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52 records · Page 3Linked to original sources
USGS1978

Shell thinning and pesticide residues in Texas aquatic bird eggs, 1970

Significant decreases in eggshell thickness were found in 15 of 22 species of aquatic birds in Texas in 1970. Shell thickness reductions of 9 to 15 percent were found in white pelicans (Pelecanus erythrorhynchos), brown pelicans (P .occidentalis), and great blue herons (Ardea herodias). DDT family compounds were found in all eggs, and mean residues ranged from 0.4 ppm in white ibis (Eudocimus albus) to 23.2 ppm in great egrets (Casmerodius albus). GDDT residues were negatively correlated with shell thickness in five species; PCBs were negatively correlated in two. Residues in marine birds were generally lower and more uniform than levels in birds feeding in fresh and brackish water. DDT and dieldrin residues were higher in eggs from colonies near agricultural areas where these insecticides were heavily used; higher PCB residues were consistently associated with urban and industrial areas. Populations of five species have declined and deserve continued study: brown pelican, reddish egret (Dichromanassa rufescens), white-faced ibis (Plegadis chihi), laughing gull (Larus atricilla), and Forster's tern (Sterna forsteri). Population trends of four other species were undetermined and should be followed closely in future years.

Pesticides Monitoring Journal↗
USGS1979

Organochlorine residues in young herons from the upper Mississippi River-1976

Chicks of great blue herons (Ardea herodias) from four heronaries located near South St. Paul, Royalton, and Wabasha, Minnesota, and La Crosse, Wisconsin, were analyzed for organochlorines, Highest mean wet-weight concentrations, 6.43 ppm PCBs. 1.31 ppm DDE, and 1.90 ppm sigma DDT, were found in the South St. Paul chicks. Among chicks from the other three heronries, most levels were similar, but were significantly lower than levels in South St. Paul chicks. Lowest mean organochlorine levels, 0.37 ppm DDE, 0.38 ppm sigma DDT, and 0.22 ppm PCBs, were found in chicks from Royalton. All birds from South St. Paul and La Crosse contained residues of DDT and TDE whereas only one of the 10 birds from Royalton contained DDT and one contained TDE residues. Five of the 12 birds from Wabasha contained DDT; eight contained TDE. Except for PCB residues in La Crosse heron chicks, the rate of organochlorine residue accumulation in the birds was generally less than the rate of dilution caused by growth.

Pesticides Monitoring Journal↗
USGS1981

Organochlorine residues and mortality of herons

Since 1966, 72 herons found dead or moribund in the field have been analyzed for organochlorine chemicals. In addition, 36 herons were obtained through systematic collections, and carcasses were analyzed to determine sublethal exposure to organochlorines. Brains of birds found dead or moribund were analyzed to determine whether the birds had died of organochlorine poisoning. Residues of DDE were found most frequently (96 of 105 carcasses analyzed), PCBs were second (detected in 90 carcasses), and dieldrin and TDE (detected in 37 and 35 carcasses, respectively) were about equal as third and fourth most frequent. Endrin, mirex, toxaphene, and HCB were found least often (8, 9, 9, and 9 carcasses, respectively). At least one organochlorine was found in each carcass, except for six heron chicks found dead in a Maryland heronry. DDE and PCBs were present in highest concentrations; they exceeded 100 ppm in two birds each. Organochlorine concentrations were almost always higher in adult herons than in immature birds. All birds that had hazardous or lethal concentrations in the brain were adults, and most were great blue herons (Ardea herodias). Dieldrin was the chemical most often considered responsible for death. Herons died of suspected DDT and dieldrin poisoning years after the chemicals were banned in the United States. More than 20 percent of the herons found dead or moribund had lethal or hazardous concentrations of organochlorines in the brain.

