic region of eastern North America. 2. Calling surveys at breeding sites were used to document the distribution of each species. Twelve climatic models and one elevation model were generated to predict climatic and elevation values for gray treefrog breeding sites. A canonical analysis of discriminants was used to describe relationships between climatic variables, elevation and the distribution of H. chrysoscelis and H. versicolor . 3. There was a strong correlation between several climatic variables, elevation and the distribution of the gray treefrog complex. Specifically, the tetraploid species almost exclusively occupied areas of higher elevation, where climatic conditions were relatively severe (colder, drier, greater annual variation). In contrast, the diploid species was restricted to lower elevations, where climatic conditions were warmer, wetter and exhibited less annual variation. 4. Clusters of syntopic sites were associated with areas of high variation in annual temperature and precipitation during the breeding season. 5. Our data suggest that large-scale climatic conditions have played a role in the establishment of the polyploid H. versicolor in at least some portions of its range. The occurrence of the polyploid and absence of the progenitor in colder, drier and more varied environments suggests the polyploid may posses a tolerance of severe environmental conditions that is not possessed by the diploid progenitor. 6. Our findings support the hypothesis that increased tolerance to severe environmental conditions is a plausible mechanism of polyploid establishment.

Maryland, Virginia↗
USGS2008

A quantile count model of water depth constraints on Cape Sable seaside sparrows

1. A quantile regression model for counts of breeding Cape Sable seaside sparrows Ammodramus maritimus mirabilis (L.) as a function of water depth and previous year abundance was developed based on extensive surveys, 1992-2005, in the Florida Everglades. The quantile count model extends linear quantile regression methods to discrete response variables, providing a flexible alternative to discrete parametric distributional models, e.g. Poisson, negative binomial and their zero-inflated counterparts. 2. Estimates from our multiplicative model demonstrated that negative effects of increasing water depth in breeding habitat on sparrow numbers were dependent on recent occupation history. Upper 10th percentiles of counts (one to three sparrows) decreased with increasing water depth from 0 to 30 cm when sites were not occupied in previous years. However, upper 40th percentiles of counts (one to six sparrows) decreased with increasing water depth for sites occupied in previous years. 3. Greatest decreases (-50% to -83%) in upper quantiles of sparrow counts occurred as water depths increased from 0 to 15 cm when previous year counts were 1, but a small proportion of sites (5-10%) held at least one sparrow even as water depths increased to 20 or 30 cm. 4. A zero-inflated Poisson regression model provided estimates of conditional means that also decreased with increasing water depth but rates of change were lower and decreased with increasing previous year counts compared to the quantile count model. Quantiles computed for the zero-inflated Poisson model enhanced interpretation of this model but had greater lack-of-fit for water depths > 0 cm and previous year counts 1, conditions where the negative effect of water depths were readily apparent and fitted better with the quantile count model.

Journal of Animal Ecology↗
USGS2008

Fidelity and breeding probability related to population density and individual quality in black brent geese Branta bernicla nigricans

1. Patterns of temporary emigration (associated with non-breeding) are important components of variation in individual quality. Permanent emigration from the natal area has important implications for both individual fitness and local population dynamics. 2. We estimated both permanent and temporary emigration of black brent geese (Branta bernicla nigricans Lawrence) from the Tutakoke River colony, using observations of marked brent geese on breeding and wintering areas, and recoveries of ringed individuals by hunters. We used the likelihood developed by Lindberg, Kendall, Hines & Anderson 2001 (Combining band recovery data and Pollock's robust design to model temporary and permanent emigration. Biometrics, 57, 273-281) to assess hypotheses and estimate parameters. 3. Temporary emigration (the converse of breeding) varied among age classes up to age 5, and differed between individuals that bred in the previous years vs. those that did not. Consistent with the hypothesis of variation in individual quality, individuals with a higher probability of breeding in one year also had a higher probability of breeding the next year. 4. Natal fidelity of females ranged from 0.70 ?? 0.07-0.96 ?? 0.18 and averaged 0.83. In contrast to Lindberg et al. (1998), we did not detect a relationship between fidelity and local population density. Natal fidelity was negatively correlated with first-year survival, suggesting that competition among individuals of the same age for breeding territories influenced dispersal. Once females nested at the Tutakoke River, colony breeding fidelity was 1.0. 5. Our analyses show substantial variation in individual quality associated with fitness, which other analyses suggest is strongly influenced by early environment. Our analyses also suggest substantial interchange among breeding colonies of brent geese, as first shown by Lindberg et al. (1998).

Journal of Animal Ecology↗
USGS2008

Trade-offs of predation and foraging explain sexual segregation in African buffalo

1. Many studies have investigated why males and females segregate spatially in sexually dimorphic species. These studies have focused primarily on temperate zone ungulates in areas lacking intact predator communities, and few have directly assessed predation rates in different social environments. 2. Data on the movement, social affiliation, mortality and foraging of radio-collared African buffalo (Syncerus caffer) were collected from 2001-06 in the Kruger National Park, South Africa. 3. The vast majority of mortality events were due to lion (Panthera leo) predation, and the mortality hazard associated with being an adult male buffalo in a male-only 'bachelor' group was almost four times higher than for adult females in mixed herds. The mortality rates of adult males and females within mixed herds were not statistically different. Mortality sites of male and female buffalo were in areas of low visibility similar to those used by bachelor groups, while mixed herds tended to use more open habitats. 4. Males in bachelor groups ate similar or higher quality food (as indexed by percentage faecal nitrogen), and moved almost a third less distance per day compared with mixed herds. As a result, males in bachelor groups gained more body condition than did males in breeding herds. 5. Recent comparative analyses suggest the activity-budget hypothesis as a common underlying cause of social segregation. However, our intensive study, in an area with an intact predator community showed that male and female buffalo segregated by habitat and supported the predation-risk hypothesis. Male African buffalo appear to trade increased predation risk for additional energy gains in bachelor groups, which presumably leads to increased reproductive success. ?? 2008 The Authors.

Journal of Animal Ecology↗
USGS2008

Flower power: Tree flowering phenology as a settlement cue for migrating birds

1. Neotropical migrant birds show a clear preference for stopover habitats with ample food supplies; yet, the proximate cues underlying these decisions remain unclear. 2. For insectivorous migrants, cues associated with vegetative phenology (e.g. flowering, leaf flush, and leaf loss) may reliably predict the availability of herbivorous arthropods. Here we examined whether migrants use the phenology of five tree species to choose stopover locations, and whether phenology accurately predicts food availability. 3. Using a combination of experimental and observational evidence, we show migrant populations closely track tree phenology, particularly the flowering phenology of honey mesquite (Prosopis glandulosa), and preferentially forage in trees with more flowers. Furthermore, the flowering phenology of honey mesquite reliably predicts overall arthropod abundance as well as the arthropods preferred by migrants for food. 4. Together, these results suggest that honey mesquite flowering phenology is an important cue used by migrants to assess food availability quickly and reliably, while in transit during spring migration. ?? 2008 The Authors.

Journal of Animal Ecology↗