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The Hakalau Forest National Wildlife Refuge was established to protect native Hawaiian forest birds, particularly endangered species. Management for forest restoration on the refuge has consisted mainly of removing feral ungulates, controlling invasive alien plants, and reforesting former pastures. To assess effects of this habitat improvement for forest birds, we estimated density annually by distance sampling and examined population trends for native and alien passerines over the 21 years since the refuge was established. We examined long-term trends and recent short-term trajectories in three study areas: (1) reforested pastureland, (2) heavily grazed open forest that was recovering, and (3) lightly grazed closed forest that was relatively intact. Three species of native birds and two species of alien birds had colonized the reforested pasture and were increasing. In the open forest, densities of all eight native species were either stable or increasing. Long-term trends for alien birds were also generally stable or increasing. Worryingly, however, during the most recent 9 years, in the open forest trajectories of native species were decreasing or inconclusive, but in the reforested pasture they generally increased. The closed forest was surveyed in only the most recent 9 years, and trajectories of native species there were mixed. Overall, long-term population trends in Hakalau are stable or increasing, contrasting with declines in most other areas of Hawai'i over the same period. However, more recent mixed results may indicate emergent problems for this important bird area.
Surveys are essential for conservation of populations, because they provide estimates of density that may reveal a change in status. Managers therefore depend on analysis of density estimates to evaluate effectiveness of their actions. However, statistical problems associated with trend analysis make declines caused by environmental change difficult to detect. Here we apply piecewise regression to analysis of trends in eight species of Hawaiian passerines to determine if slopes following a year of documented change differ from slopes estimated before the change. Using estimates of densities of forest birds at Hakalau Forest National Wildlife Refuge, we show that estimates of trends based on 21 years of survey data and expressed as a single slope missed an environmental change in year 14. Analysis of residuals indicates that analysis of trends as a single slope violated the model's assumptions, giving a false impression that populations were either stable or increasing and at variance with independent indicators of population decline based on banded birds. In contrast, piecewise regression shows that the slopes of all eight species were positive before the change and negative after it and meets the model's assumptions. The Hawai'i ‘Ākepa (Loxops coccineus coccineus) and the ‘I’iwi (Vestiaria coccinea) are declining significantly, with several other species likely declining. On the basis of the structure of the data, the positive slopes were anomalous. We suggest that piecewise regression may be the more appropriate technique for dealing with environmental changes toward the end of long-term data sets.
The use of social information has recently been documented in a wide variety of animals, with potentially diverse consequences for those living in heterogeneous landscapes. Here we review and synthesize investigations on the use of social information in heterogeneous landscapes and provide a conceptual framework for interpreting the role social information plays for birds living in human-modified landscapes. Our framework emphasizes the spatial components of the availability and value of social information and how landscape change can alter the availability and perceived value of social information to individuals. As an example of the utility of an information-based perspective, we discuss investigations of habitat use by the Bobolink (Dolichonyx oryzivorus) in agricultural landscapes. Overall, work to date provides unique insight into the importance of social-information use at large scales but also underscores that much uncertainty remains regarding the role of social information in driving distributions and dynamics across landscapes.
Correlations between habitat measures and animal distributions are not always applicable outside the study area that generated them. In such cases, the particularities of these correlations likely arise because only use of the local habitat has been quantified, rather than actual habitat selection, as the distribution models do not account for the behavior of animals in choice. The addition of covariates accounting for selection strategies could improve the precision and accuracy of correlative models of habitat use, but this conjecture has received little empirical attention. To evaluate this possibility, we re-assess previously developed habitat-use models for abundance of males of three grassland birds by explicitly including two measures of selection behavior: the “propensity to aggregate” and “propensity to use social information.” Habitat-use models for Nelson's Sharp-tailed Sparrow (Ammodramus nelsoni) were not improved by either behavioral variable. However, models for two other species, the Bobolink (Dolichonyx oryzivorus) and Savannah Sparrow (Passerculus sandwichensis), improved substantially through reduced prediction error (assessed with cross-validation) and were much more likely to be an appropriate model (by reducing the deviance of the fitted models). These results indicate that habitat-selection models can be an improvement over correlative habitat-use models. In our case, these improvements were limited to two species in which individuals use their conspecifics as cues of local habitat quality. However, numerous other measures of selection behavior can be included to improve upon certain habitat-use models, particularly when those models depart unexplainedly from optimality theory.
