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We studied the gonadal and molt cycle of three species of Columbina ground-doves in seasonal savannas of Venezuela to explore whether gonadal development is related to seasonal patterns of rainfall. The study site is a strongly seasonal, open woodland-savanna displaying a predictable alternation of dry and wet seasons, providing strong cues on environmental periodicity and on the variation in the level of food resources associated with those fluctuations. All three species of doves had an extensive breeding season, bred (although not exclusively) during the dry season, had fully functional testes independent of rainfall, and displayed a relatively small seasonal fluctuation in testes size. Nevertheless, even though all three species were exposed to similar ambient conditions and their diets overlapped extensively, there were noticeable differences in their testicular cycles, and no simple generalizations regarding the possible stimulatory roles of photoperiod or rainfall were apparent. The testes of most Scaled Doves (C. squammata) remained fully or partly recrudesced throughout the year, but a significant reduction in testes size occurred in May, at the beginning of the rainy season, when the size of the soil seed bank should be smallest. Maximum testes sizes were reached from November through March during the beginning and peak of the dry season, when the seed bank is expected to be large. In this species, neither rainfall nor longer photoperiod had a stimulatory effect on testicular development. Ruddy Ground-Doves (C. talpacoti) had an extensive, and seemingly double, breeding season encompassing the latter part of the dry season and the rainy season. Male C. talpacoti had fully functional testes in the peak of the dry season, some three months ahead of the first rains. Hence, as in the Scaled Dove, the influence of rainfall on testicular development was not apparent, but testicular recrudescence coincided with the period of increasing day length and we cannot rule out a possible degree of photoresponsiveness in Ruddy Ground-Doves. Males of Plain-breasted Ground-Doves (C. minuta) underwent testicular regression in April and May, at the beginning of the rainy period, and testicular recrudescence progressed along with the advancement of the rainy season. In this species, rainfall, but not increasing photoperiod, could have a stimulatory effect on gonadal development, since testes became smaller as day length increased towards May, but testis size increased until November while days were becoming shorter. In none of the species was the molt of primary wing feathers synchronous, but in all of them there was a tendency not to molt primaries during the peak of the rainy season. At the level of the population, the periods of breeding and wing molt overlapped. The molt was spread over most of the year in all species. An unforeseen finding was a highly biased sex ratio in favor of males among adults in all species.
We used original data (mist net captures) from San Salvador and Andros Island and literature sources for 19 islands in the Bahamas to conduct a biogeographic analysis of the permanent resident, terrestrial bird communities in the archipelago. The number of regularly occurring resident species on the 19 islands varied between 6 and 35 species. Island area accounted for 76% of the variation in richness, but multivariate analyses showed that vegetation and topography also affected richness: the richest communities were found on large islands that were dominated by pines and which had the highest topographic relief. Separate analyses of raptors, pigeons and doves, near-passerines, and passerines confirmed the primacy of island area as a determinant of richness among all groups except passerines. Among the latter species, area effects were secondary to vegetation type and elevation. The number of bird species on San Salvador conformed closely to the number predicted based on island area alone, but habitat use by the current avian community is at odds with the taller broadleaf forests that covered most of the island little more than 300 yr ago. We suspect that San Salvador has experienced numerous extinctions since that time.
Locating and monitoring nests are among the most widely used approaches in studies of avian ecology, evolution, and conservation. While several papers outline “best practices” for nest studies, nest-searching techniques are seldom standardized in field investigations because observers generally use strategies that work best for them. In this study, I examined if field observers differed in the cues they used to locate nests, the species they found, and the fate of their nests (i.e., successful or failed), and the extent to which nest-searching cues were associated with either nest fate or vegetation characteristics surrounding the nest. My field assistants and I monitored 355 songbird nests on 10 forested sites in central Pennsylvania in 1998–99. Parental behavior was the most frequently used cue for locating nests (41%), followed by systematic searching of nesting substrate (37%). Accidental flushing of the parent (5%) and luck (17%) were involved in fewer located nests. Field observers differed in the cues they used to find nests, and these nest-searching cues were associated with finding certain species. In addition, estimates of nesting success (percentage of nests fledging young) differed among field observers by up to 2.35×. Nest-searching cues were related to nest-placement (e.g., nest height) and vegetation characteristics (e.g., leaf litter) within nest-patches for Ovenbirds (Seiurus aurocapillus), Red-eyed Vireos (Vireo olivaceus), and Scarlet Tanagers (Piranga olivacea), although cues were not significantly related to the fate of nests. Overall, nest-searching cues were associated with nest placement, nest-patch habitat, and species composition of nest samples, all of which can ultimately influence findings from nesting studies. Consequently, investigators should exercise caution when allocating individual effort across experimental units and consider assigning each observer to ≥1 treatment, multiple observers to each site, and addressing nest-patch and nest-placement differences among cues through training and data analysis.
