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We examined the relationship between singing behavior and breeding status in the American Redstart (Setophaga ruticilla) by analyzing song rates, singing mode (Repeat or Serial), and variability of song delivery in relation to the age and breeding status of 129 males in the Hubbard Brook Experimental Forest, New Hampshire. Unpaired males spent most of their time (>90%) after dawn singing in Repeat mode, whereas paired males sang sporadically, in Serial as well as Repeat mode (51% of their singing time). Males who lost their mates sang in Repeat mode at rates indistinguishable from males who had not yet obtained a mate. Overall, unpaired males sang in Repeat mode at significantly higher and less variable rates than did paired males. Although a larger proportion of second-year males were unpaired than after-second-year males, we found no evidence that age affected singing behavior.
We also assessed the effect of pairing status on male detectability in song-based monitoring surveys (e.g., point counts), and we suggest a field protocol for identifying unpaired males. Simulations of 5-min field samples, obtained from continuous samples >3 hr in duration, suggest that human listeners would be twice as likely to detect unpaired males as paired males. This result suggests that surveys based on aural detections may be biased in favor of unpaired males. In our population, >90% of males who sang >40 Repeat songs in 5 min were unpaired. Unpaired males were >3 times as likely as paired males to sing only Repeat songs in a given 5-min period. These results suggest that it may be possible to identify unpaired male American Redstarts by their high singing rates of exclusively Repeat songs.
At early breeding stages, male woodpeckers invest heavily in nest construction and defense, but parental contributions to brood defense among Picidae are not well known. We studied the Northern Flicker (Colaptes auratus) to determine whether sex, age, brood size, body size, or body condition influenced defense behavior. When presented with a model predator (red squirrel, Tamiasciurus hudsonicus) during the brood-rearing period, parents exhibited a range of behaviors, such as blocking the nest hole, diving at the model, and striking the model; however, defense scores did not differ between males and females aged 1, 2, or 3 years old. Although we predicted that defense level would be positively correlated with brood size, we found no such relationship. Adult body size and condition also were not related to defense intensity. We conclude that the sexes may exhibit similar levels of defense because they have similar apparent annual survival rates and males are only slightly larger than females. If flickers optimize clutch size according to the number of offspring they can rear, then there may be no relationship between defense and brood size.
Balsam fir (Abies balsamea) understory may be an important predictor of Black-throated Blue Warbler (Dendroica caerulescens) and Veery (Catharus fuscescens) distributions in northern hardwood forests that are heavily browsed by white-tailed deer (Odocoileus virginianus). We examined the abundance and age ratios of Black-throated Blue Warblers, and the abundance of Veerys, in 16 plots of hardwood forest with different understory composition within a heavily browsed region of the Hiawatha National Forest in Michigan's eastern Upper Peninsula. Four of these 36-ha plots had minimal understory and 12 had dense understory with variable amounts of balsam fir. Black-throated Blue Warbler abundance was significantly greater in plots with an average of 27% balsam fir understory cover than in plots dominated by deciduous understory; no Black-throated Blue Warblers were detected on the minimal understory plots. Age ratios did not differ significantly relative to balsam fir understory density. Veery abundance also did not vary with balsam fir understory density, but it increased with overall understory density. In forests such as these, where deer are abundant but rarely browse balsam fir, active management of balsam fir understory could provide key habitat for sustaining populations of Black-throated Blue Warblers and Veerys. We recommend that managers consider the presence of balsam firs in the understory when planning forest harvests in deer-impacted areas, so that they leave some balsam fir and stagger the cutting of stands with balsam fir over time to create and maintain heterogeneous understory structure.
