Kirtland's Warblers Research Articles

Non-breeding conditions induce carry-over effects on survival of migratory birds

Identifying the processes that limit populations is a foundational objective of ecology and an urgent need forconservation. For migratory animals, researchers must study individuals throughout their annual cycles to determine how environmental conditions limit demographic rates within each period of the annual cycleand also between periods through carry-over effects and seasonal interactions.1,2,3,4,5,6 Our poor understanding of the rates and causes of avian migration mortality7 hinders the identification of limiting factors and the reversal of widespread avian population declines.8,9 Here, we implement new methods to estimate apparent survival (hereafter survival) during migration directly from automated telemetry data10 in Kirtland's Warblers (Setophaga kirtlandii) and indirectly from mark-recapture data in Black-throated Blue Warblers (S.caerulescens). Previous experimental and observational studies of our focal species and other migratory songbirds have shown strong effects of Caribbean precipitation and habitat quality on food availability,11,12,13,14 body condition,12,13,14,15,16,17,18,19 migration timing,11,12,15,16,20,21,22,23 natal dispersal,24,25 range dynamics,26 reproductive success,20,22,27 and annual survival.18,19,20,23,28,29,30,31 Building on this research, we test the hypotheses that environmental conditions during the non-breeding period affect subsequent survival during spring migration and breeding. We found that reduced precipitation and environmental productivity in the non-breeding period strongly influenced survival in both species, primarily by reducing survival during spring migration. Our results indicate that climate-driven environmental conditions can carry over to affect survival in subsequent periods and thus likely play an important role in year-round population dynamics. These lethal carry-over effects may be widespread and are likely magnified by intensifying climate change.

Comparative detection, density, and reproductive performance of Kirtland's warbler in jack and red pine

AbstractThe formerly endangered Kirtland's warbler (Setophaga kirtlandii) is among a growing number of conservation‐reliant species that depend on active management to avoid reverting to endangered status. Because the Kirtland's warbler is a habitat specialist of young, even‐aged jack pine (Pinus banksiana), managers of the recovery effort stressed creating new jack pine stands and monitoring numbers of singing males through an annual census using single visits to individual stands. Kirtland's warbler will occupy and breed in red pine (P. resinosa), but red pine has not been surveyed for Kirtland's warblers in the annual population census. Furthermore, the current monitoring approach cannot determine their species detection probability or individual detection probability, which is essential to evaluate both red pine use and the accuracy of the census. From 2016–2018 we estimated density and detection probabilities in jack pine and red pine stands through repeated visits to a limited number of stands rather than single visits to many stands. Estimates of species detection probability indicated that ≥1 male Kirtland's warbler would be detected on most sites when any were present, but individual detection probabilities were less and varied by stand type, indicating that single visits to sites would underestimate numbers and that accurate estimation of detection probability was important for estimation of density in different stand types. We offer quantitative estimates of detection probabilities for determination of Kirtland's warbler population size in jack pine versus red pine stands in the same areas and breeding seasons. Managers of Kirtland's warblers should incorporate detection probabilities into population surveys to achieve more accurate estimates of population size.

Repeated sampling of individuals reveals impact of tropical and temperate habitats on microbiota of a migratory bird.

Migratory animals experiencing substantial change in diet and habitat across the annual cycle may have corresponding shifts in host-associated microbial diversity. Using automated telemetry and radio tags to recapture birds, we examined gut microbiota structure in the same population and often same individual of Kirtland's Warblers (Setophaga kirtlandii) initially sampled on their wintering grounds in The Bahamas and subsequently resampled within their breeding territories in Michigan, USA. Initial sampling occurred in March and April and resampling occurred in May, June and early July. The composition of the most abundant phyla and classes of the warblers' microbiota is similar to that of other migratory birds. However, we detected notable variation in abundance and diversity of numerous bacterial taxa, including a decrease in microbial richness and significant differences in microbial communities when comparing the microbiota of birds first captured in The Bahamas to that of birds recaptured in Michigan. This is observed at the individual and population level. Furthermore, we found that 22 bacterial genera exhibit heightened abundance within specific sampling periods and are probably associated with diet and environmental change. Finally, we described a small, species-specific shared microbial profile that spans multiple time periods and environments within the migratory cycle. Our research highlights that the avian gut microbiota is dynamic over time, most significantly impacted by changing environments associated with migration. These results support the need for full annual cycle monitoring of migratory bird microbiota to improve understanding of seasonal host movement ecologies and response to recurrent physiological stressors.