Nesting success of Dusky Canada Geese (Branta canadensis occidentalis) has declined greatly since a major earthquake affected southern Alaska in 1964. To identify nest predators, we collected predation data at goose nests and photographs of predators at natural nests containing artificial eggs in 1997–2000. To document feeding behavior by nest predators, we compiled the evidence from destroyed nests with known predators on our study site and from previous studies. We constructed a profile for each predator group and compared the evidence from 895 nests with unknown predators to our predator profiles using mixture-model analysis. This analysis indicated that 72% of destroyed nests were depredated by Bald Eagles and 13% by brown bears, and also yielded the probability that each nest was correctly assigned to a predator group based on model fit. Model testing using simulations indicated that the proportion estimated for eagle predation was unbiased and the proportion for bear predation was slightly overestimated. This approach may have application whenever there are adequate data on nests destroyed by known predators and predators exhibit different feeding behavior at nests.
Brown-headed Cowbirds (Molothrus ater), and to a lesser extent Shiny Cowbirds (M. bonariensis), have expanded their breeding ranges to include that of the Florida Scrub-Jay (Aphelocoma coerulescens). We added the eggs of seven bird species to Florida Scrub-Jay nests to determine if scrub-jays eject foreign eggs. Most eggs were ejected within 1 d of addition, probably by grasp-ejection. Ejection of foreign eggs by Florida Scrub-Jays suggests that cowbirds, should they begin to parasitize scrub-jays, would waste their reproductive effort. Fossil evidence suggests that Florida Scrub-Jays may be former cowbird hosts that have sustained the ability to eject.
Bachman's Sparrow (Aimophila aestivalis) at Fort Pickett Maneuver Training Center, Virginia, was regularly observed mimicking the primary song of an Indigo Bunting (Passerina cyanea). Two Bachman's Sparrows were observed mimicking the primary song of a Common Yellowthroat (Geothlypis trichas). Although there is a previous report of a Bachman's Sparrow mimicking the song of a Field Sparrow (Spizella pusilla), there are no prior reports of Bachman's Sparrows mimicking Indigo Bunting or Common Yellowthroat songs.
We used a combination of standardized audio-visual surveys, made over nine years, and vegetation analysis to determine habitat associations of Marbled Murrelets (Brachyramphus marmoratus) breeding in inland coniferous forests. Our study, in Carmanah-Walbran watersheds on southwestern Vancouver Island, British Columbia, covered extensive contiguous tracts (>16,000 ha) of apparently suitable habitat that supported a large breeding population of murrelets, a threatened species. Indicators of stand occupancy (circling and subcanopy flights) from audio-visual surveys were consistently associated with known nest habitat indicators (availability of platform limbs, cover and thickness of epiphytes on tree limbs, variable canopy structure). Both murrelet detections and nest microhabitat indicators were associated with a suite of macrohabitat variables, indicating that the most suitable habitat was low-elevation old-growth forest with widely spaced large trees. Biogeoclimatic productivity units, based on soil moisture and nutrient regimes, were useful proxies of habitat suitability, but tree species composition, timber volume, and tree height, variables commonly available in timber inventory maps, were not. Using two principle component factors derived from habitat characteristics, we clustered the 27 survey stations into three groups that differed significantly in occupied and subcanopy detections of murrelets and in nest habitat indicators. This is a useful method for combining multivariate measures for classifying and mapping habitat for murrelets.
Roost counts may be a useful method for assessing and monitoring parrot populations as long as counting regimes can detect real differences in abundance above the noise of daily variability in roost size. We studied a roost of up to 85 Red-tailed Amazons (Amazona brasiliensis) for 28 consecutive mornings and evenings from 12 July to 7 August 2001, and recorded bird behavior and associated weather data. The roost declined significantly over the survey period as the breeding season drew nearer. There was no significant difference between evening and morning roost counts, but we suggest that as long as misty mornings are avoided, morning roost counts were more effective as birds left more quickly and predictably. It took longer for birds to arrive on evenings when the roost was large, but birds left quicker in the morning when there were large numbers in the roost. Weather influenced both roost size and timing of arrival, with larger than expected numbers in the roost, and birds arriving later in the afternoons of sunny, warm days. We tested the reliability of four roost counting regimes: counts from five consecutive nights, five counts, one every fourth night, five nights picked at random, and 10 randomly picked nights. Counts from every fourth day performed significantly worse than all the other regimes in estimating the mean numbers in the roost. The 10-d random sampling regime performed significantly better than the 5-d regime in detecting very large roosts which occurred occasionally through the month.