Wetlands are dynamic and can be destroyed and created quickly by natural forces. Therefore, birds inhabiting these wetlands may need to locate new suitable sites quickly. We investigated the cues wetland birds use when selecting a breeding site. Many species may use both habitat (e.g., vegetation structure) and social cues (presence of conspecifics and/or heterospecifics) when selecting a location for breeding. Using a two-species occupancy-modeling approach, we found certain wetland birds more likely to occur with other species, suggesting the presence of heterospecifics may influence settlement. Species that preferred wetlands with a roughly 50:50 ratio of open water to vegetation (hemi-marsh) occurred more frequently than expected with the Pied-billed Grebe (Podilymbus podiceps), suggesting these species may use the presence of grebes when selecting a habitat. Conversely, Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) do not appear to use the presence of other species when selecting a breeding location. Previous research supports this finding, in that the number of young produced at a site the previous year (patch reproductive success) was important in how this species selected its breeding sites. Because settling on the basis of patch reproductive success requires occupancy or visiting a site the previous year, individuals new to a population must use other cues such as habitat. In this population inexperienced Yellow-headed Blackbirds were more prone than experienced individuals to colonize recently created wetlands. Several wetland species we investigated used social cues to select breeding sites, and this behavior may help explain the occurrence and distribution of wetland birds.
Numerous studies have confirmed that when selecting habitat birds can use social information acquired from observing other individuals, and many aspects of this social information can be capitalized upon to manage bird populations. The conservation implications of attraction to conspecifics are especially promising for management, and as research progresses it is important to consider how this behavior can be applied to conservation practice. The biological underpinnings of conspecific attraction and the repercussions of manipulating species' distributions with attraction methods are not well understood, but conservation decisions often cannot wait for scientific research. Here we synthesize the current research on manipulation of songbirds by conspecific-attraction methods and review our knowledge gaps critically. We reviewed the published literature on conspecific-attraction experiments in songbirds and found that of 24 studies in which they were attempted, 20 were successful in attracting birds. Although many experiments have been successful in attracting conspecifics with various cues, we outline issues to be considered before songbirds are manipulated by attraction methods, and we highlight areas of research necessary to enhance the understanding of conspecific attraction and its use in conservation.
Birds may colonize new habitats because of introduction, changing environmental conditions, and/or altered social or environmental cues. However, aside from introduced (often invasive) species, little is known about the consequences of such colonizations for members of existing communities. If the realized niche is influenced by the presence or absence of heterospecific competitors, then addition of a species to a novel habitat or location could result in extirpation or avoidance if members of the existing community are subdominant. Alternatively, if for some species heterospecific cues are the primary means for collecting information about a site's quality, heterospecific attraction could occur. To test these predictions, we experimentally induced free-living Black-throated Blue Warblers (Dendroica caerulescens) to colonize a novel environment within their existing range. We used dynamic occupancy modeling to test for the dynamics of colonization and local extinction as a function of our experimental treatment. We found strong evidence for dynamic occupancy by birds during the breeding season; colonizations and extirpations were common. Although dynamics were not generally well explained by our experimental introduction of Black-throated Blue Warblers, we found some support for the heterospecific-avoidance hypothesis; three of the four species we examined that prefer early seral forests tended to abandon a site once Black-throated Blue Warblers occupied it. We suggest that heterospecific interactions should be considered when species' distributions are projected in relation to climate change. Our results provide a caution that managers broadcasting a species' song to increase its abundance should consider the technique's effects on the broader community.