Alabama, Michigan, Vermont↗
USGS2001

Changes in the nesting populations of colonial waterbirds in Jamaica Bay Wildlife Refuge, New York, 1974-1998

The Jamaica Bay Wildlife Refuge (JBWR) represents the largest protected area for over 300 species of migratory and resident birds on Long Island (LI), New York, and occupies a key position along the Atlantic flyway. We identified changes in nesting populations for 18 species of colonial waterbirds in JBWR and on LI, during 1974 - 1998, to provide a basis for future wildlife management decisions in JBWR and also at nearby John F. Kennedy International Airport. None of the populations was stable over the past 25 years in JBWR or on LI. Some populations in JBWR increased (Laughing Gull L. atricilla Linnaeus, Great Black-backed Gull L. marinus Linnaeus, Forster's Tern Sterna forsteri Nuttall) while others decreased (Herring Gull Larus argentatus Coues, Snowy Egret Egretta thula Molina), but only Cattle Egrets (Bubulcus ibis Linnaeus) have disappeared from the refuge. Common Tern (S. hitundo Linnaeus), Least Tern (S. antillarum Lesson), Roseate Tern (S. dougallii Montagu), Black Skimmer (Rynchops niger Linnaeus), Black-crowned Night Heron (Nycticorax nycticorax Linnaeus) and Great Egret (Ardea alba Linnaeus) populations all increased on LI over the sampling period although the Common Tern colonies in JBWR have been declining since 1986. The continued protection of the colony sites, particularly saltmarsh islands, in JBWR will be important to the conservation efforts of many colonial waterbird populations on Long Island. The JBWR colonies may serve as a source of emigrants to other Long Island colonies, and in some cases, act as a 'sink' for birds immigrating from New Jersey and elsewhere.

Northeastern Naturalist↗
USGS2009

Species differences in the sensitivity of avian embryos to methylmercury

We injected doses of methylmercury into the air cells of eggs of 26 species of birds and examined the dose-response curves of embryo survival. For 23 species we had adequate data to calculate the median lethal concentration (LC50). Based on the dose-response curves and LC50s, we ranked species according to their sensitivity to injected methylmercury. Although the previously published embryotoxic threshold of mercury in game farm mallards (Anas platyrhynchos) has been used as a default value to protect wild species of birds, we found that, relative to other species, mallard embryos are not very sensitive to injected methylmercury; their LC50 was 1.79 ug/g mercury on a wet-weight basis. Other species we categorized as also exhibiting relatively low sensitivity to injected methylmercury (their LC50s were 1 ug/g mercury or higher) were the hooded merganser (Lophodytes cucullatus), lesser scaup (Aythya affinis), Canada goose (Branta canadensis), double-crested cormorant (Phalacrocorax auritus), and laughing gull (Larus atricilla). Species we categorized as having medium sensitivity (their LC50s were greater than 0.25 ug/g mercury but less than 1 ug/g mercury) were the clapper rail (Rallus longirostris), sandhill crane (Grus canadensis), ring-necked pheasant (Phasianus colchicus), chicken (Gallus gallus), common grackle (Quiscalus quiscula), tree swallow (Tachycineta bicolor), herring gull (Larus argentatus), common tern (S terna hirundo), royal tern (Sterna maxima), Caspian tern (Sterna caspia), great egret (Ardea alba), brown pelican (Pelecanus occidentalis), and anhinga (Anhinga anhinga). Species we categorized as exhibiting high sensitivity (their LC50s were less than 0.25 ug/g mercury) were the American kestrel (Falco sparverius), osprey (Pandion haliaetus), white ibis (Eudocimus albus), snowy egret (Egretta thula), and tri-colored heron (Egretta tricolor). For mallards, chickens, and ring-necked pheasants (all species for which we could compare the toxicity of our injected methylmercury with that of published reports where methylmercury was fed to breeding adults and was deposited into the egg by the mother), we found the injected mercury to be more toxic than the same amount of mercury deposited naturally by the mother. The rank order of sensitivity of these same three species to methylmercury was, however, the same whether the methylmercury was injected or maternally deposited in the egg (i.e., the ring-necked pheasant was more sensitive than the chicken, which was more sensitive than the mallard). It is important to note that the dose-response curves and LC50s derived from our egg injections are useful for ranking the sensitivities of various species but are not identical to the LC50s that would be observed if the mother bird had put the same concentrations of mercury into her eggs; the LC50s of maternally deposited methylmercury would be higher.

