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Henslow's Sparrow (Ammodramus henslowii) is a declining, disturbance-dependent grassland bird that winters in the longleaf pine (Pinus palustris) ecosystem of the southeastern United States. During two winters (2001, 2002), we estimated the relative abundances, movement patterns, and habitat associations of Henslow's Sparrows wintering in habitat patches differing in time since last burn (burn treatment). We conducted our study in southeastern Louisiana in Andropogon spp.-dominated longleaf pine savanna habitat. Henslow's Sparrows were most abundant in savannas burned the previous growing season, with a mean relative abundance of 2.6 individuals/ha. The most dramatic decline occurred between burn year 0 and year 1 (first and second winters after burning), when mean relative abundance dropped to 1.0 individual/ha. Home-range size of radio-tagged birds was not correlated with burn treatment. All radio-tagged individuals maintained stable home ranges, with a mean size of 0.30 ha. Vegetation characteristics differed significantly among burn treatments. Sites burned the previous growing season had low vegetation density near the ground, vegetation taller than 1.0 m, and high seed abundance. These variables were all highly correlated with Henslow's Sparrow relative abundance, but seed density best predicted Henslow's Sparrow numbers. We recommend a biennial, rotational burn regime to maintain habitat characteristics correlated with Henslow's Sparrow abundance.
We examined nest-site spacing and selection of nesting cliffs by Peregrine Falcons (Falco peregrinus) in central West Greenland. Our sample included 67 nesting cliffs that were occupied at least once between 1972 and 1999 and 38 cliffs with no known history of Peregrine Falcon occupancy. We measured 29 eyrie, cliff, and topographical features at each occupied nesting cliff and unused cliff in 1998–1999 and used them to model the probability of peregrines occupying a cliff for a breeding attempt. Nearest-neighbor distance was significantly greater than both nearest-cliff distance and nearest-occupied distance (the distance between an occupied cliff and one occupied at least once, 1972–1999). Thus, spacing among occupied cliffs was probably the most important factor limiting nesting-cliff availability, and, ultimately, peregrine nesting densities. Although some unused cliffs were unavailable in a given year because of peregrine spacing behavior, physical characteristics apparently made some cliffs unsuitable, regardless of availability. We confirmed the importance of several features common to descriptions of peregrine nesting habitat and found that peregrines occupied tall nesting cliffs with open views. They chose nesting cliffs with eyrie ledges that provided a moderate degree of overhang protection and that were inaccessible to ground predators. Overall, we concluded that certain features of a cliff were important in determining its suitability as a nest site, but within a given breeding season there also must be sufficient spacing between neighboring falcon pairs. Our habitat model and information on spacing requirements may be applicable to other areas of Greenland and the Arctic, and can be used to test the generalities about features of Peregrine Falcon nesting cliffs throughout the species' widespread distribution.
Sharp-shinned Hawks (Accipiter striatus) and Cooper's Hawks (A. cooperii) are important predators of birds in North America, but little is known about their natural history during the winter. Even basic survival information is not well documented in these species and is generally unknown during the winter. Therefore, we examined survivorship and causes of mortality among wintering Cooper's and Sharp-shinned hawks. We radio-tracked 27 Cooper's and 40 Sharp-shinned hawks during 5 winters from 1999 to 2004. Neither species nor sex was a significant covariate of survivorship, but the probability of adult survival (75.4%) over 110 days was significantly higher than that of juveniles (9.4%). Our estimate of adult survivorship is comparable with those published for other accipiters, but our estimate for juveniles is lower. Age differences in survivorship may be attributed to risk taking or inexperience in juveniles and/or difficulties in dealing with transmitter attachments. Two types of mortality (predation and collisions) were observed in the study. Whereas predation by owls was a major source of mortality in rural habitat, no predation was observed in the urban habitat. Our results suggest that predation by owls may have important implications for the behavioral interactions between accipiters and their prey.