To evaluate factors influencing a suburban bird community in coastal northern California, we examined the abundance, richness, and diversity of wintering birds at 75 locations within the city of Arcata. For each location we used aerial imagery to determine the proportions of vegetation and impervious surfaces within 75 m and the distances to various landscape features. Human activity was measured and used as a covariate in model selection. Total bird abundance, species richness, and diversity declined with the proportion of road surface at a site, while the abundance of non-native species increased with the cover of roads and structures. The proportions of shrub and tree cover at a site predicted total abundance and richness positively. The effect of trees on species diversity was increasingly positive in more urban areas. Several species, such as Steller's Jay (Cyanocitta stelleri), Ruby-crowned Kinglet (Regulus calendula), Wrentit (Chamaea fasciata), Winter Wren (Troglodytes troglodytes), Yellow-rumped Warbler (Dendroica coronata), and Red-breasted Nuthatch (Sitta canadensis), were positively associated with tall vegetation and negatively associated with road and structure cover. The White-tailed Kite (Elanus leucurus) was positively correlated with grass and trees and negatively correlated with road and structure cover. These species may be the most vulnerable to the habitat changes associated with urbanization in this study area. We recommend maintaining/restoring native shrubs and or trees to enhance bird communities in developed areas or to improve planning where development is inevitable. Increasing shrub and tree cover may be especially valuable in newer neighborhoods and far from forest edges.
The Neotropical Region has experienced large habitat transformations as a result of intensified agriculture. These changes have affected the populations of many species of birds in the Espinal ecoregion of Argentina. However, relationships between birds and agriculture in the neotropics are poorly known. We assessed the effects of crops and field margins in an area of agriculture typical for the Espinal ecoregion on the structure of the bird community, and we assessed the value of this habitat for species of conservation concern. Birds and vegetation were sampled in and along the margins of fields of soybean (the most widespread crop) and alfalfa. Twenty-five of the 41 species recorded—including all the species of conservation concern—were found almost exclusively in field margins; only five species occurred almost exclusively in within the fields. All other species were found in both margins and fields. Density, richness, and diversity were much greater on the margins than in the fields. Bird density in soy fields was lower than in alfalfa. Bird richness and diversity in the two crops, however, did not differ. Density in field margins increased with the cover and height of trees and shrubs. Most of the field-margin species were woodland-border species, but the species of conservation concern were all associated with grassy field margins. Because further declines in the populations of these threatened species are expected, their conservation in agricultural areas would benefit from a broad policy of habitat-conservation plans for field margins such as those practiced in North America and Europe.
When a communal roost is large relative to foraging distances, variance in foraging success may affect the positioning of the birds within the roost and we should expect fidelity to positions that improve foraging success. We explored fidelity of Snow Geese (Chen caerulescens) to three sections of a 5-km2 roost in flooded lowlands during their spring stopover in Quebec. From 1998 to 2000, we located 166 radio-tagged geese on 1077 occasions. Fidelity rates were higher than expected by chance in all sections in 1998, in two in 2000, but in none in 1999. Fidelity increased with the number of birds using a section, suggesting a positive effect of conspecific attraction. We tracked 292 foraging trips of 108 radio-tagged geese; birds from different sections tended to forage in specific directions. Average distance to foraging sites saved by appropriate choice of a section varied between 7 and 17%, depending on the section. However, distance traveled over 2 successive days did not decrease when geese switched from roosting in one section to another, suggesting that minimization of foraging-trip distance may stem simply from the spatial organization of foraging trips in order to reduce travel distance to food patches. Higher fidelity rates were associated with shorter travel distance in only one section of the roost, and dominant birds arriving early in the season tended to be more faithful to this section. We conclude that conspecific attraction, reduction in travel costs to foraging sites, and individual variation in dominance determine roost positioning and fidelity concurrently.
