Summer Tanager Research Articles

Avian haemosporidians of the genera Plasmodium and Haemoproteus from resident and Neotropical migrant birds in Colombia

Avian haemosporidians of the genera Plasmodium and Haemoproteus are a group of widely distributed blood parasites that can negatively affect the fitness of their hosts. Colombia contains the greatest diversity of birds on the planet, but knowledge about the associations between haemosporidian and its avifauna is scarce and fragmented. We collected blood samples from 255 birds (203 residents and 52 neotropical migrants) belonging to 27 families and 108 species. The study was conducted in six localities in the inter-Andean valleys of the Cauca and Magdalena rivers. Parasites of the genera Plasmodium and Haemoproteus were identified in the samples by morphological and molecular analysis of a fragment of the mitochondrial gene cyt b. Among the samples, 9.3% (n = 24) were positive for Plasmodium or Haemoproteus. Co-infection with Plasmodium and Haemoproteus was found in Red-eyed Vireo. Seventeen haemosporidian lineages were identified, five of which were reported for the first time in resident birds (Common Ground Dove, Checker-throated Stipplethroat, Tropical Kingbird, Pale-breasted Thrush, and Ruddy-breasted Seedeater) and one in the Summer Tanager (neotropical migrant). The research results confirm the wide diversity of haemosporidian present in tropical lowlands and the possible role of neotropical migratory birds in dissemination on haemosporidian along their migratory routes.

Potential Breeding Habitat for Forest Birds Suburbs

Studies report that urban areas may not provide breeding habitat for interior-forest birds, but some residential neighborhoods do have sufficient vegetation to serve as breeding habitat for forest birds and even interior-forest specialists. The authors investigated the occurrence of three interior-forest specialists, pileated woodpeckers, summer tanagers, and northern parulas, during the breeding season in two suburban neighborhoods of Gainesville, Florida, and found that all three species occurred in residential neighborhoods consistently throughout the breeding season and were most often detected in local areas with dense vegetation from the ground to the canopy. Our results suggest that decision-makers can improve the amount of breeding habitat in southeastern cities for these three species by retaining forest fragments, tree canopy, large trees, tree snags, and understory vegetation.

Using Bioacoustics to Examine Vocal Phenology of Neotropical Migratory Birds on a Wild and Scenic River in Arizona

Passive acoustic recorders have been used successfully as automated survey tools to detect terrestrial wildlife. However, few studies have monitored Neotropical migratory bird use of riparian forest habitat using this technology. Within dryland ecosystems, the forests along rivers support high bird diversity. Many bird species of conservation concern require these floodplain forest habitats for foraging, migration stop-overs, and breeding. Few studies have explored the use of acoustic records in riverine systems designated for conservation for their natural resource value via the Wild and Scenic Rivers Act in the USA. Using acoustic recorders, we document vocal activity of four riparian-obligate species (Bell’s Vireo, Vireo bellii; Summer Tanager, Piranga rubra; Yellow Warbler, Setophaga petechial; and Yellow-billed Cuckoo, Coccyzus americanus) to determine species occurrence along a Wild and Scenic River. We established three study reaches along the perennial Lower Verde River, in the Sonoran Desert of central Arizona, USA. Nine acoustic recorders were used over the period of 80–120 days during the summer of 2018. We measured vegetation composition and structure in 100 m2 plots paired with acoustic recorders. Visualizing vocal activity showed that three species were calling and singing at each reach; whereas, one species, the cuckoo, had fewer recordings and occurred later in the summer. We demonstrate the utility of acoustic monitoring even when applied to rare birds in complex riparian habitats. This information is important for land management and conservation efforts concerning these species of interest and identifying important habitat features in Southwestern US riparian woodlands.

A New Isospora (Apicomplexa: Eimeriidae) from the Summer Tanager, Piranga rubra (Aves: Passeriformes: Cardinalidae), in Oklahoma.

