Tennessee Warbler Research Articles

Temporal trends in bird abundance in response to thinning of lodgepole pine (Pinus contorta)

Forest managers are continually seeking methods of extracting more timber from the land base while maintaining appropriate wildlife habitat. A 7-year before–after control–impact (BACI) experiment was conducted in 60-year-old lodgepole pine (Pinus contorta Douglas ex Loudon) to determine the effects of thinning on forest birds. One year of pretreatment data was compared with 6 years of posttreatment data. Using linear trend analysis corrected for detection error, we found that Chipping Sparrow (Spizella passerina (Bechstein, 1798)), Dark-eyed Junco (Junco hyemalis (Linnaeus, 1758)), and Red-eyed Vireo (Vireo olivaceus (Linnaeus, 1766)) increased over time in thinned stands and declined in control stands. Alder Flycatcher (Empidonax alnorum Brewster, 1895), Hermit Thrush (Catharus guttatus (Pallas, 1811)), Rose-breasted Grosbeak (Pheucticus ludovicianus (Linnaeus, 1766)), and Blue-headed Vireo (Vireo solitarius (Wilson, 1810)) decreased in control and thinned stands but declined more in controls. Varied Thrush (Ixoreus naevius (J. F. Gemlin, 1789)) and White-winged Crossbill (Loxia leucoptera Gemlin, 1789) increased in control stands relative to thinned areas, where they were stable or declined. A significant interaction between treatment and year was observed for 24 species when year was categorical. Of these, species more likely found in control stands were Boreal Chickadee (Poecile hudsonicus (J.R. Forster, 1772)), Golden-crowned Kinglet (Regulus satrapa Lichtenstein, 1823), and Least Flycatcher (Empidonax minimus (W.M. Baird & S.F. Baird, 1843)), whereas Ruffed Grouse (Bonasa umbellus (Linnaeus, 1766)), Tennessee Warbler (Vermivora peregrina (A. Wilson, 1811)), and Warbling Vireo (Vireo gilvus (Vieillot, 1808)) were more likely in thinned stands. Total bird abundance was higher in thinned stands, which is consistent with 83% of thinning studies in western coniferous forests. Thinning can be a useful tool for achieving wildlife management goals and increasing timber yields.

Taxonomic recommendations for British birds: seventh report

Taxonomic recommendations for British birds: seventh report

A review of the interaction between forest birds and eastern spruce budworm

The eastern spruce budworm, Choristoneura fumiferana Clem., (hereafter budworm) is responsible for the largest areas of insect-caused disturbance in North America, and as such, is an important part of spruce–fir forest change and succession. The insectivorous forest bird community shows large and rapid responses to budworm outbreaks. There is good evidence that there are budworm-linked species (bay-breasted, Cape May, and Tennessee warblers) that respond to budworm outbreak much more strongly and consistently than other species, probably through increased productivity of local populations when budworm are abundant. There also appears to be a more widespread positive bird community response to budworm outbreak that involves many more species. The response is evident in local and regional scale studies, but individual species’ responses across studies are not always consistent, probably because of the relatively small number of studies conducted in a wide variety of contexts. Budworm outbreaks provide a short-term increase in food supply for birds, but also result in longer-term habitat change due to budworm-induced defoliation and tree mortality. Birds appear to influence budworm cycles, mostly at endemic population levels, through predation of large larvae and pupae. There are good arguments suggesting that bird predation is not the primary cause of budworm oscillations, but may play a role in determining the mean level of oscillations. Climate change is expected to change the bird–budworm relationship through changes in fire regimes, spruce–fir distributions, bird distributions, and budworm and bird phenology.