Waterfowl lay large clutches of eggs over many days, yet the offspring hatch synchronously, indicating regulatory mechanisms must aid in minimizing developmental differences among offspring. Understanding how embryos' metabolic rates vary with the sequence in which the egg is laid can provide insight into intrinsic mechanisms regulating the synchrony of hatching. Furthermore, developmental rates differing among offspring within a clutch likely have post-hatching consequences for offspring performance. We characterized variation in developmental rates within a clutch and the consequences for offspring performance in the Canada Goose (Branta canadensis maxima). We measured embryonic metabolic rates every 2 to 3 days, examined goslings' yolk reserves immediately after hatching, and conducted a cross-fostering experiment to assess plasticity in hatching synchrony and gosling survival in the 2 weeks after hatching. We found that embryonic O2 consumption rates increased with age in an S-shaped pattern, as in other birds with precocial young. Embryos in eggs laid later in the sequence had higher metabolic rates than those in eggs laid earlier in the sequence. Yolk reserves at hatching were also lower in goslings hatched from eggs laid later in the sequence. We altered the incubation period by 1–2 days, with no effect on gosling survival in the 2 weeks after hatching. Embryos in eggs laid later consume O2 at a higher rate and develop faster, reducing yolk reserves available at hatching. Reduced yolk reserves did not have immediate consequences for gosling survival. We hypothesize that maternal influences regulate the synchrony of hatching.
Implanted transmitters have become an important tool for studying the ecology of sea ducks, but their effects remain largely undocumented. To address this, we assessed how abdominally implanted transmitters with percutaneous antennas affect the vertical dive speeds, stroke frequencies, bottom time, and dive duration of captive Common Eiders (Somateria mollissima). To establish baselines, we recorded video of six birds diving 4.9 m prior to surgery, implanted them with 38- to 47-g platform transmitter terminals, and then recorded their diving for 3.5 months after surgery to determine effects. Descent speeds were 16–25% slower and ascent speeds were 17–44% slower after surgery, and both remained below baseline at the end of the study. Dive durations were longer than baseline until day 22. On most days between 15 and 107 days after surgery, foot-stroke frequencies of birds foraging on the bottom were slower. Foot- and wing-stroke frequencies during descent and bottom time did not differ across the time series. If birds that rely on benthic invertebrates for sustenance dive slower and stay submerged longer after being implanted with a satellite transmitter, their foraging energetics may be affected. Researchers considering use of implanted transmitters with percutaneous antennas should be mindful of these effects and the possibility of concomitant alterations in diving behavior, foraging success, and migratory behavior compared to those of unmarked conspecifics.
Age- and sex-specific survival and dispersal are important components in the dynamics and genetic structure of bird populations. For many avian taxa survival rates at the adult and juvenile life stages differ, but in long-lived species juveniles' survival is logistically challenging to study. We present the first estimates of hatch-year annual survival rates for a sea duck, the King Eider (Somateria spectabilis), estimated from satellite telemetry. From 2006 to 2008 we equipped pre-fiedging King Eiders with satellite transmitters on breeding grounds in Alaska and estimated annual survival rates during their first 2 years of life with known-fate models. We compared those estimates to survival rates of adults marked in the same area from 2002 to 2008. Hatch-year survival varied by season during the first year of life, and model-averaged annual survival rate was 0.67 (95% CI: 0.48–0.80). We did not record any mortality during the second year and were therefore unable to estimate second-year survival rate. Adults' survival rate was constant through the year (0.94, 95% CI: 0.86–0.97). No birds appeared to breed during their second summer. While 88% of females with an active transmitter (n = 9) returned to their natal area at the age of 2 years, none of the 2-year old males (n = 3) did. This pattern indicates that females' natal philopatry is high and suggests that males' higher rates of dispersal may account for sex-specific differences in apparent survival rates of juvenile sea ducks when estimated with mark—recapture methods.