We studied the relationships between Red-cockaded Woodpecker (Picoides borealis) group size and nest productivity. Red-cockaded Woodpecker group size was positively correlated with fledging success. Although the relationships between woodpecker group size and nest productivity measures were not statistically significant, a pattern of increasing clutch size and number of hatchlings with increasing group size was apparent. The presence of helpers appeared to enhance the survival of nestlings between hatching and fledging. The contribution that helpers make to nestling feeding and incubation, cavity excavation, and territory defense appears to have a positive effect on nest productivity. A threshold number of helpers may be necessary before a significant benefit for fledging success is realized. Nests with four and five group members fledged more young than nests with two or three group members. Whether partial brood loss occurred or not within a nest was primarily a function of clutch size and the number of hatchlings. Although partial brood loss did affect the number of young fledged from individual nests by removing young from nests with high numbers of hatchlings, woodpecker group size appeared to be the primary determinant of fledging success.
We describe the nest-site characteristics of Northern Waterthrushes (Seiurus noveboracensis) in western Newfoundland. Based on data for 36 nests (20 in streambanks, 16 in overturned trees) and 108 comparative sites, we examined habitat features at microhabitat (a 25-cm radius around the nest) and macrohabitat (a 5-m radius, and position on the landscape) scales. Assessment of microhabitat variables indicated that features around nests in the roots of overturned trees were no different from randomly selected sites. However, streambank nest-site features at this spatial scale distinguished these nest sites from randomly selected streambank locations through the presence of greater substrate slope and less leafy vegetation around nests. At broader spatial scales, streambank nests did not differ from associated random sites, but the macrohabitat features of nests in overturned trees distinguished them from randomly selected overturned-tree sites through overturned-tree nests being closer to water and having higher numbers of small-size-class deciduous trees in the surrounding habitat. Overall, depredated nests had less concealment with lower numbers of coniferous trees within 5 m, particularly in smaller size classes. Nests in buffer strips had greater numbers of both deciduous and coniferous trees in these smaller size classes than did nests in riparian habitat adjacent to undisturbed forests.
We compared three methods to determine nest predators of the Least Bell's Vireo (Vireo bellii pusillus) in San Diego County, California, during spring and summer 2000. Point counts and tracking stations were used to identify potential predators and video photography to document actual nest predators. Parental behavior at depredated nests was compared to that at successful nests to determine whether activity (frequency of trips to and from the nest) and singing vs. non-singing on the nest affected nest predation. Yellow-breasted Chats (Icteria virens) were the most abundant potential avian predator, followed by Western Scrub-Jays (Aphelocoma californica). Coyotes (Canis latrans) were abundant, with smaller mammalian predators occurring in low abundance. Cameras documented a 48% predation rate with scrub-jays as the major nest predators (67%), but Virginia opossums (Didelphis virginiana, 17%), gopher snakes (Pituophis melanoleucus, 8%) and Argentine ants (Linepithema humile, 8%) were also confirmed predators. Identification of potential predators from tracking stations and point counts demonstrated only moderate correspondence with actual nest predators. Parental behavior at the nest prior to depredation was not related to nest outcome.
Thick-billed Parrot (Rhynchopsitta pachyrhyncha) nest sites were studied during a six-year period (1995–2000) at six nesting areas in the Sierra Madre Occidental in Mexico. Nests were found in 187 snags or live trees of seven species, which averaged 75.2 cm dbh. Most nests were in snags (59%), and only two nests occurred in trees or snags under 40 cm dbh. Most nests occurred in Douglas fir (Pseudotsuga menziesii; 32.6%), and Mexican white pine (Pinus ayacahuite; 21.9%). Nest-site characteristics observed in the study were compared with those reported in the literature 20 yrs ago by Lanning and Shiflett (1983), and important differences were found in the percentages of tree species used. Decline of the Thick-billed Parrot seems to be related to large-scale logging over a wider historical breeding range. Commercial timber harvesting appears to affect nest site availability by leaving few snags and pine trees large enough for parrots to nest in.