We compared migrating behavior of Broad-winged Hawks (Buteo platypterus) at two sites along their migration corridor: Hawk Mountain Sanctuary in eastern Pennsylvania and the Kéköldi Indigenous Reserve in Limón, Costa Rica. We counted the number of times focal birds intermittently flapped their wings and recorded the general flight type (straight-line soaring and gliding on flexed wings versus circle-soaring on fully extended wings). We used a logistic model to evaluate which conditions were good for soaring by calculating the probability of occurrence or absence of wing flaps. Considering that even intermittent flapping is energetically more expensive than pure soaring and gliding flight, we restricted a second analysis to birds that flapped during observations, and used the number of flaps to evaluate factors influencing the cost of migration. Both the occurrence and extent of flapping were greater in Pennsylvania than in Costa Rica, and during periods of straight-line soaring and gliding flight compared with circle-soaring. At both sites, flapping was more likely during rainy weather and early and late in the day compared with the middle of the day. Birds in Costa Rica flew in larger flocks than those in Pennsylvania, and birds flying in large flocks flapped less than those flying alone or in smaller flocks. In Pennsylvania, but not in Costa Rica, the number of flaps was higher when skies were overcast than when skies were clear or partly cloudy. In Costa Rica, but not in Pennsylvania, flapping decreased as temperature increased. Our results indicate that birds migrating in large flocks do so more efficiently than those flying alone and in smaller flocks, and that overall, soaring conditions are better in Costa Rica than in Pennsylvania. We discuss how differences in instantaneous migration costs at the two sites may shift the species' migration strategy from one of time minimization in Pennsylvania to one of energy minimization in Costa Rica.
Coloniality is unusual among Scolopacidae. At Churchill, Manitoba, however, the small, remnant population of Semipalmated Sandpipers (Calidris pusilla) is highly clumped, with nesting density approximating 3–4 pairs/ha, and should be considered colonial. The species exhibits high fidelity to territory, mates, and nest sites—behaviors that promote rapid pair formation and allow experienced birds to increase their reproductive success by nesting earlier than pairs forming for the first time. The value of experience and early nesting was evidenced by the fact that six of seven returning young were produced by experienced pairs and had hatched on the first day of their respective nesting seasons. Nests were placed in dry locations very near open water. Those adjacent to small shrubs had slightly greater success, and young produced from these nests had much higher rates of return than those from nests placed amid sedges. In other parts of their breeding range, Semipalmated Sandpipers are also clumped and seem likely to be colonial. If so, estimates of breeding populations derived from indirect methods, such as habitat assessment from aerial photographs, will have limited applicability and will need to be complemented by ground-truthing.
Human recreational disturbance and its effects on wildlife demographics and behavior is an increasingly important area of research. We monitored the nesting success of American Oystercatchers (Haematopus palliatus) in coastal North Carolina in 2002 and 2003. We also used video monitoring at nests to measure the response of incubating birds to human recreation. We counted the number of trips per hour made by adult birds to and from the nest, and we calculated the percent time that adults spent incubating. We asked whether human recreational activities (truck, all-terrain vehicle ;obATV;cb, and pedestrian traffic) were correlated with parental behavioral patterns. Eleven a priori models of nest survival and behavioral covariates were evaluated using Akaike's Information Criterion (AIC) to see whether incubation behavior influenced nest survival. Factors associated with birds leaving their nests (n = 548) included ATV traffic (25%), truck traffic (17%), pedestrian traffic (4%), aggression with neighboring oystercatchers or paired birds exchanging incubation duties (26%), airplane traffic (1%) and unknown factors (29%). ATV traffic was positively associated with the rate of trips to and away from the nest (β1 = 0.749, P < 0.001) and negatively correlated with percent time spent incubating (β1 = −0.037, P = 0.025). Other forms of human recreation apparently had little effect on incubation behaviors. Nest survival models incorporating the frequency of trips by adults to and from the nest, and the percentage of time adults spent incubating, were somewhat supported in the AIC analyses. A low frequency of trips to and from the nest and, counter to expectations, low percent time spent incubating were associated with higher daily nest survival rates. These data suggest that changes in incubation behavior might be one mechanism by which human recreation affects the reproductive success of American Oystercatchers.
We used implanted satellite transmitters to track the northbound (spring) and southbound (fall) migration and possible breeding locations of three Long-tailed Ducks (Clangula hyemalis) wintering on western Lake Ontario in Ontario, Canada. The birds exhibited short, rapid migration movements punctuated by extended periods of up to 30 days at staging areas. For much of the nesting period (∼10 June to 10 July), the birds remained inland of western Hudson Bay in Nunavut. During fall migration, they circumnavigated Hudson Bay to its eastern coast, opposite the coast they had followed in spring, for a mean travel distance of 6,760 km. Identification of these previously unknown, key migration sites fills some important information gaps on Long-tailed Ducks in eastern Canada, and it augments what is known about important coastal marine habitats in the Arctic.