Birds can hide from visual predators by locating nests where there is cover and from olfactory predators where habitat features create updrafts, high winds, and atmospheric turbulence, but sites optimal for hiding from visual and olfactory predators often differ. We examined how Greater Sage-Grouse (Centrocercus urophasianus) balance the dual needs of hiding from both visual and olfactory predators on Parker Mountain, Utah, where the Common Raven (Corvus corax) is the main visual predator and the striped skunk (Mephitis mephitis) and American badger (Taxidea taxus) are the main olfactory predators. By comparing nest sites to random sites during 2005 and 2006, we found that sage-grouse nest at sites where their nests were obscured from visual predators but were exposed to olfactory predators. To validate these findings, we replicated the study in southwest Wyoming during 2008. Again, we found that visual obscurity at nest sites was greater than at control sites but olfactory obscurity was less. Our results indicate that Greater Sage-Grouse select nest sites where they will be concealed from visual predators but at the cost of locating nests where they are exposed to olfactory predators. In southwest Wyoming, we found that olfactory predators (mammals) and visual predators (birds) depredated an equal number of nests. By selecting nest sites with visual obscurity, Greater Sage-Grouse have reduced the threat from visual predators to where it was similar to the threat posed by olfactory predators.
We wish to use stable-isotope analysis of flight feathers to understand the feeding behavior of pelagic seabirds, such as the Hawaiian Petrel (Pterodroma sandwichensis) and Newell's Shearwater (Puffinus auricularis newelli). Analysis of remiges is particularly informative because the sequence and timing of remex molt are often known. The initial step, reported here, is to obtain accurate isotope values from whole remiges by means of a minimally invasive protocol appropriate for live birds or museum specimens. The high variability observed in δ13C and δ15N values within a feather precludes the use of a small section of vane. We found the average range within 42 Hawaiian Petrel remiges to be 1.3‰ for both δ13C and δ15N and that within 10 Newell's Shearwater remiges to be 1.3‰ and 0.7‰ for δ13C and δ15N, respectively. The δ13C of all 52 feathers increased from tip to base, and the majority of Hawaiian Petrel feathers showed an analogous trend in δ15N. Although the average range of δD in 21 Hawaiian Petrel remiges was 11‰, we found no longitudinal trend. We discuss influences of trophic level, foraging location, metabolism, and pigmentation on isotope values and compare three methods of obtaining isotope averages of whole feathers. Our novel barb-sampling protocol requires only 1.0 mg of feather and minimal preparation time. Because it leaves the feather nearly intact, this protocol will likely facilitate obtaining isotope values from remiges of live birds and museum specimens. As a consequence, it will help expand the understanding of historical trends in foraging behavior.
Few studies at a broad geographical scale have characterized intraspecific variation in morphology of woodland hawks in the genus Accipiter. From 1999 to 2007 we investigated morphological variation in large samples of live Cooper's Hawks (A. cooperii) nesting in four study areas: coniferous woodland around Victoria, British Columbia, Canada, isolated deciduous woodlands in short-grass prairies of northwestern North Dakota, towns and rural deciduous woodlands along the border of North Dakota and Minnesota, and urban and rural mixed deciduous and coniferous landscapes of Wisconsin. These sites span 2660 km across the northern part of the species' breeding range. We measured body mass (i.e., size), wing chord, tail length, tarsus diameter, hallux length, and culmen length of breeding adults, finding significant and clinal variation in body mass (or size). The smallest and most similar-sized birds occurred in British Columbia and western North Dakota, larger birds along the border between North Dakota and Minnesota, and the largest birds in Wisconsin. Several other characters varied significantly when mass was used as a covariate. Variation by study site in mean indices of sexual size dimorphism was negligible and not significant. We speculate that the morphological differences we found, in part, are the result of geographic isolation, where diets, migratory behavior, and structural characteristics of nesting habitats vary across landscape types.