Hidden Long-Distance Movements by a Migratory Bird

Technology has revolutionized our ability to track animals across the globe, significantly advancing our understanding of animal movement [1, 2]. Technological and logistical challenges, however, have led to non-migratory movements that fall outside of the territory/home range paradigm, receiving less attention. This may have resulted in a widespread underestimation of the frequency and spatial scale at which animals either move outside of their territories and home ranges or adopt altogether different space-use strategies. We used a breeding-range-wide automated radio-telemetry system to track movements in a migratory songbird, the Kirtland's warbler (Setophaga kirtlandii). By attaching radio tags on the wintering grounds and relocating the same individuals on the breeding grounds, we were able to sample the population without regard to their eventual breeding status or space-use strategy. We found that a surprising proportion of breeders and most non-breeders made long-distance (5-77km) movements during the breeding season while conspecifics remained within their small territories. Movement frequency peaked during the nestling and fledgling periods, indicating that both breeders and non-breeders were likely prospecting to inform dispersal. A literature review revealed that Kirtland's warblers moved farther than most species in absolute distances and farther than all other species relative to normal daily movements. We argue that similarly long-distance movements likely exist in many other species but have gone undetected because of technological limitations, research biases, and logistical challenges. Underestimation of the scale of these poorly understood life history behaviors has important implications for the ecology, evolution, and conservation of animals. VIDEO ABSTRACT.

Florida's Strategic Position for Collaborative Automated Telemetry Tracking of Avian Movements Across the Americas

Florida's Strategic Position for Collaborative Automated Telemetry Tracking of Avian Movements Across the Americas

Response of early-successional Bahamian dry forest habitat to goat grazing and implications for Kirtland's warbler (Setophaga kirtlandii) wintering ground management

Limited financial resources coupled with competing social demands may require novel approaches for biodiversity conservation. Within the Bahamas archipelago, subtropical dry forest (“coppice”) provides habitat for many resident and migratory bird species including the U.S. federally endangered Kirtland's warbler (Setophaga kirtlandii). On the islands, this migratory bird relies heavily on fruits of shrubs in early-successional coppice. Evidence indicates shrub species of particular importance in the bird's diet may not be generally widespread, while attempts to actively manage for these plant species could be cost-prohibitive or counterproductive to other conservation concerns. Because these shrubs and Kirtland's warblers have been frequently found on goat farms we investigated whether periodic goat grazing might be a practical tool for maintaining suitable coppice habitat in other semi-natural areas where late-successional coppice is unsupportable (e.g. utility rights-of-way) and whether it promotes increased abundance of important fruiting shrubs. In experiments conducted in young coppice within a pipeline corridor, we found little evidence that grazing would increase abundance of these shrubs, but our grazed coppice plots returned quickly to their pre-grazing state and exhibited few subsequent structural or compositional differences from ungrazed coppice plots. Thus, if appropriately applied and managed, goat grazing may serve as an economically viable tool for biodiversity preservation because (1) suitability as habitat for Kirtland's warblers and many other birds returns quickly after a short-term grazing treatment and (2) periodic, short-term grazing will forestall succession toward a vegetation type less appropriate for these birds or for certain types of human land use.

A probabilistic risk assessment for the Kirtland's warbler potentially exposed to chlorpyrifos and malathion during the breeding season and migration.

Two organophosphate pesticides, chlorpyrifos and malathion, are currently undergoing reregistration in the United States and were recently used by the US Environmental Protection Agency (USEPA) as case studies to develop a national procedure for evaluating risks to endangered species. One of the endangered bird species considered by the USEPA was the Kirtland's warbler (Setophaga kirtlandii). The Kirtland's warbler is an endangered migratory species that nests exclusively in young jack pine stands in Michigan and Wisconsin, and winters in the Bahamas. We developed probabilistic models to assess the risks of chlorpyrifos and malathion to Kirtland's warblers during the breeding season and the spring and fall migrations. The breeding area model simulates acute and chronic exposure and risk to each of 10 000 birds over a 60-d period following initial pesticide application. The model is highly species specific with regard to the foraging behavior of Kirtland's warblers during the breeding season. We simulated the maximum application rate and number of applications allowed on the labels for representative use patterns that could be found within 3 km of the breeding areas of Kirtland's warbler. The migration model simulates 10 000 birds during the course of their 12- to 23-d migration between their breeding area and the Bahamas. The model takes advantage of more than a century of observations of when, where, and for how long Kirtland's warblers forage in different habitats during the course of their migration. The data indicate that warblers only infrequently stop over in habitats that could be treated with chlorpyrifos and malathion. The breeding area and migration models resulted in predictions of very low acute and chronic risk for both pesticides to Kirtland's warblers. These results were expected, given that field observations indicate that the Kirtland's warbler has dramatically increased in abundance in recent decades. Integr Environ Assess Manag 2018;14:252-269. © 2017 SETAC.