Migratory songbird use of shade coffee in the Venezuelan Andes with implications for conservation of cerulean warbler

Although previous studies have demonstrated high use of shade coffee plantations by Neotropical migratory birds, we still have a poor understanding of the suitability of shade coffee plantations as habitat on the wintering grounds. We studied density, body condition, and survivorship of Neotropical migrants, with emphasis on cerulean warbler ( Dendroica cerulea), in primary forest and shade coffee plantations in Venezuela. We worked in three primary forest sites and three shade coffee plantations on the western slope of the Cordillera de Mérida of the Andes Mountains. At each site, we surveyed migrants with distance-based line transects and mist-netted and banded migrants during November–February 2005/06 and 2006/07. In addition, we estimated apparent monthly survival for cerulean warbler based on 29 color-banded individuals. Densities of migrants were 3–14× higher in shade coffee plantations than primary forest sites, even after accounting for differences in detectability. Apparent monthly survival of cerulean warblers was estimated at 97% and overwinter persistence was similarly high. In addition, cerulean warblers demonstrated high between-season fidelity, with 65% of the birds banded the first year being resighted during the second year. Interestingly, immature birds returned at nearly half the rate as did adults. Banding data indicated that body condition increased significantly as the season progressed for cerulean warblers, Tennessee warblers ( Vermivora peregrina), and American redstarts ( Setophaga ruticilla). Collectively, these data provide evidence that shade coffee plantations offer high quality wintering habitat for Neotropical migrants, including cerulean warblers.

A test of Ontario's Habitat Suitability Matrix as a forest management planning tool for forest birds

We used point-count data collected as part of Bird Studies Canada's Boreal Forest Bird Program to validate habitat suitability models for 22 forest bird species in Ontario's Habitat Suitability Matrix. We found that many of the species'models performed relatively poorly in discriminating between occupied and unoccupied sites, primarily due to the high error of commission rates (false positive predictions). Since species presence and abundance were assessed by single, five minute point counts, insufficient sampling was at least partly responsible for some of the observed over-prediction. Results suggested that model parameters for at least nine of the species tested (hairy woodpecker [Picoides villosus], blueheaded vireo [Vireo solitarius], red-eyed vireo [Vireo olivaceus], red-breasted nuthatch [Sitta canadensis], Swainson's thrush [Catharus ustulatus], hermit thrush [Catharus guttatus], Tennessee warbler [Vermivora peregrina], Blackburnian warbler [Dendroica fusca] and dark-eyed junco [Junco hyemalis]) should be reviewed to improve the predictive capability of the models and to ensure appropriate consideration of the habitat needs of these species during forest management planning. Key words: boreal forest, forest birds, discrimination capacity, habitat models, habitat suitability matrix, model accuracy, model validation, relative operating characteristic curve, ROC

COMPOSITION AND TIMING OF POSTBREEDING MULTISPECIES FEEDING FLOCKS OF BOREAL FOREST PASSERINES IN WESTERN CANADA

The aggregation of nonbreeding insectivorous songbirds into multispecies feeding flocks during migration and on their wintering grounds is a well-known and important aspect of their ecology. The establishment of multispecies feeding flocks on the temperate breeding grounds of North American Neotropical migrants, however, remains poorly known or understood. To address this gap, we investigated the composition and timing of flocking behavior among several species occurring in the southern boreal mixed-wood forest of western Canada. Of 67 species observed in 216 flocks, the most abundant were Tennessee Warbler (Vermivora peregrina) and several resident species: Black-capped Chickadee (Poecile atricapillus), Red-breasted Nuthatch (Sitta canadensis), and Boreal Chickadee (Poecile hudsonica). Consistent with previous work on Eurasian boreal species, residents appeared to play a pivotal role in flock occurrence and cohesion. Flocking tended to begin in late June, and flock sizes increased throughout the summer. This suggests that unsuccessful breeders, early breeders, and early migrants are the first to join flocks, whereas later-nesting species may delay joining flocks until after their young fledge. We also investigated the propensity of several species to display flocking behavior in areas with and without a superabundant food source—the spruce budworm (Choristoneura fumiferana). These data provided some support for the hypothesis that flocking facilitates foraging, as species tended to flock in areas where food abundance was lower.