Archives of Environmental Contamination and Toxico↗
USGS1991

Use of Wetland Habitats by Selected Nongame Water Birds in Maine

We examined the use of 87 palustrine and lacustrine wetlands by nongame water birds in central and eastern Maine using 3,527 h of observation (1,501 visits) made during April-August, 1977-85. Wetlands used by 15 species of water birds were distinguished from those not used, according to 20 habitat features. The species were the common loon (Gavia immer) , pied-billed grebe (Podilymbus podiceps), double-crested cormorant (Phalacrocorax auritus), American bittern (Botaurus lentiginosus), great blue heron (Ardea herodias), green-backed heron (Butorides striatus), osprey (Pandion haliaetus), bald eagle (Haliaeetus leucocephalus), northern harrier (Circus cyaneus), Virgima rail (Rallus limicola), sora (Porzana carolina), spotted sandpiper (Actitis macularia), common snipe (Gallinago gallinago), herring gull (Larus argentatus), and belted kingfisher (Ceryle alcyon). Predictive models of habitat use were developed for each species. Water birds were classified by similarity of habitats used, and species use was contrasted by wetland type. Smaller, isolated wetlands were used by fewer (P < 0.05) species than larger wetlands in complexes; many species had large area-requirements (pied-billed grebe, common loon, herring gull, double-crested cormorant, bald eagle) or preferred to use wetlands near other wetlands (common loon, herring gull, great blue heron, spotted sandpiper, osprey, bald eagle). Wetland area contributed more to overall variation in species richness on wetlands than wetland isolation, although on small wetlands (<3.6 ha) isolation was a better predictor of species richness than wetland area. Wetlands with intermediate amounts (33-66%) of emergent vegetation supported more species (P< 0.05) than closed (>66%) or open (<33%) wetlands. Low pH typified wetlands used by large-bodied piscivores (common loon, cormorant, osprey). Other water birds were associated with more densely vegetated, chemically buffered wetlands. Habitat features associated with wetland use by each waterbird species are reported, as are numerical responses of waterbird populations to wetland features and estimates of annual variation in habitat occupancy. Lacustrine wetlands supported a distinct, low diversity community of water birds, including most fish-eating species. Waterbird diversity at forested palustrine wetlands was intermediate between lacustrine communities and more species-rich assemblages at palustrine emergent and scrub-shrub wetlands. Regional variation in wetland characteristics and water bird use was associated with surficial geology, soils, and management practices. Management for nongame water birds in Maine should consider providing emergent and aquatic-bed vegetation with variable cover-to-water ratios, accommodating species-specific habitat needs, focusing on species of restricted distribution and low abundance, and maintaining wetland complexes. Bird use and habitat information from 87 wetlands and models of habitat selection for each species are provided in appendixes.

Fish and Wildlife Research↗
USGS1997

Nesting season food habits of 4 species of Herons and Egrets at Lake Okeechobee, Florida

Based on the composition of nestling regurgitations collected during 3 breeding seasons, fish were the most important prey group for Great Egrets (Ardea alba: N = 200 nest-day samples; aggregate percent biomass [APB] = 73.4%), Snowy Egrets (Egretta thula: N = 115; APB = 91.4%), and Tricolored Herons (E. tricolor. N = 68; APB = 97.3%). For Little Blue Herons (E. caerulerr. N = 57), grass shrimp (Palaemoneles paludosus; APB = 39.7%) ranked higher in overall importance than all fishes combined (APB = 36.5%). Dietary overlap, as measured by Schoener's Similarity Index, was greatest between Snowy Egrets and Tricolored Herons (77%) and lowest between Tricolored Herons and Little Blue Herons (30%). Diet diversity, as measured by Shannon's Index, was highest for Great Egrets (2.04), intermediate for Snowy Egrets (1.71) and Tricolored Herons (1.68), and lowest for Little Blue Herons (1.60). Great Egrets ate a wider variety of fish species and sizes, especially larger fishes, and more crayfish than the other species. Little Blue Herons ate fewer fish and more grass shrimp and insects, and ate smaller forage fishes than Tricolored Herons but similar-sized fish as Snowy Egrets. The coarse-scale trophic composition of Snowy Egret and Tricolored Heron diets did not differ significantly, but Tricolored Herons ate larger forage fishes than Snowy Egrets. Pronounced interannual and intercolony variation in diet composition suggested that Great Egrets and Little Blue Herons switched prey types as hydrologic conditions and habitat availability changed. Conversely, lack of such variation suggested that Snowy Egrets and Tricolored Herons adjusted their foraging tactics to ensure contin-ued encounters with preferred prey despite changing habitat conditions. These results are generally consistent with other published data, help confirm some generalizations about foraging strategies and patterns of niche differentiation among these ecologically similar species, and have implications for managing the Lake Okeechobce ecosystem.