The value of riparian habitats to birds differs among ecosystems. I tested whether riparian habitat near large streams and rivers in the Pacific Northwest supported a higher abundance and diversity of birds than adjacent upland forest. From 1996 to 1998, I surveyed breeding birds at four 9-ha sites in coastal western hemlock forest on western Vancouver Island, British Columbia. Five species of forest generalists dominated both riparian and upland bird communities: Winter Wren (Troglodytes troglodytes), Chestnut-backed Chickadee (Poecile rufescens), American Robin (Turdus migratorius), Swainson's Thrush (Catharus ustulatus), and Pacific-slope Flycatcher (Empidonax difficilis). Species richness and total abundance were similar over the riparian-to-upland gradient, whereas abundances of riparian specialists and aerial foragers declined with distance from the river. To explore whether vegetation composition and structure explained bird distribution patterns, I sampled three locations along both riparian and upland transects at each site. Riparian areas had higher densities of deciduous trees; conifer and snag densities were higher in upland areas. Salmonberry (Rubus spectabilis) cover was marginally higher in riparian areas and blueberry (Vaccinium spp.) cover was higher in upland areas. There was little effect of distance from the river on most bird species, but there were stronger associations of birds with specific vegetation attributes. Tree and snag densities explained the most variation in abundance of aerial foragers, and eight of nine individual species, whereas distance from the river and shrub cover were important predictors of Hammond's Flycatcher (Empidonax hammondii) abundance. Apart from riparian specialists and a few species with strong vegetation associations, bird assemblages in riparian and upland habitats of this moist forest type were dominated by similar sets of generalist species.
Grassland bird populations are sharply declining in North America. Changes in agricultural practices during the past 50 years have been suggested as one of the major causes of this decline. Field-border conservation practices encouraged by the U.S. Department of Agriculture's National Conservation Buffer Initiative meet many of the needs of sustainable agriculture and offer excellent opportunities to enhance local grassland bird populations within intensive agricultural production systems. Despite the abundant information on avian use of, and reproductive success in, strip habitats during the breeding season, few studies have examined the potential value of field borders for wintering birds. We planted 89.0 km of field borders (6.1 m wide) along agricultural field edges on one-half of each of three row crop and forage production farms in northeastern Mississippi. We sampled bird communities along these field edges during February–March 2002 and 2003 using line-transect distance sampling and strip transects to estimate density and community structure, respectively. We used Program DISTANCE to estimate densities of Song (Melospiza melodia), Savannah (Passerculus sandwichensis), and other sparrows along bordered and non-bordered transects while controlling for adjacent plant community. Greater densities of several sparrow species were observed along most bordered transects. However, effects of field borders differed by species and adjacent plant community types. Diversity, species richness, and relative conservation value (a weighted index derived by multiplying species-specific abundances by their respective Partners in Flight conservation priority scores) were similar between bordered and non-bordered edges. Field borders are practical conservation tools that can be used to accrue multiple environmental benefits and enhance wintering farmland bird populations. Provision of wintering habitat at southern latitudes may influence population trajectories of short-distance migrants of regional conservation concern.
The eastern Gulf of Alaska coastline is suspected of providing an important pathway for birds migrating to and from Alaska. Because no intensive study of landbird migration has been conducted in this region, we used mist nets to study the post-breeding migration of landbirds along the coast from 1994 through 1999. Over six post-breeding periods, we netted for a total of 316 days (23,538 net-hr) and captured 13,490 individuals of 46 species (57.3 birds/100 net-hr). Six species constituted >65% of all captures (ordered by abundance): Orange-crowned Warbler (Vermivora celata), Hermit Thrush (Catharus guttatus), Lincoln's Sparrow (Melospiza lincolnii), Ruby-crowned Kinglet (Regulus calendula), Fox Sparrow (Passerella iliaca), and Yellow Warbler (Dendroica petechia). Most birds captured (71%) were Nearctic-Neotropical migrants, and percentages of hatching-year (HY) birds varied from 51 to 90% among common species. Daily capture rates of all species were highest between mid-August and mid-September. Migration of HY individuals preceded that of after-hatching-year (AHY) birds in 70% of the Nearctic-Neotropical species. Masses of HY Nearctic-Neotropical migrants were significantly less than those of AHY individuals. High capture rates and consistent annual use indicate that the eastern Gulf of Alaska coast is an important pathway for many small landbird migrants, particularly Nearctic-Neotropical species, departing breeding grounds in southern Alaska.