Population Dynamics of Songbirds in the Boreal Mixedwood Forests of Alberta, Canada: Estimating Minimum and Maximum Extents of Spatial Population Synchrony

Ecological phenomena vary over space and time and interpretation of these processes also varies depending on the measurement scale. As the spatial scale of observation increases and decreases, changes in population abundance will likely exhibit alternating patterns of asynchrony and synchrony. While the study of how and why population dynamics change with spatial scale of measurement is intrinsically interesting, most population ecologists seek to study mechanisms of population change on a discrete study area. Our study develops methods that population ecologists can use to determine spatially appropriate sampling designs, and demonstrates how such spatial scales can be determined for 25 species of songbirds using longterm data from the boreal mixedwood forest of Alberta, Canada. To determine minimum scales of synchrony in population dynamics, we calculated the average correlation of changes in population abundance over time across different numbers of fixed-radius point-count samples. We then used a randomization test to remove the effect of number of replicates from the determination of appropriate spatial scale. The maximum extent of synchrony was estimated as the distance where population dynamics were no longer correlated. Estimates of the minimum scale of synchrony differed between species, ranging from 3.1 to 18.6 ha. The maximum scale of synchrony was estimated to be greater than or equal to 8 km for 14 of the 25 species examined, and to be greater than or equal to 70 km for Tennessee Warbler (Vermivora peregrina). Maximum spatial extents were significantly correlated with body mass and territory size.

TWO SPECIES OF SYNHIMANTUS (DISPHARYNX) RAILLIET, HENRY AND SISOFF, 1912 (NEMATODA: ACUARIOIDEA: ACUARIIDAE), IN PASSERINE BIRDS FROM THE AREA DE CONSERVACION GUANACASTE, COSTA RICA

Members of 2 species of Synhimantus (Dispharynx) live under the lining of the gizzard in passerine birds from the Area de Conservacion Guanacaste, Costa Rica. Synhimantus (Dispharynx) nasuta (Rudolphi, 1819) occurs in Thraupis episcopus, Turdus grayi, Caryothraustes poliogaster, Platyrinchus cancrominus, Ramphocaenus melanurus, Vermivora peregrina, and Geothlypis poliocephala. A single male, in Turdus grayi, apparently representing a new species, distinguishable from all other species of Synhimantus (Dispharynx) by having similar shaped left and right spicules, is described but not named.

Habitat Segregation Among Songbirds in Old-Growth Boreal Mixedwood Forest

The foraging behaviour of ten species of insectivorous songbirds — Boreal Chickadee (Poecille hudsonicus), Golden-crowned Kinglet (Regulus satrapa), Ruby-crowned Kinglet (R. calendula), Blue-headed Vireo (Vireo solitarius), Tennessee (Vermivora peregrina), Blackburnian (Dendroica fusca), Magnolia (D. magnolia), Yellow-rumped [Myrtle] (D. coronata), Black-throated Green (D. virens), and Bay-breasted (D. castanea) warblers — was observed in the boreal mixedwood forest of Prince Albert National Park in central Saskatchewan. Birds segregated their habitat use by preferentially foraging in different tree species, and through preferential use of different foraging locations (height and position) within trees.White Spruce (Picea glauca) was used more than expected by Yellow-rumped Warblers, and Golden-crowned and Ruby-crowned kinglets. Tennessee and Magnolia warblers used White Birch (Betula papyrifera), more than expected and Boreal Chickadees and Blue-headed Vireos used Balsam Fir (Abies balsamea) more than expected. Boreal Chickadees, Ruby-crowned Kinglets, and Tennessee, Blackburnian and Yellow-rumped warblers all used the bottom part of trees less than expected, while Blueheaded Vireos foraged near the top of trees less than expected. Large inner branches were avoided by Tennessee, Blackburnian and Yellow-rumped warblers, while Bay-breasted Warblers and Blue-headed Vireos avoided small outer twigs. In conifers, Blackburnian Warblers foraged significantly higher in the trees than all other species except Black-throated Green and Baybreasted warblers. Blackburnian Warblers also foraged significantly higher than Blue-headed Vireos and Magnolia Warblers in deciduous trees.