Tree Swallows (Tachycineta bicolor) show one of the highest levels of extra-pair mating among bird species, yet extra-pair copulations are rarely observed. Despite the suggestion that extra-pair copulations could be taking place away from nest sites, very little is known about movement patterns of individual Tree Swallows during the pre-laying and laying periods. We used radio telemetry to track movement patterns of four female Tree Swallows at dawn and dusk during the pre-laying and laying periods. Our tracking results indicate that individual females differed in their movement patterns: some remained close to their nest site on multiple nights while others were rarely detected near their nest box at night. Despite differences in movement patterns, all four females that we tracked produced extra-pair offspring for which we were unable to identify extra-pair sires, even after sampling the majority of males breeding within our nest-box grids. Despite the small sample size, our results confirmed extensive Tree Swallow movement away from nest-box grids during the pre-laying and laying periods. This highlights the need for future studies of mating behavior away from the nesting site, particularly for species that forage and/or roost in communal areas during their fertile period.
Cuban Parrots (Amazona leucocephala) on the island of Great Abaco in the Bahamas forage and nest in native pine forests. The population is unique in that the birds nest in limestone solution holes on the forest floor. Bahamian pine forests are fire-dependent with a frequent surface fire regime. The effects of fire on the parrots, especially while nesting, are not well known. We measured ambient conditions inside a cavity characteristic of the Cuban Parrot's Abaconian population as a prescribed fire passed over it. Cavity conditions were relatively benign; although temperatures immediately outside the cavity rose to >800° C, inside temperatures increased only 5° C at 30 cm inside the entrance and 0.4° C at the cavity floor (cavity depth was ∼120 cm). CO2 levels briefly rose to 2,092 ppm as the flames passed, but dropped to nearly ambient levels approximately 15 min later. Smoke levels also were elevated only briefly, with 0.603 mg of total suspended particulates filtered from 0.1 m3 of air. Smokey conditions lasted approximately 20 min.
Open population models using capture-mark-recapture (CMR) data have a wide range of uses in ecological and evolutionary contexts, including modeling of stopover duration by migratory passerines. In using CMR approaches in novel contexts there is a need to determine the conditions under which open population models may be employed effectively. Our goal was to determine whether there was a simple a priori mechanism of determining the conditions under which CMR models could be used effectively in the study of avian stopover ecology. Using banding data (n = 188 capture histories), we examined the challenges of using CMR-based models due to parameter inestimability, adequacy of descriptive power (Goodness-of-Fit, GOF), and parameter uncertainty. These issues become more apparent in studies with limited observations in a capture history, as is often the case in studies of avian stopover duration. Limited sample size and sampling intensity require an approach to reducing the number of fitted parameters in the model. Parameter estimability posed the greatest restriction on the utility of open population models, with high parameter uncertainty posing a lesser challenge. Results from our study also indicate the need for >10 observations per estimated parameter (approximately 3 birds captured or recaptured per day) to provide a reasonable chance of successfully estimating all model parameters.
We evaluated maximum diving depth and time spent at the nest of fledging Blue-footed Boobies (Sula nebouxii) at Isla El Rancho, Sinaloa, in the Gulf of California, Mexico. Within three consecutive 10-day post-fledging intervals, maximum diving depth was highly variable, but was not affected by sex, weight, or body condition. During the first days of post-fledging flight, maximum diving depth increased rapidly. By the second week after first flight, the plunge-dives of juveniles were almost as deep as those of adults. Parental care and attachment to the nest lasted several additional weeks (up to 40 days after first flight). Although their diving capacity rapidly reached a level similar to that of the adults, it appeared that juvenile boobies took much longer in acquiring other foraging skills.
