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Based on recordings and observations from the Yucatan Peninsula, I present the first formal description of the song characteristics and singing behavior of the Mangrove Warbler. Male Mangrove Warblers sing multiple song types with immediate variety, and song types are shared between neighbors. Responses from a playback experiment suggested that Mangrove Warbler song functions in an intrasexual territory defense context. I compared the fine structural characteristics of songs from one subspecies of Mangrove Warbler (Dendroica petechia bryanti) to that of a widely studied Northern Yellow Warbler subspecies (D. p. aestiva). Songs from these two subspecies are significantly different in length frequency and syllabic characteristics, and principal components analysis separates their songs entirely. These results, when taken together with geographical and morphological evidence, suggest a great separation between Mangrove Warblers and Northern Yellow Warblers.
We evaluated the magnitude of use of waste tips by Kelp Gulls (Larus dominicanus) nesting at Isla de los Pájaros, a large and growing colony in Patagonia, Argentina, and we assessed the difference in use between tips with urban and fishery waste. We marked with color dye 1347 adult breeding Kelp Gulls to determine if they fed in urban and fishery tips and to estimate the number of birds which used those tips during incubation. Kelp Gulls were present during 100% and 64% of counts at the fishery waste tip and the urban waste tip, respectively. The number of adult gulls was always larger at the fishery waste tip (mean ± SD = 1694 ± 664) than in the urban waste tip (mean ± SD = 59 ± 68). Considering the total number of gulls flying from the colony to the tips and the proportion of marked gulls in the tips, we estimated that at least 54–69% of the birds of the colony were present at the tips. The use of fishery waste may have contributed to the increase in the number of Kelp Gulls breeding at Isla de los Pájaros, and the quality, abundance, and predictability of food disposed at the fishery waste tip could foster the use of this site relative to natural food sources located closer to their breeding colony. Removal of artificial food sources may be assessed at the Puerto Madryn and other Patagonian waste tips to reduce conflicts between human and gull populations.
We evaluated the use of three direct methods (fat scoring, condition indices, and multiple regression of external morphological variables) to estimate total body fat (TBF) in Sharp-shinned Hawks (Accipiter striatus) and Cooper's Hawks (A. cooperii). All three methods explained more than 82% of the variation in actual TBF values, and all three methods required the use of multiple equations to account for the categories of species, age, and/or sex. We also evaluated an indirect method that estimates TBF by the difference between actual mass and estimated lean mass. We estimated lean mass using a multiple regression of external morphological variables, and this technique was fairly accurate (r = 0.94). The methods evaluated here, though reasonably precise, may not be accurate enough to reliably compare estimated TBF between individual birds. Using multiple regression to directly estimate TBF from mass and tarsus length measurements is the recommended technique because (1) it provides continuous estimates of TBF; (2) it requires measuring only two external morphological characters, both of which are less subjective than fat scores; and (3) it is more explanatory than the other two methods which use external characteristics as predictors. Our results also suggest that comparing groups of birds using condition indices may yield misleading results because these indices can relate to TBF differently for species, age, and or sex classes.
We describe a breeding male Blackpoll Warbler (Dendroica striata) in aberrant female-like plumage captured on Mt. Mansfield, Vermont, in 1998. The bird had been banded the previous summer in typical alternate-male plumage. This appears to represent the first documentation of a wild adult male bird in female-like alternate plumage. We suspect that the abnormal plumage reported here was due to hormonal influences on pigmentation. The bird's between-year change in appearance appeared to have no effect on its reproductive behavior.
Quantifying incubation patterns has often involved long observation periods in the field, video cameras, or the use of other electronic devices that sometimes require the partial destruction of clutches and insertion of artificial eggs. In this study, we used an inexpensive, nondestructive method involving temperature probes combined with data loggers to examine the incubation rhythm of female Black-throated Blue Warblers (Dendroica caerulescens). The method provided detailed records of on–off patterns for females for selected 24-h periods during incubation. Female warblers spent an average (±SE) of 64.0% of daylight hours incubating in bouts lasting 20.5 ± 1.5 min and made 2.4 ± 0.1 departures from the nest/h on trips that lasted 10.6 ± 0.7 min. Incubation bouts were longer and females spent more time incubating per hour in the mornings and late afternoons than at mid-day. Older females had longer incubation bouts and tended to have shorter incubation periods than did yearling females, suggesting that experienced individuals were more effective incubators. Because of its ease of use and because nests with probes were not depredated at a higher rate than controls, we suggest that the temperature probe/data logger method is an efficient and effective way to quantify incubation rhythms for open-cup nesting birds.