The summer tanager, Piranga rubra (L., 1758) is a medium-sized songbird formerly belonging to the tanager family Thraupidae but now has been placed within the family Cardinalidae. Nothing is known about the coccidian parasites of this stunningly colorful bird. Feces from 2 P. rubra found dead in McCurtain County, Oklahoma were collected and examined for coccidia; 1 was found to be passing a new species of Isospora. Oocysts of Isospora mccurtainensis n. sp. are subspheroidal to ovoidal with a smooth bilayered wall, measure (length × width [L × W]) 21.7 × 19.5 μm, and have a L/W ratio of 1.1; a micropyle and oocyst residuum were absent but a bilobed and refractile polar granule is present. Sporocysts are ellipsoidal and measure 13.9 × 8.6 μm, L/W 1.6; a knoblike Stieda body is present as well as a distinct sub-Stieda body. The sporocyst residuum is composed of a granular compact cluster with a dense, irregular mass of granules lying between and dispersed among the sporozoites. This is the first coccidian reported from P. rubra and, most important, only the first known from the Cardinalidae in the mainland of the United States.

Effects of pine-oak woodland restoration on breeding bird densities in the Ozark-Ouachita Interior Highlands

Restoration is underway to restore lost or degraded remnants of savanna and woodland in the Midwestern United States in the hopes of restoring floristic and wildlife diversity. Information is needed on the effects of pine savanna-woodland restoration on bird abundance to inform management decisions. We conducted point-count surveys for 19 bird species across the gradient of savanna, woodland, and forest in restored and non-restored areas throughout the Ozark-Ouachita Highlands in parts of Missouri, Arkansas, and Oklahoma during the 2013–2015 breeding seasons. We estimated densities of 16 species using distance sampling to account for detection probability, and we determined relationships of bird abundance with management and vegetation variables by evaluating support for a priori models. Densities of early-successional and generalist species were positively related, and interior-forest species negatively related, to restoration. Densities of Brown-headed Nuthatch (Sitta pusilla), Eastern Towhee (Pipilo erythrophthalmus), Eastern Wood-Pewee (Contopus virens), Pine Warbler (Setophaga pinus), Prairie Warbler (Setophaga discolor), Red-headed Woodpecker (Melanerpes erythrocephalus), White-eyed Vireo (Vireo griseus), and Yellow-breasted Chat (Icteria virens) were positively related to prescribed fire activity. Blue-winged Warbler (Vermivora cyanoptera), Kentucky Warbler (Geothlypis formosa), and Yellow-breasted Chat densities were positively related to tree thinning. Many species had higher densities in areas with less canopy cover, tree density, and forest cover. Acadian Flycatcher (Empidonax virescens), Black-and-white Warbler (Mniotilta varia), Ovenbird (Seiurus aurocapilla), Wood Thrush (Hylocichla mustelina), and Worm-eating Warbler (Helmitheros vermivorum) were negatively related to one or more aspects of restoration treatment and generally preferred areas with greater tree density and canopy cover. Summer Tanagers (Piranga rubra) were abundant but density was not strongly related to management or vegetation variables. Restoration provided breeding habitat for disturbance-dependent species and woodland generalists, many of which are species of conservation concern, but canopy cover generally remained too great for species that require more open savanna.

Songbird Nest Survival in Managed Oak Savannas and Woodlands in the Missouri Ozarks

Midwestern savannas and woodlands were once dominant transitional communities but are now considered endangered. Savanna and woodland habitat is being restored and managed throughout the central and east-central portions of the United States, but few studies have investigated the effects of management on songbird nest survival. We monitored songbird nests in managed savanna and woodland sites in southern Missouri from 2009 to 2011 to estimate nest survival and to predict the relationships between nest survival and temporal, vegetation structure, and nest site variables. Daily nest survival was generally higher for the four canopy-nesting species, Acadian flycatcher (Empidonax virescens), blue-gray gnatcatcher (Polioptila caerulea), Eastern wood-pewee (Contopus virens), and summer tanager (Piranga rubra) (0.96-0.98), than the four shrub-nesting species, field sparrow (Spizella pusilla), indigo bunting (Passerina cyanea), prairie warbler (Setophaga discolor), and yellow-breasted chat (Icteria virens) (0.92-0.95), for which we estimated survival rates. In general vegetation structure around the nest had little influence on nest survival, except for canopy cover, which occurred in the top model set for four species. Nest survival of yellow-breasted chats was much higher in areas of lower canopy cover, whereas nest survival of Eastern wood-pewees was moderately lower and indigo buntings and summer tanagers peaked at low and intermediate levels of canopy cover, respectively. Therefore, savanna and woodland management, which tends to open the tree canopy, may benefit some bird species in this forested landscape, particularly those associated with low to intermediate canopy cover.

