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From 1936 to 1998 I kept notes on the Riverside Wren (Thryothorus semibadius) in southern Pacific Costa Rica. Early in this interval, the wrens lived chiefly along the shores of wider streams flowing through the forests and around marshy openings in the lowlands. With widespread deforestation, they tended to forage and nest farther from the watercourses. Their short, ringing songs sound above the clamor of mountain torrents. They eat small invertebrates. At all seasons they sleep alone or two or three together, in well-enclosed nests such as they occupy for breeding. Only the female incubates the two eggs, taking long sessions and long recesses. The incubation period is 18 or 19 d. Both parents attend the young. The nestling period is 16 d. The parents lead newly emerged fledglings to sleep in a nest with their mother. For a year and a half, I followed the activities of a family of four that lived in our garden.
We determined the best plumage and morphometric variables for ageing and sexing the Maui Parrotbill (Pseudonestor xanthophrys), an endangered Hawaiian honeycreeper found only on east Maui, Hawaii, by examining and measuring 30 museum specimens and 71 live birds captured in mist nets. Juvenal plumage was identified by the presence of pale-tipped wing bars on the middle and greater coverts, grayish olive dorsal plumage, and dingy white underparts and superciliaries. Birds undergoing first prebasic molt retained the juvenal remiges, rectrices, and wing coverts. Birds in first basic plumage possessed juvenal wing bars and a dull juvenal-like plumage. Subsequent molts were complete, and adults lacked wing bars. Adult males had bright yellow plumage on the cheeks, throat, and superciliaries, as did 27% of adult females. All other adult females had less yellow in the underparts. The dorsal plumage of adult females was more variable than adult males and was either yellow-olive like the males or grayish olive. Adult males had longer wing, bill, tail, and tarsometatarsus and greater mass than adult females. Virtually all males and females could be distinguished by wing length. Morphometrics of immature birds were significantly smaller than for adult males. Only immature male wing chord was significantly larger than that of adult females. Although it was difficult to distinguish between immatures and some adult females based on plumage coloration or measurements, a cut-off point of 70.4 mm for wing chord separated 91% of females from 93% of males, regardless of age.
Sexual differences in maximum diving depths and in the composition of the diet of the Antarctic Shag (Phalacrocorax bransfieldensis) were investigated at Harmony Point, Nelson Island, South Shetland Islands, Antarctica, during the 1995–1996 and 1996–1997 breeding seasons. The mean maximum diving depth estimated by the capillary-tube depth gauge technique was 37.8 m which, when compared with other studies, reflects good food availability in shallow waters around Harmony Point. Females dived significantly deeper than males and also reached the maximum dive depth registered (112.6 m). The analysis of the stomach contents recovered when the individuals with capillary-tubes returned to the nest from foraging trips indicated that males ingested almost exclusively large Notothenia coriiceps specimens, whereas females preyed more intensively on smaller fish. The differences observed suggest that individuals of both sexes partitioned foraging depths and food resources.
The effect of investigator activity on Black-legged Kittiwakes (Rissa tridactyla) was assessed using the disturbance caused by an ongoing intensive study investigating chick growth and adult mass loss. Though the effects were small, investigator disturbance decreased adult nest attendance and increased daily chick loss rates. Whereas overall chick survival until day 18 post-hatch was significantly lower in the high-disturbance plot in the first year of the study, it was substantially higher in the second year. We hypothesize that changes in predator activity as an indirect consequence of disturbance were responsible for this pattern. Herring Gulls (Larus argentatus) which nested near the high-disturbance plot and are the main predator of kittiwakes in our study area may have been more susceptible to the effect of disturbance than the kittiwakes themselves. There was otherwise no statistically significant impact of disturbance on chick growth, or on adult kittiwakes extending into the following year. Biases in studies of kittiwakes due to investigator disturbance may thus be negligible when the study is carefully designed. Future studies investigating effects of disturbance on birds should, however, include data concerning potential predators of the focal species and include more than one low-disturbance plot, and should be carried out over two or more years.
We describe 14 nest predation events witnessed in the Republic of Panama during studies of avian nesting success. Eight predations were by birds, including two species of toucan and one forest-falcon. Four predations were by two species of snake, one was by monkeys, and one near-predation was by army ants (Eciton burchelli). In three cases, a fraction of the nest contents were removed by the predator on one visit before it returned later to consume the remaining items. Our observations, although limited to diurnal encounters, indicate that Neotropical birds are susceptible to a diverse array of potential predators. Detailed investigations using cameras are needed to quantify the potential importance of each predator species and to identify the occurrence and importance of nocturnal predation.
Several studies have attempted to calculate the energy intake rates of shorebirds. One method of doing this involves estimating the size of prey items in relation to the bird's bill length, and then using an average bill length to convert this value to actual prey length in order to estimate prey mass/energy content. Although this technique is used regularly, no study has assessed under what conditions the method is reliable. Observer distance from the bird, prey type, size and orientation in the bill, bill length and shape, and light conditions may all affect the accuracy of the estimates. This study sought to establish the conditions under which the method is or is not reliable, using three different prey types (“thin” and “fat” polychaetes, and crabs) and a long- and short-billed bird. It was found that if observers were calibrated, and their results were corrected using regression analysis, errors in estimates of food intake were much reduced. The time allowed to view the prey significantly affected the error in the result, with shorter viewing time causing greater errors, and observations made into the light were less accurate than those made with the light behind the observer. Large prey types, or even larger size classes of the same species, are estimated less accurately than small prey, and accuracy decreases as prey size increases as multiples of bill length. If separate calibration regressions are generated for large and small prey, food intake estimates are more accurate than if a single regression is used incorporating the full range of prey sizes. Sexual dimorphism in bill length can also increase error in the results.