We examined vegetative and thermal aspects of roost-site selection in urban Yellow-billed Magpies (Pica nuttalli) in Sacramento, California, from winter 2003 to spring 2004. Vegetation used for roosting included cultivated species such as glossy privet (Ligustrum lucidum), English ivy (Hedera helix), and white mulberry (Morus alba), and native species such as interior live oak (Quercus wislizeni), valley oak (Q. lobata), and California laurel (Umbellularia californica). Percent canopy cover was consistently high (mean = 94% ± 1.9 SD). Mean roost height was 9.7 m ± 3.5 SD and the mean height at which magpies roosted was 6.6 m ± 2.0 SD. Communal roosts were generally located within or near riparian corridors. Magpies roosted in relatively warm microhabitats, but they did not appear to obtain a thermal advantage by roosting communally. The timing of roost occupancy was restricted primarily to times when the roost was thermally advantageous.
Reclaimed surface coal mines in southwestern Indiana support many grassland and shrub/ savanna bird species of conservation concern. We examined the nesting success of birds on these reclaimed mines to assess whether such “unnatural” places represent productive breeding habitats for such species. We established eight study sites on two large, grassland-dominated mines in southwestern Indiana and classified them into three categories (open grassland, shrub/savanna, and a mixture of grassland and shrub/savanna) based on broad vegetation and landscape characteristics. During the 1999 and 2000 breeding seasons, we found and monitored 911 nests of 31 species. Daily nest survival for the most commonly monitored grassland species ranged from 0.903 (Dickcissel, Spiza americana) to 0.961 (Grasshopper Sparrow, Ammodramus savannarum). Daily survival estimates for the dominant shrub/savanna nesting species ranged from 0.932 (Brown Thrasher, Toxostoma rufum) to 0.982 (Willow Flycatcher, Empidonax traillii). Vegetation and landscape effects on nesting success were minimal, and only Eastern Meadowlarks (Sturnella magna) showed a clear time-of-season effect, with greater nesting success in the first half of the breeding season. Rates of Brown-headed Cowbird (Molothrus ater) parasitism were only 2.1% for grassland species and 12.0% for shrub/savanna species. The nesting success of birds on reclaimed mine sites was comparable to that in other habitats, indicating that reclaimed habitats on surface mines do not necessarily represent reproductive traps for birds.
We examined age- and sex-related differences in the timing of Wilson's Warbler (Wilsonia pusilla pileolata) migration at four locations in Alaska: Fairbanks, Tok, Mother Goose Lake, and Yakutat. We captured Wilson's Warblers with mist nets for ≥5 years during spring (northbound) and autumn (southbound) migration. In spring, males passed through our two northernmost sites—Tok and Fairbanks—earlier than females. During autumn, timing of adult migration did not differ by sex, but immatures passed through earlier than adults at all four sites. During previous studies of autumn passage sampled at lower latitudes, the lack of age-related differences in migration timing could be attributed to adults migrating faster than immatures (i.e., if immatures from higher latitudes began migration earlier than the adults, then the adults may have caught up to them at lower latitudes) or to the mixing of breeding populations from different locales. Autumn migration of adults and immatures netted at our two southernmost sites, both coastal locations, preceded migration at our two interior sites. These site-specific differences in the timing of autumn migration are likely the result of our coastal stations sampling birds that breed farther south and arrive earlier than birds breeding in more northerly regions of Alaska (and sampled at our interior stations). Early-arriving populations are likely able to complete their breeding season activities earlier and, subsequently, initiate their autumn migration earlier.
Loss of oak woodlands to vineyard development in California is a growing concern to conservationists. Analyzing breeding performance of birds that nest in and around vineyards versus those that nest in nearby native habitat can provide information on the suitability of vineyard environments to birds. We placed predator-protected nest boxes in vineyard and oak-savannah habitats and monitored nest-box occupancy, nesting success, and life history characteristics of Western Bluebirds (Sialia mexicana) that used the boxes. Western Bluebirds were common occupants in both habitats, occupying >50% of available nest boxes. Analysis using program MARK revealed that nest survival was not associated with habitat type; however, clutch size was greater and nests were initiated earlier in vineyard than in oak-savannah habitat. Our results suggest that when naturally occurring nest sites are limiting, vineyards could be converted to good breeding habitat for Western Bluebirds with the addition of nest boxes. Nest boxes, however, should not be viewed as a remedy for the chronic problem of habitat loss and degradation.