Patterns of change in body condition in wintering Neotropical-Nearctic migratory birds in shaded plantations in the Andes

Despite the widely accepted idea that shaded plantations are valuable habitats for Neotropical migrants in disturbed landscapes, little empirical evidence is available in relation to the quality of this habitat for Neotropical-Nearctic migratory birds in the Andes. We evaluated the suitability of shaded plantations for overwintering Neotropical-Nearctic migratory birds by examining diurnal and seasonal variation in body condition of migrants in these agroforests in the Andes. During October to April 2008–2009 and 2009–2010, we mist-netted eight species of Neotropical-Nearctic migrants in shaded plantations in the Colombian Andes. Body condition improved throughout the day for Cerulean Warbler (Setophaga cerulea), Blackburnian Warbler (Setophaga fusca), Tennessee Warbler (Leiothlypis peregrina), and especially Canada Warbler (Cardellina canadensis). Similarly, body condition improved across the season for Tennessee Warbler, Rose-breasted Grosbeak (Pheucticus ludovicianus), and Summer Tanager (Piranga rubra). Our results provided additional evidence that several common Neotropical migrants, including species of conservation concern such as Cerulean Warbler and Canada Warbler, may improve their body condition in shaded plantations. However, about half of the species assessed in this study did not show any significant improvement in their body condition (e.g. Red-eyed Vireo). Furthermore, on average, species were in poor condition (i.e. body condition scores were negative) in these coffee farms. Thus, the extent to which this agricultural system represents high quality habitat should be looked with caution. Overall, our data contribute novel information on daily patterns of body condition improvement for several common migratory birds in Andean shaded plantations.

Unusual Concentration of Summer Tanagers (Piranga rubra) in Northwestern Arkansas during Winter 2015-2016

Unusual Concentration of Summer Tanagers (Piranga rubra) in Northwestern Arkansas during Winter 2015-2016

John William Hardy, 1930–2012

John William Hardy, Elective Member of the AOU since 1964 and Fellow since 1971, died in Gainesville, Florida, on October 1, 2012, at 82 years of age. Like many ornithologists, Bill Hardy was a boy birdwatcher, collecting Fuertes cards from his mother’s Arm & Hammer baking soda packages, finding a Summer Tanager nest in the backyard when he was four years old, and at seven helping his uncle install a Purple Martin house in the yard. He would go on to pioneer the creation, collection, and distribution of sound recordings that are now considered as important as study skins and skeletons in many museums’ bird collections. Bill was born on January 12, 1930, in Murphysboro, Illinois, a small town 50 miles north of the junction of the Ohio and Mississippi rivers. His biology teacher at Murphysboro Township High School, E. Esther Smith, was an important influence. She had a strong interest in natural history, and The Auk was among the journals and books available in her laboratory. She joined the AOU in 1949 and remained a member at least until 1985. Through the school’s Audubon Biology Club, which Ms. Smith began, students were encouraged to prepare projects for yearly state science fairs sponsored by the Illinois Academy of Science. These were often small research projects based on original data. One of Bill’s projects, with two other students, was a study of the seasonal occurrence of birds in the state’s seven southernmost counties. Although not publicly distributed, this was the first checklist of the birds of southern Illinois. While in high school, Bill spent a few hours a day closely observing martins, putting into practice the outline for a life-history study in Joseph J. Hickey’s (1943) Guide to Bird Watching. Years later, in 1962, Bill and Richard Johnston, his graduate advisor at the University of Kansas, coauthored a paper based mainly on a Kansas martin colony but including Bill’s Illinois observations from the 1946 season, when a few pairs had double brooded. From 1948 to 1952, Bill continued his education at Southern Illinois University, Carbondale, less than 10 miles from home. He majored in zoology, worked part-time in the Cooperative Fisheries laboratory, and founded the Southern Illinois Bird Club, which held a few meetings but did not survive his graduation. For graduate work, Bill moved out of state, taking a teaching assistantship at Michigan State College (soon, University) in East Lansing. His advisor was George W. Wallace, a fine ornithologist who was at that time working on the first college ornithology textbook, An Introduction to Ornithology (Macmillan 1955). When Bill wrote Wallace’s memorial for The Auk, he noted that Wallace had ‘‘nurtured my qualities and gently began the process of sharpening me for the tough road ahead.’’ The two years in Michigan provided good experiences beyond the connections with Wallace and fellow MSU JOHN WILLIAM HARDY in Florida in 1985. Photo credit: Thomas A. Webber