BOREAL FOREST SONGBIRD COMMUNITIES OF THE LIARD VALLEY, NORTHWEST TERRITORIES, CANADA

Abstract Songbird communities in the boreal forest of the Liard Valley, Northwest Territories, Canada, are described after three years of study. Point count stations (n = 195) were placed in six types of forest (mature deciduous, coniferous, and mixedwood; young forests; wooded bogs; clearcuts) in a 700-km2 area. Vegetation characteristics at each station were also measured. Eighty-five species of birds (59 passerine species) occurred in 11 647 detections. Mixedwood forests had the highest richness of songbirds (∼41 species per 800 individuals) of the six forest types, and contained approximately 30% more individuals than nearly pure coniferous or deciduous forests. Species richness and relative abundance was 10–50% lower than in comparable forests farther south and east, and the difference was most pronounced in deciduous forests. Communities were dominated by a few species, especially Tennessee Warbler (Vermivora peregrina), Magnolia Warbler (Dendroica magnolia), Swainson's Thrush (Catharus ustulatus), ...

PATTERNS OF CONSUMPTION AND DIET DIFFERENTIATION FOR THREE BREEDING WARBLER SPECIES DURING A SPRUCE BUDWORM OUTBREAK

Abstract Diets of three warbler species were analyzed during a spruce budworm outbreak in the boreal forest of northern Ontario. Beetles constituted a large portion of the food items consumed by Cape May (Dendroica tigrina), Bay-breasted (Dendroica castanea), and Tennessee (Vermivora peregrina) warblers early in the breeding season (7–11 June), and caterpillars were the most frequently used food category shortly later (18–24 June). Differences in diet served to differentiate the warbler species in the earlier period when Bay-breasted Warblers consumed more beetles, Tennessee Warblers consumed more caterpillars, and Cape May Warblers consumed more flies than the other species. Only Bay-breasted Warblers' continuing preference for beetles differentiated the warblers' diets in the later period. Food-niche overlaps increased for two of the three warbler species pairs between the two periods in June, but there was no change in the overlap between Bay-breasted and Cape May warbler diets.

Patterns of Consumption and Diet Differentiation for Three Breeding Warbler Species During a Spruce Budworm Outbreak

Abstract Diets of three warbler species were analyzed during a spruce budworm outbreak in the boreal forest of northern Ontario. Beetles constituted a large portion of the food items consumed by Cape May (Dendroica tigrina), Bay-breasted (Dendroica castanea), and Tennessee (Vermivora peregrina) warblers early in the breeding season (7–11 June), and caterpillars were the most frequently used food category shortly later (18–24 June). Differences in diet served to differentiate the warbler species in the earlier period when Bay-breasted Warblers consumed more beetles, Tennessee Warblers consumed more caterpillars, and Cape May Warblers consumed more flies than the other species. Only Bay-breasted Warblers' continuing preference for beetles differentiated the warblers' diets in the later period. Food-niche overlaps increased for two of the three warbler species pairs between the two periods in June, but there was no change in the overlap between Bay-breasted and Cape May warbler diets.