We studied the breeding ecology of Taiwan Yuhinas (Yuhina brunneiceps) at the Highlands Experiment Farm at Meifeng, National Taiwan University, in 1995 and from 1997–2002. The Taiwan Yuhina is a joint-nesting, cooperatively breeding species endemic to Taiwan. Males had significantly longer wing chords and tail lengths than females, probably due to sexual selection. Males also had a longer residence time at Meifeng than their female mates, which could be explained by philopatry being greater in males. Alpha males had a significantly longer residence time at Meifeng than beta males, but this was not the case for females, because females did not remain in the same group as males did after their mates disappeared. The breeding season was approximately 6 months long and multiple brooding was common. Nest building took 3 days, egg laying occurred over 3–4 days, the average incubation period was 14 days, and the nestling period was 12 days. Breeding success did not decrease later in the breeding season. Maximum longevity was 12 years, and the estimate of average annual overwinter survival rate for adults at Meifeng was 74%.
Nest predation is the main cause of reproductive failure in birds, yet the factors that drive predation pressure, as well as the avian strategies to minimize it, are poorly understood. There is a well-known commensal relationship between ants and birds nesting in acacia trees, but the direct benefit in terms of avian reproductive success has not been tested properly. We used artificial nests to compare success and survival probability of nests placed in Hinds' acacia trees (Acacia hindsii) associated with ants (Pseudomyrmex spp.) with those of nests placed in trees without ants. Nesting success and the probability of daily survival were greater in acacias than in antless trees. All cases of nest failure were due to egg predation, but none resulted from wren activities, as has been reported in previous studies. The results of this experimental study indicate that the presence of ants in acacias may enhance avian reproductive success by reducing the probability of nest predation.
Carolina Wrens (Thryothorus ludovicianus), which maintain lifetime pair bonds and year-round territories, huddle in pair or communal roosts during the non-breeding season, particularly during cold winter nights. Pair roosting during the nesting season, however, is not known to occur. Here, we report huddled pair roosting by Carolina Wrens in Florida. The dates of pair roosting took place during nest construction through laying of the first egg (9–20 March 2004), and also on the date the fourth egg was laid in a clutch of five (24 March). The wrens roosted in a hanging flower basket located 2.4 m from their nest site. Although huddled pair roosting by wrens during periods of low ambient temperatures in the non-breeding season likely achieves thermal conservation, the benefits derived during the breeding season remain unclear. We discuss the possible thermoregulatory and pair-bond maintenance functions of pair roosting.
I report predation of an American Crow (Corvus brachyrhyncos) by a Bald Eagle (Haliaeetus leucocephalus) in Washington state. The crow was attacked and killed while it was chasing a Red-tailed Hawk (Buteo jamaicensis). To the best of my knowledge, this is the first report of a bird of one species killing a bird of a second species that was chasing a bird of a third species.
We observed a Puerto Rican Spindalis (Spindalis portoricensis, Thraupidae) rapidly move through an area of dense vines by grasping vines in its beak and swinging from vine-to-vine without the use of its wings or feet. This behavior appears to be unique in birds.
For corvids, the decision to cache is a complex behavior likely influenced by many interacting factors. On 8 April 2004, I observed an American Crow (Corvus brachyrhynchos) caching eastern cottontail (Sylvilagus floridanus) kits taken from a rabbit nest on the Missouri State University campus in Springfield, Missouri. The crow cached at least three kits and flew away with at least one other. Caches were covered with dead leaves and landscape mulch. During the ensuing 3-day period, some caches disappeared, were partially eaten, or were moved to a different nearby location. To my knowledge, this is the first documented case of caching numerous rabbit kits from a single nest, and it is one of the few documented cases of cache-moving by American Crows.
A Gray-crowned Yellowthroat (Geothlypis poliocephala) nest was discovered in Texas during June 2005, providing the first documentation of nesting in the United States since 1894. The nest was located within the Sabal Palm Grove Audubon Center and Sanctuary in Cameron County, but was depredated within 4 days of discovery. Gray-crowned Yellowthroats are fairly common breeders in northeastern Mexico, but are currently listed as accidental in Texas. The future of this species in the United States is uncertain.