Using audio playback to expand the geographic breeding range of an endangered species

AbstractAimSpecies conservation typically involves working within the existing geographic range of that species. However, with habitat loss and climate change, conservationists may need to extend their work beyond the current range of a species. We attempted to extend the geographic breeding range of a rare species, Kirtland's warbler (Setophaga kirtlandii), by 225 km from the closest known breeding pairs and 550 km from its core population.LocationWisconsin, USA.MethodsWe broadcast conspecific song playbacks at two sites in landscapes that were far from source populations but had a history of intermittent occupancy. We also estimated the potential effect that additional use of this technique in Wisconsin could have on the global Kirtland's warbler population.ResultsThe sites were initially unoccupied, but over the course of three years, playback attracted individuals to sites where they successfully reproduced. In the final year of the study, at least seven males and four females occupied the sites, producing 15 young. Based on the amount of appropriate habitat (5‐ to 15‐year‐old jack [Pinus banksiana] and red pine [P. resinosa]) in Wisconsin, we estimated that a population of approximately 250 pairs of Kirtland's warblers (10% of the global population) could eventually be supported.Main conclusionAlthough conspecific playback has often been reported to work over short distances, our success here demonstrates the potential for effectiveness over much greater distances, even in species with a small global population. This highlights that conspecific playback can be a powerful tool to assist recovery of endangered species, particularly in migratory species with a restricted distribution and high dispersal capabilities. As climate changes and humans alter the landscape, it may become crucial to attempt to establish populations beyond a species' current geographic range.

Seasonal survival estimation for a long-distance migratory bird and the influence of winter precipitation.

Conservation of migratory animals requires information about seasonal survival rates. Identifying factors that limit populations, and the portions of the annual cycle in which they occur, are critical for recognizing and reducing potential threats. However, such data are lacking for virtually all migratory taxa. We investigated patterns and environmental correlates of annual, oversummer, overwinter, and migratory survival for adult male Kirtland's warblers (Setophaga kirtlandii), an endangered, long-distance migratory songbird. We used Cormack-Jolly-Seber models to analyze two mark-recapture datasets: 2006-2011 on Michigan breeding grounds, and 2003-2010 on Bahamian wintering grounds. The mean annual survival probability was 0.58±0.12 SE. Monthly survival probabilities during the summer and winter stationary periods were relatively high (0.963±0.005 SE and 0.977±0.002 SE, respectively). Monthly survival probability during migratory periods was substantially lower (0.879±0.05 SE), accounting for ~44% of all annual mortality. March rainfall in the Bahamas was the best-supported predictor of annual survival probability and was positively correlated with apparent annual survival in the subsequent year, suggesting that the effects of winter precipitation carried over to influence survival probability of individuals in later seasons. Projection modeling revealed that a decrease in Bahamas March rainfall >12.4% from its current mean could result in negative population growth in this species. Collectively, our results suggest that increased drought during the non-breeding season, which is predicted to occur under multiple climate change scenarios, could have important consequences on the annual survival and population growth rate of Kirtland's warbler and other Neotropical-Nearctic migratory bird species.

Characteristics of the breeding habitat of Endangered Kirtland's Warblers on a Canadian Military Installation