Breeding bird response to habitat and landscape factors across a gradient of savanna, woodland, and forest in the Missouri Ozarks

Savanna and woodland were once common in the Midwest, but land use changes have led to increasing scarcity of these communities. These transitional habitats are being restored across the Midwest, but few studies have evaluated the response of wildlife to restoration or the vegetative gradient created by management. We conducted point counts for 25 songbirds at sites undergoing savanna or woodland restoration and nearby non-managed forest sites across the Ozark Highlands of Missouri during the 2009–2011 breeding seasons; these sites represented a gradient of canopy cover and tree density from savanna to woodland to forest. We estimated density of 17 species with ⩾50 detections using distance-based models, which adjust estimates by the detection probability. Bird densities were more strongly related to habitat structure, fire history, and landscape composition than simply whether a site was managed or non-managed. Mature forest species such as Acadian Flycatcher (Empidonax virescens), Northen Parula (Setophaga americana), Red-eyed Vireo (Vireo olivaceous), and Worm-eating Warbler (Helmitheros vermivorum) were generally more abundant at points with more trees, higher canopy cover, lower shrub density, and less frequent or no fire in the 20years prior to surveys. Woodland generalists such as Blue-gray Gnatcatcher (Polioptila caerulea), Eastern Wood-Pewee (Contopus virens), Great Crested Flycatcher (Myiarchus crinitus), Summer Tanager (Piranga rubra), and Yellow-billed Cuckoo (Coccyzus americanus) were generally more abundant at points with less landscape forest cover (10-km scale), more large and fewer small trees, intermediate to high canopy cover, lower shrub density, and recent or frequent fire. Early-successional species such as Eastern Towhee (Pipilo erythrophthalmus), Field Sparrow (Spizella pusilla), Indigo Bunting (Passerina cyanea), Kentucky Warbler (Geothlypis formosa), Prairie Warbler (Setophaga discolor), White-eyed Vireo (Vireo griseus) and Yellow-breasted Chat (Icteria virens) were generally more abundant at points with lower canopy cover, recent or frequent fire, and higher shrub density. Brown-headed Cowbirds (Molothrus ater) were more abundant at points with intermediate landscape forest cover and lower canopy cover. Restored sites provided breeding habitat for woodland generalists and early-successional species of conservation concern; however, managed landscapes with more open canopy and herbaceous ground cover may be required for species more indicative of open savannas.

Variation in carotenoid–protein interaction in bird feathers produces novel plumage coloration

Light absorption by carotenoids is known to vary substantially with the shape or conformation of the pigment molecule induced by the molecular environment, but the role of interactions between carotenoid pigments and the proteins to which they are bound, and the resulting impact on organismal coloration, remain unclear. Here, we present a spectroscopic investigation of feathers from the brilliant red scarlet ibis (Eudocimus ruber, Threskiornithidae), the orange-red summer tanager (Piranga rubra, Cardinalidae) and the violet-purple feathers of the white-browed purpletuft (Iodopleura isabellae, Tityridae). Despite their striking differences in colour, all three of these feathers contain canthaxanthin (β,β-carotene-4,4'-dione) as their primary pigment. Reflectance and resonance Raman (rR) spectroscopy were used to investigate the induced molecular structural changes and carotenoid-protein interactions responsible for the different coloration in these plumage samples. The results demonstrate a significant variation between species in the peak frequency of the strong ethylenic vibration (ν(1)) peak in the rR spectra, the most significant of which is found in I. isabellae feathers and is correlated with a red-shift in canthaxanthin absorption that results in violet reflectance. Neither polarizability of the protein environment nor planarization of the molecule upon binding can entirely account for the full extent of the colour shift. Therefore, we suggest that head-to-tail molecular alignment (i.e. J-aggregation) of the protein-bound carotenoid molecules is an additional factor.