To document nest survival and habitat differences in the nesting habitats of Clapper Rails (Rallus longirostis) in tidal marshes of the northern Gulf of Mexico, we monitored 76 active nests within the Pascagoula River Marsh Coastal Preserve (a freshwater-dominated estuary) and the Grand Bay National Estuarine Research Reserve (a marine-influenced estuary) in coastal Mississippi from 2005 to 2007. During 2006, we measured the height of each Clapper Rail nest, sampled vegetation at active Clapper Rail nests and at random locations, and measured the distance to the nearest tidally influenced body of water and the average height and density of vegetation. Early in the breeding season, the average nest height was lower at the Pascagoula (36 cm) than at Grand Bay (60 cm), but, as the season progressed, nest height increased at the Pascagoula only. Within both estuaries, Clapper Rail nest sites were more structurally complex than at random locations and were associated with a greater diversity of vegetation. Overall, daily survival rates of Clapper Rail nests were relatively high (0.97–0.99), with the majority of nest loss apparently the result of tidal flooding. Our results suggest that where diverse habitat was available, Clapper Rails varied the height of their nests as a mechanism to avoid nest loss from tidal flooding. Habitat alteration from factors such as sea-level rise and coastal development may lead to lower nest success because of a loss of diverse nesting habitat.
While there is growing awareness in ecology of spatial dependency, the application in avian ecology of spatially explicit statistical methods is rare in areas such as habitat—reproduction relationships. We compared nonspatial vs. spatially explicit tests of correlation between a measure of reproduction and a habitat attribute associated with Northern Flicker (Colaptes auratus) nests. While nonspatial tests showed a significant positive correlation between nest productivity and canopy cover of conifers, Dutilleul's modified t-test, which reduces the effective sample size by accounting for dependence among sampled nest trees, showed no significant correlation. These results may be due to spatial autocorrelation of both canopy cover of conifers and nest productivity at the scale of <1 km. Low localized nest productivity appears to have been driven by nest predation rather than by conifer cover. Our results illustrate the utility of this simple modification for examining ecological correlations in the presence of spatial structure. We encourage further use of this and other spatially explicit statistical tests in avian ecology. Failing to do so risks potentially spurious and overstated conclusions, as we demonstrate here.
Survival is a critical life-history trait, and among cooperative breeders survival may be linked to the evolution of social organization. We used multi-state models in the program MARK to estimate apparent survival in the Splendid Fairy-wren (Malurus splendens), a cooperatively breeding species in which most pairs are assisted by male offspring from previous generations. We examined survival as it relates to sex, age, and social status (nestling, auxiliary, breeder), and quantified the probabilities of transition between social states. The best-supported model was one in which survival rates differed by social state, survival of auxiliaries and breeders varied annually in the same manner, and the effect of sex varied annually but influenced the survival rate of each group in the same manner. In both males and females overall survival estimates of auxiliaries were similar to those of breeders, whereas survival estimates of adult males were higher than those of females, although the effect of sex varied annually. The probability of transition between categories of social status varied in a manner expected for a cooperatively breeding species: nestling males were more likely than nestling females to become auxiliaries, whereas females were more likely to become breeders in the subsequent year. Similarly, among auxiliaries, females were more likely than males to become breeders. Survival of males being higher that of females likely contributes to the male-biased sex ratio observed in adults of this species and, indirectly, the propensity of younger males to delay dispersal.
The choices animals make in dispersal are of interest because they describe in part how populations adjust to a changing environment. We investigated which factors influence whether adult Florida Scrub-Jays delay breeding within their natal territories or disperse to breed. Factors considered included those pertaining to individuals (sex, age, parents), breeding opportunities (territory densities, deaths of breeders, potential mates, competitors [nonbreeders of the same sex]), and habitat (quality, suburb versus conservation area). We compared models that included each of these factors or combinations of them. We used nomograms to evaluate effect sizes and post hoc analyses to explore nonlinear effects of variables and their relative importance. Models that included only one or a few variables had little support. Models with the greatest support included parents, deaths of breeders, age, sex, and potential mates. Increased deaths of breeders greatly decreased the probabilities of delayed dispersal. Dispersal was most delayed if both parents or just the father remained breeders. One-year-olds and males delayed dispersal more than older nonbreeders and females. Post hoc analyses revealed that the probability of delayed breeding was lowest at intermediate densities of potential mates, suggesting that at low densities Florida Scrub-Jays delay breeding because they can't find mates and at high densities because of high competition to fill vacancies left by breeders' deaths. Our results are important because they suggest that no single factor explains delayed breeding adequately and that factors important for population recovery and minimizing extinction risk are also important for maintaining cooperative breeding behavior.