Waterbirds↗
USGS1979

Dispersal and migratory patterns of San Francisco Bay produced herons, egrets, and terns

San Francisco Bay, California, including its fringing marshes, supports a large and diverse water related avifauna (Grinnell and Wythe 19271 Sibley 1952, Gill 1973, 1977). Certain of man's alterations of the Bay's shallower wetlands have resulted in increased habitat diversity which has allowed colonization by several species of birds including some colonial nesting species. The extensive dikes associated with salt production and some areas of higher ground created by dredge spoils have provided increased tide-free substratum, some of it insular, suitable for nesting. The resulting numbers of Snowy Egrets ( Egretta thula) , Black-crowned Night Herons ( Nycticorax nycticorax ), Forster's Terns ( Sterna forsteri ), and Caspian Terns ( Sterna caspia ) using these areas now represent a significant portion of the northern California breeding populations of these species. In conjunction with a study of the breeding birds of the South San Francisco Bay Estuary (Gill 1973, 1977) from 1971 to 1973, we banded 187 Great Blue Herons ( Ardea herodias ), 1499 Snowy Egrets, 1615 Black-crowned Night Herons, 2943 Forster's Terns, and 743 Caspian Terns; often this represented a substantial portion of these species banded in the western states during these years (Table 1). Recoveries from these bandings through 1977 plus additional recoveries from a few earlier and some more recent bandings provide the data for this report on dispersal patterns and migration of San Francisco Bay produced herons, egrets, and terns.

California↗
USGS2025

Predation of Lost River and Shortnose suckers by piscivorous colonial waterbirds in the Upper Klamath Basin: An analysis of predation effects during 2021–2023

Previously published research indicated that predation by piscivorous colonial waterbirds in the Upper Klamath Basin was a source of mortality for Lost River suckers ( Deltistes luxatus ) and Shortnose suckers ( Chasmistes brevirostris ), including mortality of Sucker Assisted Rearing Program (SARP) fish. Avian predation on recently released Chinook Salmon ( Oncorhynchus tshawytscha ) in the Upper Klamath Basin has not been studied. To provide fisheries managers with the most up-to-date information, we estimated predation rates on passive integrated transponder tagged (PIT) suckers and Chinook Salmon by breeding colonies of American White Pelicans ( Pelecanus erythrorhynchos ), Double-crested Cormorants ( Nannopterum auritum ), Caspian Terns ( Hydroprogne caspia ), California Gulls ( Larus californicus ), Ring-billed Gulls ( L. delawarensis ), Great Blue Herons ( Ardea herodias ), and Great Egrets ( A. alba ) during 2021–2023. Predation rate estimates were variable depending on the fish species, age-class (juvenile, adult), waterbody (Upper Klamath Lake, Clear Lake Reservoir, Sheepy Lake), and year. Results indicated that avian predation rates were highest on juvenile suckers and on suckers in Clear Lake Reservoir. SARP fish released in the spring/summer were more likely to be consumed by breeding birds than those released in the fall/winter, although the number of fish released in the fall/winter that survived to the spring/summer was unknown and could bias predation estimates low. Predation rate estimates on Chinook Salmon varied by year, release timing (spring/summer, fall/winter), release location (Upper Klamath Lake, Klamath River), and age-class (subyearling, yearling). The location and size of colonies were also highly variable depending on the bird species, colony location, and year. Future research could focus on identifying biotic and abiotic factors associated with sucker susceptibility to piscivorous colonial waterbirds and determining to what degree avian predation limits the recovery of suckers in the Upper Klamath Basin.

California, Oregon↗
USGS2025

Predation of Lost River and Shortnose Suckers by piscivorous colonial waterbirds in the Upper Klamath Basin: An analysis of predation effects in 2024