Bird–habitat relationships in jack pine boreal forests

We made counts of 42 bird species at 217 points in 44 jack pine Pinus banksiana stands in the boreal region of north-central Saskatchewan, Canada because of concerns about the impact of forestry on avian biodiversity. Using multivariate analyses we describe the main patterns of bird species abundance and composition in relation to local habitat variables. According to canonical correspondence analysis (CCA), 17 of 43 explanatory variables accounted for significant variation in the bird species matrix: stand age, tamarack Larix laricina canopy cover, percentage shrub Alnus glutinosa, canopy closure (6–30%), percentage herbs, maximum canopy height, percentage shrub Alnus viridis, percentage moss, percentage shrub Betula nana, minimum and mean shrub height, percentage litter, white birch Betula papyrifera canopy cover, white spruce Picea glauca canopy cover, mean canopy height, canopy closure (71–100%), and zero shrub canopy closure. Habitat and stand identity (clustering of sites within stands) explained 55.5% of the total variation; 16.3% was unique to habitat alone (29.4% of the total variance explained — TVE) and 17.2% (31.0% TVE) was shared with stand identity. Stand identity accounted for 22.0% (39.6% TVE) of the variation. Neotropical migrants associated with “overmature” mixed-wood stands included Cape May Dendroica tigrina, bay-breasted D. castanea and Tennessee Vermivora peregrina warblers. Conversely, orange-crowned V. celata, palm D. palmarum, and Nashville V. ruficapilla warblers were found in young stands. The presence of several Neotropical migrants was associated with mixed-wood attributes (ovenbird Seiurus aurocapillus with aspen, red-eyed vireo Vireo olivaceus with shrub alder, and Swainson’s thrush Catharus ustulatus with canopy white birch). Jack pine stands older than commercial rotation age must be maintained in a managed landscape because their bird communities differ from younger stands. Because of the limited avian diversity in pure jack pine stands, sufficient mixed-wood stands must be regenerated to sustain populations of characteristic bird species. We recommend that jack pine mixed-woods be managed using shelterwood or selective cuts, and that rotation age of some stands be increased. Without the extensive fires necessary for regeneration, cutting patterns at the landscape level should approximate natural disturbance patches both in size and frequency of disturbance.

Breeding Bird Communities in Boreal Forest of Western Canada: Consequences of “Unmixing” the Mixedwoods

Abstract Silvicultural practices following clearcutting in boreal forest may encourage the creation of monospecific, single-aged stands having less vegetation heterogeneity and diversity than original stands. We conducted point counts in central Saskatchewan, Canada, 1993–1995, in pure and mixedwood stands dominated by black spruce (Picea mariana), jackpine (Pinus banksiana), trembling aspen (Populus tremuloides), or white spruce (Picea glauca). Mixedwood stands supported more individuals and more species than pure stands. Higher abundance in mixedwood stands relative to pure stands was consistent among nesting guilds and migration strategies. Rarefaction revealed similar patterns, although pure trembling aspen stands were predicted to support more species than aspen-dominated mixedwood stands. Increased avian diversity in mixedwood stands was not solely the result of the mixing of bird species associated with coniferous or deciduous forest types. Chipping Sparrow (Spizella passerina), Pine Siskin (Carduelis pinus), White-winged Crossbill (Loxia leucoptera), Red-breasted Nuthatch (Sitta canadensis), Swainson's Thrush (Catharus ustulatus), and Tennessee Warbler (Vermivora peregrina) were more abundant in mixedwood stands than pure stands. Black-throated Green Warbler (Dendroica virens), Magnolia Warbler (D. magnolia), and Blackburnian Warbler (D. fusca) were abundant in stands dominated by white spruce but were absent from jackpine or black spruce. Other species such as American Redstart (Setophaga ruticilla) and Chestnut-sided Warbler (D. pensylvanica) relied exclusively on pure trembling aspen, particularly stands with dense shrub cover. Several bird species in the boreal forest will be adversely affected by forestry practices that target mature to old aspen and white spruce mixedwoods and promote reduction in mixedwood compositions of regenerating stands.

Nesting biology of the Tennessee Warbler, Vermivora peregrina, in northern Ontario

Nesting biology of the Tennessee Warbler, Vermivora peregrina, in northern Ontario