The study of birds killed at the WCTV television tower in north Florida over a 29-yr period is one of the longest of its kind. A total of 44,007 individuals in 186 species was collected, and over 94% of the total number of individuals were Neotropical migrants with the Red-eyed Vireo (Vireo olivaceous) the most frequent. Two factors changed during the study period: tower height and scavenger control. Analysis of these factors suggests that towers approximately 94 m or lower may not pose as great a threat of avian mortality as caused by towers 200 m or greater. Absence of scavenger control corresponded with a 71% decline in the number of individuals found beneath the tower. Any study of avian mortality at communication towers must directly address the problem of removal of injured and dead birds by scavengers to make an accurate measure of mortality.
Of 233 Ferruginous Hawks (Buteo regalis) banded as nestlings in three study areas of Montana over 16 years, 15 (6.4%) were recovered. Recovery rate increased with duration of study and cumulative number banded. Mean age at recovery for all Ferruginous Hawks banded in Montana was 390 days. Analysis indicated 50% of banded hawks were dead by between 0.35 and 0.70 yr and 95% were dead by between 1.5 and 3 yr of age. Survival estimated at one year was higher if hawks recovered by radio-tracking were excluded. Mean distance from the natal nest of all recoveries of Ferruginous Hawks banded in Montana was 824 km. Eight recoveries were of recently fledged hawks <1 km from the natal nest, but the remainder were recovered ≥1500 km from the natal nest. Recoveries that were outside natal 10-minute blocks (n = 7) occurred in northern Mexico, central New Mexico, southern California, and Texas; most (86%) were southeast of the natal nest. Six hawks died from unknown causes, five from predation, two from conflicts with power lines or towers, and two from injuries. Predation occurred more often within than outside natal 10-minute blocks. Recovery locations suggested that breeding populations of Ferruginous Hawks remain segregated during winter. Mortality from electrocution and collision may be important for some populations, especially those that winter in populated areas of California and Mexico.
Raptor attacks in the wild can be difficult to observe and impossible to manipulate in a repeatable manner. We have developed a bicycle-driven model raptor (“Veloci-Raptor”) to test prey responses to raptor attacks. Previous researchers have used wind-up toy birds or gravity-driven model raptors, which typically have low maximum velocities and often require cumbersome equipment. Our method's combination of portability, flexibility, and high maximum speed (19.4 m/s) make it superior to previous methods in many field situations.
We compared two fixed-radius point count sampling regimes using two abundant breeding species, the Dark-eyed Junco (Junco hyemalis) and the Chestnut-sided Warbler (Dendroica pensylvanica), in a forested landscape in the southern Appalachian mountains of Virginia. The same 20 points were counted three times under each of two revisiting schedules, either hourly or weekly, and the maximum and mean number of males recorded. Revisit schedule had no detectable effect on numbers of either species recorded, regardless of whether the fixed radius was 50 or 100 m or whether count duration was 5 or 10 min. For juncos, the maximum number of birds detected using an hourly revisit schedule with a 100-m fixed-radius count circle and a 5-min or 10-min count duration provided close matches to the density estimated by intensive territory mapping of this color-banded population (0.398 males/ha or 0.298 males/ha respectively, versus 0.325 known breeding males/ha). When revisiting count stations is desirable, the use of a 1-h revisit schedule provides an economical way to increase number of visits, with no apparent reduction in precision or accuracy of the estimate.
Studies that quantify parental care in birds are often faced with the confounding effects of variation in brood size. That is, nestlings from broods of varying sizes may receive different quantities of food for reasons not entirely related to the parental quality of the adults. To control for variation in brood size, researchers often divide feeding visitation rates by brood size to yield a per-nestling feeding rate. This presents problems, however, if adults adjust food load size in response to variation in brood size. We examined the relationship between brood size and parental care in the herbivorous House Finch (Carpodacus mexicanus) by considering not only visitation rates but also food load sizes. As brood size increased, the overall visitation rate increased but both per-nestling visitation rate and per-nestling food load rate decreased. The relationship between brood size and per-nestling care was similar regardless of whether we considered visitation rate or actual food loads, suggesting that in the House Finch per-nestling visitation rate serves as a reasonable index of total mass of food received by the nestlings. However, we urge caution in assuming that per-nestling visitation rate is an adequate measure of parental care in other species.
Hourly mass gain during migratory stopover in spring and fall was estimated for 48 species at three sites on Long Point, Ontario. Estimates were based on regression of size-corrected mass at first capture on time of day. Mean mass gain for all species and sites in fall was 0.61% of lean body mass/h, well over the mean estimated gain required to maintain daily energy balance (0.27% of lean body mass/h). In spring, mass gain was 0.50% of lean body mass/h at two sites but only 0.17% at the extreme tip of Long Point, where cold lake temperature in spring probably affects insect abundance. While most species fared well in fall, thrushes did not, perhaps due to poor habitat for ground foraging on Long Point. Except for the eastern tip of Long Point in spring, the study sites compare favorably in mass gain to two other stations in the northern U.S. where similar analyses have been done.