Kirtland's Warblers (Setophaga kirtlandii) are an endangered species with specialized habitat requirements, and the only documented nesting location in Canada is an Army installation. From 2007 to 2010, I compared habitat characteristics of sites occupied and not occupied by Kirtland's Warblers at Garrison Petawawa located ∼200 km northwest of Ottawa, Ontario. Sites occupied by Kirtland's Warblers (N = 11) had greater percent cover of low sweet blueberry (Vaccinium angustifolium; 31.2%), coral lichen (Cladina stellaris; 0.4%), reindeer lichen (Cladonia rangiferina; 8.3%), and wavy-leaved moss (Dicranum polysetum; 4.3%) than unoccupied sites (N = 6). I found no difference in tree species composition between sites, with jack pine (Pinus banksiana), white pine (Pinus strobus), red pine (Pinus resinosa), poplar (Populus tremuloides), and red maple (Acer rubra) present in both used and unused sites. Forest stands occupied by Kirtland's Warblers were significantly younger (< 20 yr old) than unoccupied sites, and most occupied sites were within former fire zones and sites where jack pines had been planted. Thus, breeding habitat of Kirtland's Warblers in Canada consisted of young pine trees, with more red pine than in their breeding habitat in Michigan, and ground cover including blueberry, lichens, and mosses. These results suggest that Kirtland's Warblers may be less selective in their habitat requirements than previously thought, and should provide guidance for recovery groups and regulatory agencies in accurately delineating suitable habitat for these warblers in Canada. RESUMEN Caracteristicas del habitat de reproduccion de la especie amenazada, Setophaga kirtlandii, en una base militar Canadiense Setophaga kirtlandii es una especie amenazada con requerimientos de habitat especializados y la unica localidad de anidamiento registrada en Canada se encuentra en una base militar de la Armada. Entre 2007 y 2010, compare las caracteristicas del habitat de los sitios ocupados y no ocupados por Setophaga kirtlandii en Garrison Petawawa, ubicado 200 km al noroeste de Ottawa, Ontario. Sitios ocupados por Setophaga kirtlandii (N = 11) tienen un mayor porcentaje de cobertura de Vaccinium angustifolium (31.2%), Cladina stellaris (0.4%), Cladonia rangiferina (8.3%) y Dicranum polysetum (4.3%) que sitios no ocupados (N = 6). No encontre diferencia en la composicion de especies de arboles entre sitios, con Pinus banksiana, Pinus strobus, Pinus resinosa, Populus tremuloides y Acer rubra presentes en los sitios ocupados y no ocupados. Parches de bosque utilizados por Setophaga kirtlandii resultaron ser significativamente mas jovenes (< 20 anos) que los sitios no ocupados, y la mayoria de los sitios ocupados se encontraron dentro de zonas que fueron incendiadas y donde habia plantaciones de Pinus banksiana. En consecuencia, el habitat de reproduccion de Setophaga kirtlandii en Canada consistio de pinos jovenes con una mayor proporcion de Pinus resinosa que en los habitats de reproduccion en Michigan, y el recubrimiento del suelo incluyo arandano, liquenes y musgos. Estos resultados sugieren que Setophaga kirtlandii puede ser menos selectiva en los requerimientos de habitat de lo que se pensaba en el pasado, y provee una guia para las grupos de recuperacion y agencias regulatorias para delinear con precision habitats propicios para estas reinitas en Canada.

Recent sightings of Kirtland's Warblers on San Salvador Island, The Bahamas

Abstract Here we present recent accounts of Kirtland's Warblers (Setophaga kirtlandii) captured and observed on San Salvador Island, The Bahamas, in 2012 and 2013. We conducted mist-netting and passive surveys in a variety of habitats on the island from January–March 2012 and December 2012–March 2013. We captured four warblers and sighted at least six other individuals, including two color-banded birds originally banded near Mack Lake, Michigan, USA. Warblers were primarily captured or observed in coastal scrub and inland low coppice habitats where vegetation height is <4 m, has a scrub/shrub appearance, and openings because of disturbance. These habitats are similar to habitats preferred by Kirtland's Warblers on other Bahamian islands, and also have fruiting plants the warblers prefer such as Erithalis fruticosa, and Lantana involucrata. To our knowledge, these are the first confirmed cases of observed Kirtland's Warblers on the island in over 46 years.

Duration and Rate of Spring Migration of Kirtland's Warblers

Abstract The duration of migration of the endangered Kirtland's Warbler (Setophaga kirtlandii) has not been previously documented. We estimated the average duration of spring migration for five male Kirtland's Warblers by observing uniquely color-banded individuals at or near both the beginning and end of spring migration in Eleuthera, The Bahamas, and Michigan, respectively. We estimated the average duration of spring migration for these five individuals to have been no more than 15.8 days (range 13–23 days) and the average distance traveled to have been 144.5 km/day (96.1–169.1 km/day).

Temporal patterns of genetic diversity in Kirtland's warblers (Dendroica kirtlandii), the rarest songbird in North America.