The recovery of endangered fish requires an understanding of the sources of mortality that regulate population dynamics. We estimated avian predation rates on passive integrated transponder tagged (PIT) adult and juvenile Lost River Suckers ( Deltistes luxatus ), Shortnose Suckers ( Chasmistes brevirostris ), Klamath Largescale Suckers ( C. snyderi ), and juvenile Chinook Salmon ( Oncorhynchus tshawytscha ) by breeding colonies of American White Pelicans ( Pelecanus erythrorhynchos ), Doublecrested Cormorants ( Nannopterum auritum ), Caspian Terns ( Hydroprogne caspia ), California Gulls ( Larus californicus ), Ring-billed Gulls ( L. delawarensis ), Great Blue Herons ( Ardea herodias ), Great Egrets ( A. alba ), and Black-crowned Night Herons ( Nycticorax nycticorax ) in the Upper Klamath Basin (UKB) in 2024. Results from 2024 contribute to a long-term (2009–2023) avian predation dataset in the UKB. The largest numbers of pelicans (1,422 breeding adults), terns (610 adults), and gulls (3,872) were nesting on islands in Clear Lake Reservoir. The largest number of cormorants (787 adults) were on an island in Sheepy Lake, and the largest numbers of herons (450 adults) were at arboreal sites adjacent to Upper Klamath Lake. Predation rate estimates in 2024 were highly variable depending on the fish species, ageclass (juvenile, adult), waterbody (Upper Klamath Lake, Clear Lake Reservoir, Sheepy Lake, Tule Lake), and release or reencounter date for juveniles (spring/summer or fall/winter). Results indicated that avian predation rates were the highest on wild and hatchery juvenile suckers. An estimated 19.4% (95% credible interval = 15.7–24.0) of Sucker Assisted Rearing Program (SARP) hatchery fish, released or reencountered during the spring and summer, were consumed by the colonies included in the study. Predation was also appreciable on wild juvenile suckers in Clear Lake Reservoir (13.4% [9.9–17.7]) and SARP juvenile suckers released or reencountered in Upper Klamath Lake in the fall/winter (11.2% [9.6 13.2]). Predation rate estimates on adult suckers were significantly lower than those of juvenile suckers, with predation ranging from 0.3% (0.2–0.7) to 2.6% (1.9–3.5) in adult Lost River Suckers in Upper Klamath Lake and adult Shortnose Suckers-Klamath Largescale Suckers in Clear Lake Reservoir, respectively. Predation rate estimates on juvenile Chinook Salmon, which were released in fall/winter, were 2.9% (2.2–3.6). Pelican (312 adults) and cormorant (301 adults) colonies in Upper Klamath Lake, however, were not scanned for fish PIT tags following breeding season, resulting in minimum estimates of predation on UKB fishes (both suckers and Chinook Salmon) in 2024. Future research could examine what biotic and abiotic factors best explain variation in predation and to what degree predation limits sucker survival using the long-term dataset that has been complied to date.

California, Oregon↗
USGS2007

Space use by Black-necked Stilts Himantopus mexicanus in the San Francisco Bay estuary

We examined space use by Black-necked Stilts Himantopus mexicanus in the San Francisco Bay estuary, USA, to better understand how shorebirds use their Pacific Flyway landscape. These efforts are particularly important in the San Francisco Bay estuary where ongoing large-scale restoration projects are rapidly changing the mosaic of wetland habitats. We radio-marked 59 stilts and tracked individuals for up to four months and found no difference in home range size by sex or between North and South Bay subregions. We did find differences in home range size by capture site. Mean home range was 283.5 ha and movement from capture sites was 4.5 km. We used cluster analysis to calculate number of focal areas for individuals and found that overall space requirements were larger for stilts with multiple centres of activity. Birds with multiple use areas were often those that bred in vegetated marshes and moved into salt ponds when their nests failed or after chicks hatched. In the South Bay subregion, salt pond use was greater than availability in core use home range areas despite comprising the largest proportion of available habitat. Tidal salt marsh restoration from former salt ponds may reduce available habitat of invertebrate prey species that depend on hypersaline habitats; retention of some shallow, mid-salinity managed ponds may mitigate this loss. A better understanding of the space use and habitat requirements of stilts will provide for more specific habitat and management recommendations in areas targeted for wetland restoration, contributing to better conservation of shorebird populations along the Pacific Flyway.