We determined proportions and return rates of yearling and older territorial male Ovenbirds (Seiurus aurocapillus) in 12 forest fragments and two contiguous forest sites in southern Ontario to test the hypothesis that a larger proportion of second-year birds occupied forest fragments. There was no significant difference in the proportion of yearling birds among small, large and contiguous forests (small, 54.0%; large, 54.3%; contiguous, 44.9%). Return rates did not vary among fragment size categories (small, 34.4%; large, 35.7%; contiguous, 42.8%). Males in small fragments did not differ significantly from those in large fragments or contiguous forest in mass, wing length, or a condition index. Males also did not vary in these characteristics between age groups. These results suggest that male Ovenbirds in our study area do not occupy sites in proportion to their quality as breeding locations.
Scavenging of carcasses may bias estimates of mortality following the use of pesticides. To assess carcass survival, we monitored bird carcasses in crop stubble, grassland, roadside, and wooded habitats in east-central South Dakota in early spring. Survival rates differed among habitat types in 1997 but not in 1998. Survival rates did not differ among species of carcass in 1997 or 1998. Within 5 d, 66% of carcasses had been scavenged in both 1997 and 1998. To assess potential hazards of secondary poisoning, we used infrared-triggered camera systems to identify predators and scavengers of live birds and bird carcasses. Mammals (n = 164), particularly striped skunks (Mephitis mephitis) (n = 105), were the most commonly photographed predators and scavengers. The number of avian predators and scavengers photographed (n = 39) was relatively low. Because scavenger activity can be high, we recommend that searches for carcasses should be conducted within 24 h of pesticide application to ensure accurate estimates of mortality.
The O'ahu ‘Elepaio (Chasiempis sandwichensis ibidis) is an endangered monarch flycatcher endemic to the Hawaiian Island of O'ahu. One of the main causes of the decline of this forest bird is low nest success. This study investigated whether introduced rodents might be important nest predators in ‘Elepaio habitat by conducting artificial nest experiments before and during a rodent control program. In each experiment, 20 artificial nests with two quail eggs each were placed on the ground and in trees, then checked at 5, 10, and 15 day intervals. The size, appearance, location, and odor of artificial nests were considered in order to make the experiment more realistic. Rodent control decreased predation on artificial tree and ground nests by 45% and 55%, respectively. Predation on ground nests was higher than predation on tree nests before rodent control, but afterwards predation was low on both ground and tree nests and did not differ between them. Survival of artificial tree nests after rodent control (80%, 0.985 daily) was similar to survival through incubation of ‘Elepaio nests (82%, 0.989 daily), indicating the artificial nests provided a realistic measure of predation on ‘Elepaio nests. Automatic cameras wired to nests documented the identity of the predator in 10 events; in every case it was a black rat (Rattus rattus). Black rats appear to be the primary nest predator in O'ahu ‘Elepaio habitat, and rodent control is a valuable management technique that can be used to increase the reproductive success of ‘Elepaio.
Loggerhead Shrikes (Lanius ludovicianus) have rarely been observed engaging in kleptoparasitism. We describe two observations of kleptoparasitism by Loggerhead Shrikes on soil-foraging songbirds. We suggest that kleptoparasitism of these birds gives shrikes access to resources that are otherwise unavailable. Kleptoparasitism may help to explain aggressive behavior by shrikes toward birds too large to be depredated successfully.
Smith et al. (1999) assessed nest-site selection in Great Horned Owls (Bubo virginianus) by comparing data from 75 owl nests with data recorded at random sites derived from computer-generated map coordinates. The authors concluded that owl nests differed significantly from random sites for nearly all of the eight habitat variables that they measured. I consider some of these conclusions tentative because Smith et al. (1999) ignored the fact that owls do not build their own nests and thus are not free to nest wherever they choose. Although Smith et al.'s approach is adequate for assessing selection of some macrohabitat features, a more appropriate way to assess nest-site selection per se would have been to compare characteristics of owl nests with those measured at a similar number of suitable but unused stick nests in the same study area where the owls nested.
Marks (2001) is critical of our study of nest-site selection in Great Horned Owls (Bubo virginianus) because we compared our data from owl nests with data from random points rather than from unused stick nests. We argue that Great Horned Owls have so many options for nesting in eastern forests that there is little constraint on nest-site selection. Therefore, their choice of nest sites is determined largely by vegetation characteristics in the surrounding landscape, and comparison of owl nests with random points is the best way to assess nest-site selection. Furthermore, we believe that use of unused nests as controls, as advocated by Marks, is biased because control nests will have many of the same characteristics as nests used by owls.
Bosakowski and Smith's (2001) reply to my commentary reflects a fundamental difference in our views of what constitutes “nest-site selection” and a misunderstanding of why random points would have been inadequate for assessing this trait in one of my previous studies. Their study of Great Horned Owls (Bubo virginianus) focused on macrohabitat selection, whereas my study of Long-eared Owls (Asio otus) focused on nest-site selection as the term is generally applied by ornithologists.