We studied incubation patterns and hatchability of Red-winged Blackbirds (Agelaius phoeniceus) nesting in two different wetland habitats—beaver ponds and sewage lagoons—in eastern Ontario during 1999–2001. We presumed that, if incubating Red-winged Blackbirds could acquire food more readily at sewage lagoons than at beaver ponds, they should respond by taking fewer and shorter foraging bouts, which would result in longer bouts of attentiveness, shorter incubation periods, and higher hatchability of eggs. Although differences were small, female foraging bouts were shorter and bouts of attentiveness were longer at sewage lagoons than they were at beaver ponds. Incubation constancies were subsequently greater, and, ultimately, incubation periods at sewage lagoons were shorter. Shorter incubation periods at sewage lagoons, however, did not result in increased hatchability. Our results suggest that, in habitats where incubating Red-winged Blackbirds can acquire food more readily, incubation periods may become shorter and incubation constancies may become higher.
We used radio-telemetry techniques to determine hourly activity patterns of 29 juvenile Lilac-crowned Parrots (Amazona finschi) during 1996–2000 in tropical dry forest of Jalisco, Mexico. Parrots had two peak activity periods—early morning and late afternoon—for both overall activity and local movement. Individuals were generally inactive and did not change location for 5–6 hr during the middle of the day. Parrots were more active in the dry season than in the rainy season, although movements resulting in a change of location did not vary between seasons. Seasonal variations in activity of Lilac-crowned Parrots may be related to variations in food availability or temperature. Activity patterns of parrots need to be considered when evaluating habitat use or survey data.
Parrots that inhabit tropical lowland forests are difficult to study, are poorly known, and little information is available on their nesting habits, making analysis of community-wide nesting patterns difficult. I present nesting records for 15 species of psittacids that co-occur in southeastern Peru. The psittacid breeding season in this area lasted from June to April, with smaller species nesting earlier than larger species. Why smaller species bred earlier is uncertain, though it may be related to interspecific competition for nest sites or variations in food availability. This study identified two keystone plant resources used by nesting parrots: Dipteryx micrantha (Fabaceae) and Mauritia flexuosa (Arecaceae). Local threats to these plant species are discussed.
We investigated the effect of group size on incubation effort in Taiwan Yuhinas (Yuhina brunneiceps) at the Highlands Experimental Farm of National Taiwan University at Meifeng, Nantou County, central Taiwan, during 2000 and 2001. The Taiwan Yuhina is a joint-nesting, cooperatively breeding species endemic to Taiwan. We compared differences in parental investment among individuals of different sexes and status, explored the effect of group size on group incubation effort, and investigated whether individuals show compensatory reductions in care with respect to the number of females laying. Constancy of incubation increased as group size increased. Alpha females exhibited a significantly greater incubation effort than other individuals, but effort was similar among other group members. Both alpha males and females decreased their relative and absolute incubation effort as group size increased (i.e., there was a compensatory reduction in parental effort). However, beta pairs maintained a consistent but low incubation effort when either gamma pairs or an extra individual joined the group. Our study also demonstrated a new potential group-size benefit for cooperatively breeding birds—an increase in the constancy of incubation.
I describe an unusual food-handling behavior performed by juvenile Northern Mockingbirds (Mimus polyglottos). In the course of one morning, I observed juvenile Northern Mockingbirds repeatedly roll several prey items down the incline of a roof in Charlottesville, Virginia. I discuss this behavior in the context of the development of aerial foraging skills.
The Northern Bobwhite (Colinus virginianus) is one of the most studied game birds in North America. It is a ground-nesting galliform capable of producing multiple nests during a single season. Since 1993, personnel of the Albany Quail Project have radio-tagged >6,000 bobwhites and monitored >2,000 nests via radio telemetry on private lands in southwestern Georgia. We have observed nests in some peculiar places; however, every nest that we have monitored has been on the ground. Previously, no case of above-ground nesting has been documented for this species. Here, we report an above-ground nest, found in June 2001.
I report the first case of divorce for the Canary Islands Stonechat (Saxicola dacotiae), an endemic bird species of the semiarid island of Fuerteventura (Canary Islands, Spain). I studied 72 pairs during three breeding seasons (2000–2001, 2001– 2002, and 2002–2003). In 2001–2002, a female divorced after a successful first nesting. This female settled in a neighboring territory where the owner was unpaired, built a new nest, and laid four eggs. The low rate of divorce (1.4%) suggests that unforced mate changes by Canary Islands Stonechats are rare.
I describe a Yellow-crowned Tyrannulet (Tyrannulus elatus) nest built largely of mistletoe seeds, which differs from the cup of plant matter typically constructed by this species. Mistletoe seeds have been observed in the nests of at least two other bird species, but this observation is the first where the nest appeared to be purposely constructed from seeds, possibly to take advantage of their adhesive properties.