CHANGES IN BIRD COMMUNITIES IN BOREAL MIXEDWOOD FOREST: HARVEST AND WILDFIRE EFFECTS OVER 30 YEARS

A current paradigm in conservation biology is that forest harvest practices that better approximate natural disturbance processes are more likely to conserve biodi- versity. We contrasted bird communities in three replicate stands in each of 1, 13-15, and 22-28 yr old forests following wildfire and harvest in north-central Alberta, Canada. Stands were chosen from old (>120 yr) boreal mixedwood forests having -95% of the canopy trees killed during fire, and harvested sites retaining an average of 6% of the pre-harvest canopy trees. For all age classes, postharvest sites tended to have greater bird abundance. Species composition also differed between these treatment types. Two-Way Indicator Spe- cies Analysis (TWINSPAN) identified five major ecological groupings of species that dif- fered between wildfire and harvest, and among stand ages. Correspondence analysis (CA) identified similar bird communities. Greatest differences between bird communities oc- curred immediately following disturbance, and gradual convergence of communities oc- curred throughout the first 28 yr after disturbance. Species associated with open shrub and grassland or riparian habitats were associated primarily with 1-yr postharvest stands. Three- toed Woodpeckers (Picoides tridactyla) and Black-backed Woodpeckers (P. arcticus), to- gether with other species that use snags for foraging or nesting, occurred primarily in 1- yr postwildfire stands. Convergence in avian communities was correlated with the loss of standing snags on postwildfire sites. However, differences in bird communities were ap- parent up to 28 yr following disturbance, and this lack of complete convergence has im- portant consequences for sustainable forestry practices designed to maintain biodiversity in the boreal mixedwood forest. Notably, Connecticut Warbler (Oporornis agilis), Brown Creeper (Certhia americana), Winter Wren (Troglodytes troglodytes), and American Robin (Turdus migratorius) had higher densities on postwildfire than on postharvest stands. Lin- coln's Sparrow (Melospiza georgiana), Alder Flycatcher (Empidonax alnorum), Tennessee Warbler (Vermivora peregrina), Black-and-white Warbler (Mniotilta varia), American Red- start (Setophaga ruticilla), Mourning Warbler (Oporornis philadelphia), Rose-breasted Grosbeak (Pheucticus ludoviciana), Canada Warbler (Wilsonia canadensis), and Pine Siskin (Carduelis pinus) had higher densities on postharvest stands, possibly due to the greater abundance, after harvest, of larger live residual trees and a taller and more dense shrub layer. Harvest designed to approximate stand-replacing fires may require the retention of more snags than is currently practiced. New approaches to fire salvage logging are also required to ensure adequate retention of standing dead trees on the landscape.

Reproduction and nest behaviour of Tennessee warblers Vermivora peregrina in forests treated with Lepidoptera‐specific insecticides

1. This study was designed to test the hypothesis that food reductions caused by forest spraying with Lepidoptera‐specific insecticides would affect Tennessee warbler behaviour and reproduction. 2. Tennessee warbler nests and parental behaviour were monitored in two spray blocks, one treated with Bacillus thuringiensis (Bt) and the other with tebufenozide (MIMIC), and in an untreated control area. 3. Nestling survival and growth were unaffected by the insecticide treatments. Nests in the treated blocks had smaller clutches, smaller broods and lower hatch rates than nests in the control block, but these differences were not statistically significant. 4. Nestling diets were similar in the MIMIC and control blocks. 5. There were slight differences in the behaviour patterns of female Tennessee warblers in the MIMIC and control blocks. MIMIC females spent less time at the nest and more time foraging. 6. The results of this study suggest that the indirect effects of forest spraying with Lepidoptera‐specific insecticides pose little risk to forest songbirds.

Spruce budworm outbreaks and the incidence of vagrancy in eastern North American wood-warblers

The Tennessee (Vermivora peregrina), Cape May (Dendroica tigrina), Bay-breasted (D. castanea), and Canada (Wilsonia canadensis) warblers and the Ovenbird (Seiurus aurocapillus) appear to show positive numerical responses to outbreaks of the spruce budworm (Choristoneura fumiferana); the Magnolia (D. magnolia), Black-throated Green (D. virens), and Blackburnian (D. fusca) warblers may occur in lower numbers during outbreaks because of increased competition with these budworm specialists. Thus, we predicted that the number of fall vagrant Tennessee, Cape May, and Bay-breasted warblers and Ovenbirds occurring in California are highly positively associated with budworm population density, positively intercorrelated, and negatively associated with numbers of Magnolia, Black-throated Green, and Blackburnian warblers. A 23-year (1972-1994) data set of budworm population densities and vagrant warbler occurrences in California showed that (i) budworm population density was an excellent predictor (explaining about 50% of the variance) of numbers of occurrences of the Cape May, Bay-breasted, and Canada warblers; (ii) all warbler species tended to co-occur, with numbers of Cape May and Bay-breasted warblers especially highly intercorrelated; and (iii) Magnolia Warbler numbers were negatively associated with those of budworm specialists, but Black-throated Green and Blackburnian warblers showed no association.