Because investment in eggs is costly for birds, a female's body condition is expected to influence that investment. Moreover, resource availability can fluctuate by year, and older and younger females may acquire or allocate resources differently. Additionally, in multi-brooded species, females may make tradeoffs in investment in first and second clutches. Because environmental conditions often change through the breeding season and early maturity of second broods can increase females' fitness, patterns of females' allocation to each clutch may change with the season. Here, we investigate associations between females' quality (body condition and age) and pre-hatching investment (clutch size, average egg mass per clutch, and total clutch mass) in adults of the Eastern Bluebird (Sialia sialis), a multi-brooded cavity-nesting passerine. In the two years of the study, relationships differed; in 2003, females that were heavier relative to their body size produced larger eggs and invested in greater overall clutch mass, and younger females produced larger clutches. In 2004, females whose first clutches were greater in overall mass laid second clutches proportionately smaller in overall clutch mass. Together, these data suggest that overall clutch mass may represent a significant cost to female bluebirds. We also found variation in investment in eggs within the season; second clutches tended to be smaller than first clutches but tended to contain larger eggs. By increasing resources per offspring late in the breeding season, females may increase the survival or condition of late-season offspring.
In response to brood parasitism by the Brown-headed Cowbird (Molothrus ater), some female Yellow Warblers (Dendroicapetechia) bury cowbird eggs and sometimes their own eggs, whereas other females desert parasitized nests and renest at new sites. We identified circumstances that elicit burial or desertion by analyzing the histories of 132 naturally parasitized nests inspected over 13 breeding seasons in Manitoba. Damaged nests and clutches reduced to zero, one, or two host eggs were deserted, whereas the clutch was buried when zero, one, or two host eggs were present the morning cowbirds laid and the probability of hatching was high. Response times for burial (2.3 ± 0.1 [SE] days) and desertion (2.5 ± 0.3 days) were similar, but the variance differed significantly (1.29 days2 for burial versus 2.58 days2 for desertion). Burial is the Yellow Warbler's more frequent method of rejection, though desertion is used about one-third of the time, and it may be elicited by factors unrelated to brood parasitism, such as interference by predators and inclement weather.
Brood-parasitic cowbirds (Molothrus spp.) can cause total nest failure directly by inducing nest desertion or by destroying the host's clutch or indirectly by facilitating nest predation. We examined the relationship between brood parasitism and nest survival in the Bay-winged Cowbird (Agelaioides badius), the primary host of the Screaming Cowbird (M. rufoaxillaris) and a secondary host of the Shiny Cowbird (M. bonariensis). We used the program MARK to model daily nest-survival rates, including hypothesized effects of intensity of parasitism, egg losses caused by cowbirds, and total clutch size. Support for each model was evaluated by an information-theoretic approach. More than 50% of the nests failed before incubation was completed, mainly because of the ejection or desertion of parasitized clutches. The model of daily nest survival with best support included the additive effects of intensity of parasitism and number of eggs lost, which were negatively related to nest survival. The model including the effect of clutch size did not receive support. The predicted probability of a nest surviving the entire nesting cycle was 35% for unparasitized nests without egg loss, whereas under the levels of parasitism observed during this study the probability of nest survival varied between 0 and 32%. Nest predation during the egg and nestling stages was positively related to the number of cowbird eggs and chicks, respectively, suggesting that parasitism by Screaming and Shiny Cowbirds may also facilitate depredation of Bay-winged Cowbird nests.