Breeding Bird Relationships to Landscape Metrics in Coastal Plain Georgia

Some avian species in the southeastern United States are declining, and population decreases may arise from changes in vegetation type area or structural condition. Our objective was to compare abundance of conservation priority bird species with landscape variables. We found, even in the highly forested Coastal Plain of Georgia, that areal extent and core area of cover types were related to abundance for certain bird species. Acadian flycatcher and field sparrow had models that incorporated positive area variables. Downy woodpecker, northern parula, orchard oriole, prairie warbler, and summer tanager had models that included positive area and edge associations with varying scales and vegetation types. Edge appeared in models for red-bellied woodpecker, blue jay, and brown-headed cowbird. More than half of all species did not have models that met prediction thresholds. Systematic assessment of area requirements for declining species provides information for management, conservation, and research.

Avifauna of the Easternmost Tepui, Tafelberg in Central Suriname

We conducted ornithological surveys of Tafelberg mountain, Suriname, in November–December 2005 and August–September 2008 and also surveyed the adjacent lowlands in both forest and savanna habitats around the Kappel Airstrip. We recorded a total of 283 species, including 27 Guianan endemics and 2 tepuian endemics (Heliodoxa xanthogonys and Hylophilus sclateri). Four species were new for Suriname and represented significant range extensions (Chamaeza campanisona, Hemitriccus inornatus, Turdus leucops and Turdus olivater). First specimen vouchers were collected for Dysithamnus leucostictus, Heliodoxa xanthogonys, Hylophilus sclateri, Catharus minimus and Piranga rubra, and first vocal evidence was obtained for the presence of Aeronautes montivagus in Suriname. The Kappel avifauna (266 species) was composed of species that would be expected to occur in the Guianan lowlands, but the presence of Hemitriccus inornatus was unexpected and implies a much wider distribution for this elusive tody-tyrant. The avifauna of the Tafelberg summit was composed of 170 species including 18 montane species, most of which are known disjunctly from the Guianan highlands further west, but one (Dysithamnus leucostictus) is otherwise known only from the coastal mountains of Venezuela and the Andes. The isolated Tafelberg population of the latter may represent an undescribed species.

Width of riparian buffer and structure of adjacent plantations influence occupancy of conservation priority birds

Conservation of biodiversity on forest landscapes dominated by plantations has become an increasingly important topic, and opportunities to maintain or enhance biodiversity within these forests need to be recognized and applied. Riparian buffers of mature forest retained along streams in managed forest landscapes offer an opportunity to enhance biodiversity across these landscapes. However, influence of buffer width and structure of adjacent plantations on habitat use by birds is not well understood. We modeled probability of occupancy, while accounting for variable detection probabilities, for 16 bird species of regional conservation importance in the Ouachita Mountains of Arkansas, USA. We examined occurrence of breeding birds in streamside management zone (SMZ) buffers embedded in three structural classes of pine plantation: young open-canopy, closed-canopy, and older thinned plantations. Our occupancy models included a positive association with SMZ width for nine bird species associated with mature forests. Models for three early successional species (prairie warbler [Dendroica discolor], white-eyed vireo [Vireo griseus], and northern bobwhite [Colinus virginianus]) included a negative association with SMZ width. Occupancy models for Acadian flycatcher (Empidonax virescens), summer tanager (Piranga rubra), pine warbler (Dendroica pinus), prairie warbler, and northern bobwhite also included structural condition of adjacent plantations, but most species did not appear affected by condition of surrounding plantations. We found diverse responses among species to width of retained SMZs and structure of adjacent plantations; some species apparently benefitted from SMZs >100 m wide, while others benefitted from narrow buffers. Furthermore, most species traditionally associated with mature forests were common in narrow SMZs, regardless of width. Thus, optimal width of SMZs relative to avian conservation depends on the species of greatest conservation interest.

Taxonomic recommendations for British birds: Sixth report

Taxonomic recommendations for British birds: Sixth report

Intraspecific genetic analysis of the summer tanagerPiranga rubra: implications for species limits and conservation

The summer tanagerPiranga rubrais a Neotropical migrant that has experienced noted declines in the southwestern United States caused by extensive habitat loss of native riparian woodlands. This species is composed of two morphologically and behaviorally distinct taxa that traditionally have been recognized as subspecies, each occupying unique habitats in the southern part of North America. Genetic analyses of intraspecific variation are important in studies of threatened or endangered species because they can indicate whether smaller management units exist below the species level and they also provide estimates of within population variability. Using a mitochondrial DNA marker, the intraspecific genetic variation of this species is explored to determine whether the morphologically and behaviorally distinct subspecies are also genetically unique. By using traditional phylogenetic methods and building haplotype networks, results from this study indicate that the subspecies represent two phylogenetic species and should be managed as separate units. In addition, the level of gene flow among geographically isolated populations of the western subspecies is explored using Nested Clade Phylogeographic Analysis and population genetic tests. These analyses show that populations are genetically diverse and that haplotypes are shared across populations. Newly colonized populations are as diverse as older populations. This suggests that as habitat degrades in traditional breeding areas of the summer tanager, if suitable habitat elsewhere becomes available for new populations, these new colonies should be genetically diverse.