BackgroundKirtland’s warblers are the rarest songbird species in North America, rarity due in part to a reliance on early successional Jack Pine forests. Habitat loss due to fire suppression led to population declines to fewer than 200 males during the 1970s. Subsequent conservation management has allowed the species to recover to over 1700 males by 2010. In this study, we directly examine the impact that low population sizes have had on genetic variation in Kirtland’s warblers. We compare the molecular variation of samples collected in Oscoda County, Michigan across three time periods: 1903–1912, 1929–1955 and 2008–2009.ResultsIn a hierarchical rarified sample of 20 genes and one time period, allelic richness was highest in 1903–1912 sample (ar = 5.96), followed by the 1929–1955 sample (ar = 5.74), and was lowest in the 2008–2009 sample (ar = 5.54). Heterozygosity measures were not different between the 1929–1955 and 2008–2009 samples, but were lower in the 1903–1912 sample. Under some models, a genetic bottleneck signature was present in the 1929–1955 and 2008–2009 samples but not in the 1903–1912 sample.ConclusionsWe suggest that these temporal genetic patterns are the result of the declining Kirtland’s warbler population compressing into available habitat and a consequence of existing at low numbers for several decades.

Characteristics of a Red Pine Plantation Occupied by Kirtland's Warblers in Wisconsin

Abstract We studied a newly established population of Kirtland's Warbler (Dendroica kirtlandii) in Adams County, Wisconsin, nesting in a red pine (Pinus resinosa) plantation. We found eight males and five females in Adams County in 2008 and 10 males and 10 females in 2009. Five of seven (71%) males color-banded in 2008 returned in 2009, and at least eight successful nests produced an estimated 33 young over the 2 years. Red pine comprised 66.9% of trees on the main site, 20.6% were northern pin oak/black oak (Quercus ellipsoidalis/Q. velutina), and 12.5% were jack pine (Pinus banksiana). Total tree density at the main site was 1,876 trees/ha, lower than generally reported in Michigan. Percent canopy cover and ground cover types were similar to Michigan sites. Lowest live branch height of jack pine was similar to Michigan sites, but lowest live branches of red pine at our site were closer to the forest floor. Significant red pine die-off at our site combined with substantial natural jack pine recruitment c...

Kirtland's Warblers in Anthropogenically Disturbed Early-Successional Habitats on Eleuthera, the Bahamas

Abstract. To characterize the nonbreeding habitat of Kirtland's Warbler (Dendroica kirtlandii) on Eleuthera, The Bahamas, we quantified the habitat at sites where we captured the warblers and compared these traits with those of random sites and sites of tall coppice. On the basis of a chronosequence of satellite imagery, 153 capture sites ranged in age from 3 to 28 years after human disturbance, mean 14.6 years ± 6.3 (SD). Capture sites had been abandoned after clearing (65%), converted to goat pasture (26%), burned (2%), or were young second growth following unknown disturbance (7%). Canopies in 104 capture plots were lower (mean 1.8 m) than canopies in random plots (mean 2.7 m) and plots of late-successional tall coppice (mean 6.3 m). At seven sites mean foliage density in capture plots was consistently greatest at 0.5 to 1.0 m height, but the sites were heterogeneous for other foliage-height classes <3 m and for time since disturbance, canopy height, stem density, and five ground-cover traits. Plots di...

Male Kirtland's Warblers’ patch-level response to landscape structure during periods of varying population size and habitat amounts

Forest planners must evaluate how spatiotemporal changes in habitat amount and configuration across the landscape as a result of timber management will affect species’ persistence. However, there are few long-term programs available for evaluation. We investigated the response of male Kirtland's Warbler ( Dendroica kirtlandii) to 26 years of changing patch and landscape structure during a large, 26-year forestry-habitat restoration program within the warbler's primary breeding range. We found that the average density of male Kirtland's Warblers was related to a different combination of patch and landscape attributes depending on the species’ regional population level and habitat amounts on the landscape (early succession jack pine ( Pinus banksiana) forests; 15–42% habitat cover). Specifically, patch age and habitat regeneration type were important at low male population and total habitat amounts, while patch age and distance to an occupied patch were important at relatively high population and habitat amounts. Patch age and size were more important at increasing population levels and an intermediate amount of habitat. The importance of patch age to average male density during all periods reflects the temporal buildup and decline of male numbers as habitat suitability within the patch changed with succession. Habitat selection (i.e., preference for wildfire-regenerated habitat) and availability may explain the importance of habitat type and patch size during lower population and habitat levels. The relationship between male density and distance when there was the most habitat on the landscape and the male population was large and still increasing may be explained by the widening spatial dispersion of the increasing male population at the regional scale. Because creating or preserving habitat is not a random process, management efforts would benefit from more investigations of managed population responses to changes in spatial structure that occur through habitat gain rather than habitat loss to further our empirical understanding of general principles of the fragmentation process and habitat cover threshold effects within dynamic landscapes.