Bird Populations in Rustic and Planted Shade Coffee Plantations of Eastern Chiapas, México

ABSTRACTMuch of the remaining “forest” vegetation in eastern Chiapas, Mexico is managed for coffee production. In this region coffee is grown under either the canopy of natural forest or under a planted canopy dominated by Inga spp. Despite the large differences in diversity of dominant plant species, both planted and rustic shade coffee plantations support a high overall diversity of bird species; we recorded approximately 105 species in each plantation type on fixed radius point counts. We accumulated a combined species list of 180 species on repeatedly surveyed transects through both coffee plantation types. These values are exceeded regionally only by moist tropical forest. Of the habitats surveyed, shade coffee was second only to acacia groves in the abundance and diversity of Nearctic migrants. The two plantation types have similar bird species lists and both are similar in composition to the dominant woodland—mixed pine‐oak. Both types of shade coffee plantation habitats differ from other local habitats in supporting highly seasonal bird populations. Survey numbers almost double during the dry season—an increase that is found in omnivorous migrants and omnivorous, frugivorous, and nectarivorous resident species. Particularly large influxes were found for Tennessee warblers (Vermivora peregrina) and northern orioles (Icterus galbula) in Inga dominated plantations.

Lack of Sperm Storage by Female Migrants and the Significance of Copulations En Route

In all species studied thus far, female birds store sperm in specialized sperm storage tubules (SSTs) located at the junction of the uterus and vagina (Fujii 1963, Shugart 1988, Birkhead et al. 1990, Briskie and Montgomerle 1993). SSTs appear necessary for the long-term survival of spermatozoa within the female reproductive tract, because sperm not stored within SSTs soon die or are displaced by the passage of subsequent eggs (Lorenz 1966). The length of time that sperm can remain viable in storage varies from species to species, ranging from about six days in the Ring Dove (Streptopelia roseogrisea; Zenone et al. 1979) to over 72 days in the domestic Turkey (Meleagris gallopavo; Lorenz 1950). Why sperm storage duration varies among species is not clear, but it may be related to the proximity of males and females during pre-laying (Birkhead and Moiler 1992a). For example, sperm storage durations are particularly long in seabirds, in which females are separated from their mates for prolonged periods prior to laying. In contrast, males of many passerine species typically remain close to females throughout pre-laying and these species have correspondingly shorter durations of sperm storage (Birkhead and Moiler 1992a). In most birds, copulations are generally restricted to a brief prelaying period, with the greatest frequency occurring just prior to the laying of the first egg. For migratory species, pairing and mating are thought to take place largely on the breeding grounds after the end of spring migration. However, by using lavages to census sperm within the cloacae of male and female migrant passerines, Quay (1985a, 1989) found that many Tennessee Warbler (Vermivora peregrina) females and possibly some Blackpoll Warbler (Dendroica striata) females had been inseminated during migration and south of their nearest breeding grounds. In the Tennessee Warbler, about 25% of females contained sperm in their cloacae, indicating that copulatory behavior during migration may be widespread (Quay 1989). Recently, Moore and McDonald (1993) suggested that en route copulations may have an adaptive function, such as minimizing the time and costs of mate choice or ensuring females have an adequate supply of sperm should they not find a mate on the breeding grounds. Unfortunately, the role that en route inseminations play in the eventual fertilization of eggs remains unknown.