Intraspecific genetic analysis of the summer tanager Piranga rubra: implications for species limits and conservation

Intraspecific genetic analysis of the summer tanager Piranga rubra: implications for species limits and conservation

BLOOD PARASITES OF NEARCTIC–NEOTROPICAL MIGRANT PASSERINE BIRDS DURING SPRING TRANS-GULF MIGRATION: IMPACT ON HOST BODY CONDITION

To test the hypothesis that migrants infected with blood parasites arrive on the northern coast of the Gulf of Mexico in poorer condition than uninfected birds, we examined 1705 migrant passerine birds representing 54 species of 11 families from 2 Gulf Coast sites for blood parasites. Three hundred and sixty (21.1%) were infected with 1 or more species of 4 genera of blood parasites. The prevalence of parasites was as follows: Haemoproteus spp. (11.7%), Plasmodium spp. (6.7%), Leucocytozoon spp. (1.3%), and Trypanosoma spp. (1.2%). Both prevalence and density of Haemoproteus spp. infection varied among species. We found no relationship of gender or age with the prevalence of Haemoproteus spp. infection or Plasmodium spp. infection, with the exception of the orchard oriole (Icterus spurius) for which older birds were more likely to be infected with Haemoproteus spp. than younger birds. We also found that scarlet tanagers and summer tanagers infected with species of Haemoproteus have lower fat scores than uninfected individuals and that rose-breasted grosbeaks and Baltimore orioles infected with Haemoproteus spp. have a smaller mean body mass than uninfected individuals. Blood parasites do seem to pose a physiological cost for Neotropical migrant passerines and may be important components of the ecology of these species.

Combining Breeding Bird Survey and Distance Sampling to Estimate Density of Migrant and Breeding Birds

Abstract We combined Breeding Bird Survey point count protocol and distance sampling to survey spring migrant and breeding birds in Vicksburg National Military Park on 33 days between March and June of 2003 and 2004. For 26 of 106 detected species, we used program DISTANCE to estimate detection probabilities and densities from 660 3-min point counts in which detections were recorded within four distance annuli. For most species, estimates of detection probability, and thereby density estimates, were improved through incorporation of the proportion of forest cover at point count locations as a covariate. Our results suggest Breeding Bird Surveys would benefit from the use of distance sampling and a quantitative characterization of habitat at point count locations. During spring migration, we estimated that the most common migrant species accounted for a population of 5000–9000 birds in Vicksburg National Military Park (636 ha). Species with average populations of >300 individuals during migration were: Blue-gray Gnatcatcher (Polioptila caerulea), Cedar Waxwing (Bombycilla cedrorum), White-eyed Vireo (Vireo griseus), Indigo Bunting (Passerina cyanea), and Ruby-crowned Kinglet (Regulus calendula). Of 56 species that bred in Vicksburg National Military Park, we estimated that the most common 18 species accounted for >8150 individuals. The six most abundant breeding species, Blue-gray Gnatcatcher, White-eyed Vireo, Summer Tanager (Piranga rubra), Northern Cardinal (Cardinalis cardinalis), Carolina Wren (Thryothorus ludovicianus), and Brown-headed Cowbird (Molothrus ater), accounted for >5800 individuals.

ACTIVITY BUDGETS OF SUMMER TANAGERS DURING SPRING MIGRATORY STOPOVER

We radio-tracked Summer Tanagers (Piranga rubra) at a Gulf coast stopover site to study en route activity budgets. Tanagers were inactive a majority of the time, with scanning, foraging, and preening being the next most commonly observed activities. When foraging, tanagers primarily used sallying to capture a variety of insect prey. Possible explanations for inactivity during stopover include the need for rest following prolonged flight, muscle repair, reduced digestive capacity, and the short distance remaining to the breeding grounds.