Influence of habitat amount, arrangement, and use on population trend estimates of male Kirtland’s warblers

Kirtland's warblers (Dendroica kirtlan- dii) persist in a naturally patchy environment of young, regenerating jack pine forests (i.e., 5-23 years old) created after wildfires and human logging activities. We examined how changing landscape structure from 26 years of forest management and wildfire disturbances influenced population size and spatial dispersion of male Kirtland's warblers within their restricted breeding range in northern Lower Michigan, USA. The male Kirtland's warbler popu- lation was six times larger in 2004 (1,322) compared to 1979 (205); the change was nonlinear with 1987 and 1994 identified as significant points of change. In 1987, the population trend began increasing after a slowly declining trend prior to 1987, and the rate of increase appeared to slow after 1994. Total amount of suitable habitat and the relative area of wildfire- regenerated habitat were the most important factors explaining population trend. Suitable habitat increased 149% primarily due to increasing planta- tions from forest management. The relative amount and location of wildfire-regenerated habitat modified the distribution of males among various habitat types, and the spatial variation in their abundance across the primary breeding range. These findings indicate that the Kirtland's warbler male population shifted its use of habitat types temporally and spatially as the population increased and as the relative availability of habitats changed through time. We demonstrate that researchers and managers need to consider not only habitat quality, but the temporal and the spatial context of habitat availability and population levels when making habitat restoration decisions.

Comparing census methods for the endangered Kirtland's Warbler

Abstract We compared transect counts used for the annual official count of male Kirtland's Warblers (Dendroica kirtlandii) to an observation-based mapping method of individually sighted males in 155 stands over 10 yrs. The annual census count almost tripled from 1990 to 1999. The transect and observation-based mapping method showed the same increasing trend in population between 1990 and 1999, except from 1992 to 1993. The annual official census transect count was consistently higher than the mapping method for stands censused in common. After standardizing for sample size, the annual number of Kirtland's Warblers per stand increased through time with the transect method, while there was a positive, non-significant trend with the mapping method. After 1992, the two methods began diverging in the number of males per stand. The relationship between the differences in count between methods (mapping count minus transect count) to the official transect count varied among years. At the stand level, the transect...

Re-evaluating the Bay-breasted Warbler Breeding Range: Nine Years of Presence in Lower Michigan

The breeding range of the Bay-breasted Warbler (Dendroica castanea) is thought to include only the northernmost portions of six northeastern and northcentral states in the United States. During a 10-year banding study of Kirtland's Warblers (Dendroica kirtlandii) in northern Lower Michigan, we caught 44 Bay-breasted Warblers outside of their reported migration dates during 9 of the 10 years. Two birds captured in 1997 were in breeding condition; one possessed a cloacal protuberance and the other a full brood patch. We also captured two hatching year birds with fleshy rictal flanges in 1997. We suggest that these records indicate a long term presence of Bay-breasted Warblers on breeding grounds considerably farther south than previously recorded.

KIRTLAND'S WARBLER DIET AS DETERMINED THROUGH FECAL ANALYSIS

The endangered Kirtland's Warbler (Dendroica kirtlandii) nests primarily in large (>32 ha) stands of young (5- to 25-yr-old) jack pine (Pinus banksiana) which grow on Grayling sand soil. These specific habitat requirements restrict the Kirtland's Warbler breeding range to only 13–16 counties in the northern lower peninsula of Michigan. Although the nature of the species' affinity for this habitat is poorly understood, one theory suggests that higher prey abundance in young jack pine may play a role. To explore further the hypothesis that Kirtland's Warblers choose nesting habitat due to prey abundance, a more thorough knowledge of the warblers' diet is needed. To better understand the diet, we identified arthropod and plant fragments found in 202 Kirtland's Warbler fecal samples, collected from June to September, 1995–1997. The major food items recorded were spittlebugs and aphids (Homoptera; found in 61% of all samples), ants and wasps (Hymenoptera; 45%), blueberry (Vaccinium augustifolium; 42%), beetles (Coleoptera; 25%), and moth larvae (Lepidoptera; 22%).