Ardea↗
USGS2010

Colony attendance patterns by mated Forster's Terns Sterna forsteri using an automated data-logging receiver system

In order to examine 24-hour colony attendance patterns by mated Forster's Terns Sterna forsteri in South San Francisco Bay, California, during incubation and chick-rearing stages, we radio-marked 10 individuals consisting of five pairs and recorded colony attendance using an automated data-logging receiver system. We calculated and analyzed five variables: the total attendance time by pairs and individuals, the duration of individual attendance bouts, and the duration both members of a pair either overlapped in colony attendance or were both absent from the colony. The percentage of time spent on the colony by at least one individual of a pair was highest during incubation and declined during chick rearing. Overall, male terns spent a greater proportion of time diurnally attending the colony than females. Females spent a greater proportion of time on colony at night, and without these nocturnal records, we would have reported overall female colony attendance rates as being much lower. Despite sex-specific differences in attendance rates, the length of attendance bouts did not differ between the sexes. Simultaneous colony attendance by both members of a pair was high at night, but during the day, pairs infrequently overlapped in their colony attendance and both members were frequently absent. Our datalogging system functioned well, and our data illustrates the importance of collecting 24-hour records when considering attendance rates.

Ardea↗
USGS2014

Post-breeding migration of Dutch-breeding black-tailed godwits: timing, routes, use of stopovers, and nonbreeding destinations

Conservation of long-distance migratory shorebirds is complex because these species use habitats spread across continents and hemispheres, making identification of critical habitats and potential bottlenecks in the annual cycle especially difficult. The population of Black-tailed Godwits that breeds in Western Europe, Limosa limosa limosa , has declined precipitously over the past few decades. Despite significant efforts to identify the root causes of this decline, much remains unclear. To better understand the migratory timing, use of stopover and nonbreeding sites, and the potential impact of breeding success on these parameters, we attached 15 Argos satellite transmitters and 10 geolocation tracking devices to adult godwits nearing completion of incubation at breeding sites in southwest Friesland, The Netherlands during the spring of 2009. We successfully tracked 16 adult godwits for their entire southward migration and two others for part of it. Three migration patterns and four regions of use were apparent. Most godwits left their breeding sites and proceeded south directly to stopover sites in the Mediterranean &mdash; e.g. Spain, Portugal, and Morocco &mdash; before flying on to non-breeding sites in West Africa. Other individuals spent the entire nonbreeding season in the Mediterranean. A third pattern included a few individuals that flew nonstop from their Dutch breeding sites to nonbreeding sites in West Africa. Tracking data from this study will be immediately useful for conservation efforts focused on preserving the dispersed network of sites used by godwits during their southward migration.

Friesland↗
USGS2015

Habitat edges have weak effects on duck nest survival at local spatial scales

Edge effects on nesting success have been documented in breeding birds in a variety of contexts, but there is still uncertainty in how edge type and spatial scale determine the magnitude and detectability of edge effects. Habitat edges are often viewed as predator corridors that surround or penetrate core habitat and increase the risk of predation for nearby nests. We studied the effects of three different types of potential predator corridors (main perimeter roads, field boundaries, and ATV trails within fields) on waterfowl nest survival in California. We measured the distance from duck nests to the nearest edge of each type, and used distance as a covariate in a logistic exposure analysis of nest survival. We found only weak evidence for edge effects due to predation. The best supported model of nest survival included all three distance categories, and while all coefficient estimates were positive (indicating that survival increased with distance from edge), 85% coefficient confidence intervals approached or bounded zero indicating an overall weak effect of habitat edges on nest success. We suggest that given the configuration of edges at our site, there may be few areas far enough from hard edges to be considered &lsquo;core&rsquo; habitat, making edge effects on nest survival particularly difficult to detect.

Ardea↗
USGS2004

Differential spring migration by male and female Western Sandpipers at interior and coastal stopover sites

Western Sandpipers Calidris mauri are differential migrants on their non-breeding areas, with females wintering farther south. Earlier passage of males in the spring has been explained by sexual differences in winter latitude (male-biased sex ratios at more northerly areas) and onset of migration (males departing earlier). We investigated sex differences during spring migration by capturing and radio-marking Western Sandpipers at two Pacific coast sites, San Francisco Bay, California and Grays Harbor, Washington and at a Great Basin interior wetland, Honey Lake, California. We monitored northward migration of 132 radio-marked birds at a network of 12 major stopover sites and 4 breeding areas. At the banding sites, we observed differences in sex by date and site, with males preceding females. We found sex differences in departure time of radio-marked birds from the banding site. their arrival time at the Copper River Delta, Alaska (our most frequently used stopover site), and in the likelihood that a stopover was used. Our data suggest that by mid to late April, migration timing becomes more compressed and sex differences are less pronounced and harder to detect.

Ardea↗