A Band-rumped Storm-Petrel (Oceanodroma castro) banded in the Azores was recovered in the Gulf of Mexico south of Pensacola, Florida. Research in breeding colonies in the Azores indicates that two genetically distinct breeding forms of this storm-petrel exist, one that breeds in the cool season (August–January), another that breeds in the hot season (April–September). The individual recovered from the Gulf was a representative of the cool-season breeders. Only recently has it become known that Band-rumped Storm-Petrels are regular in the western Atlantic Ocean when not breeding. Based on recent reports (sightings) and records (salvaged specimens), probably they also are regular in the Gulf of Mexico. If the two forms are determined to be separate species, a new challenge exists, namely to determine if they are distinguishable other than from measurements and genetic analysis. Careful record keeping and willingness to donate specimens to research institutions by persons in charge of rehabilitation facilities could help clarify this situation and others related to the distribution of pelagic birds.
A common assumption made when conducting broadcast surveys for owls is that vocalizations of larger owls reduce the responsiveness of smaller owls. We tested this assumption by comparing the responsiveness of Elf Owls (Micrathene whitneyi) exposed to the taped calls of conspecifics and to imitations of Great Horned Owls (Bubo virginianus). Elf Owls were more responsive to conspecific calls during courtship and incubation, but less responsive during the nestling stage, than they were to Great Horned Owl calls. This was because responsiveness to conspecific calls decreased through the nesting season, but responsiveness to Great Horned Owl calls was consistent throughout the nesting season. Singing Elf Owls switched from song to scold vocalizations more frequently after hearing a Great Horned Owl call (83.3%) than when hearing a conspecific call (33.3%). The responsiveness of Elf Owls to Great Horned Owl calls may benefit studies conducted throughout the breeding season.
We surveyed birds on seven small (<3 km2) offshore islands in the Independent State of Samoa to investigate the distribution and abundance of landbirds relative to abiotic and biotic features of these islands. We recorded 17 of the 28 indigenous species of landbirds that reside in Independent Samoa. Species richness among the islands ranged from 3 to 12 species. Other than the domestic chicken, none of the non-native species common on the large islands of ‘Upolu and Savai'i was recorded on offshore islands. Nu'utele, the third-largest of the seven islands, had the greatest proportion of native forest and harbored the most species of landbirds, including five not found on any other offshore island (Tooth-billed Pigeon [Didunculus strigirostris], West Polynesian Ground-Dove [Gallicolumba stairi], Pacific Pigeon [Ducula pacifica], Samoan Whistler [Pachycephala flavifrons], and Samoan Flycatcher [Myiagra albiventris]). The similarly sized Apolima, which is inhabited and extensively cultivated, had only six species, about the same as the much smaller but forested Namua. Manono, the largest island, also has the largest human population, and harbored 10 landbird species. These included two species not recorded on Nu'utele (Banded Rail [Gallirallus philippensis] and Purple Swamphen [Porphyrio porphyrio]), both of which prefer disturbed habitats to native forest. Habitat and human disturbance, rather than abiotic variables such as area and distance to the nearest island, are the primary factors influencing the distribution of birds on small offshore islands, especially those >1.0 km2.
By radio-tracking and recording the movements of flocks, the distribution of feeding Red Knots (Calidris canutus rufa) was studied day and night at a migration stopover site near San Antonio Oeste, Río Negro, Argentina in March and April 1998. By day, the birds fed in dense flocks of 500–4000 on an area of restinga or rock platform where there were beds of the small mussel Brachidontes rodriguezi. By night, this site was deserted, and the birds were found widely scattered over nearby sandflats. It was evident that the birds were feeding at night because variation in the signal strength of the radio-transmitters indicated that the birds were active. Also fresh knot droppings were found in an area which only became exposed by the tide after dark. The reason for the change in feeding distribution may be that the restinga is close to terrestrial habitats that harbor night-adapted predators. Therefore it is avoided at night. By day, it may be safer because better visibility means that predators can be identified more readily. Alternatively, it could be that feeding opportunities become available at night that are better than on the restinga, and this is why the birds feed elsewhere.
We quantified the extent of molt in Steppe Buzzards (Buteo buteo vulpinus), Marsh Harriers (Circus aeruginosus), Levant Sparrowhawks (Accipiter brevipes), and Eurasian Sparrowhawks (A. nisus) caught during spring migration 1985–1988 in Elat, southern Israel. Ten percent of yearling Steppe Buzzards (58 of 550) and four percent of yearling Marsh Harriers (3 of 77) were actively molting their remiges while on migration. These findings are contrary to suggestions that birds do not molt their flight feathers during migration when they should avoid extra energy expenditure and maintain flight performance. Active molt of primaries and secondaries, however, was not found among adult buzzards and harriers, or in any individual of the two Accipiter species. Molt strategies may be related to differences in flight mode during migration between species that primarily soar (buzzards and harriers) and species that flap more (sparrowhawks). Body condition was not related to the state of molt. This, and the fact that only yearling birds were in active molt of remiges, suggest that a delay in arrival on the breeding grounds may be a significant cost of molt during spring migration.
Male Red-winged Blackbirds (Agelaius phoeniceus) are known to give alarm calls in response to the approach of a predator, and to encode information about the level of threat in their calling behavior. To determine whether such sentinel males alert females, we conducted a simple field experiment in which we measured the distances at which incubating females flushed from their nests in response to the approach of a human observer. Using a matched-pairs design, we measured flushing distances with a sentinel male present (mean 19.8 m), and when the same male was absent from his territory (mean 10.4 m). Female Red-winged Blackbirds flushed from their nests at significantly greater distances when males were present than when males were absent. These results and those of other studies support the existence of a “predator early warning system” in the Red-winged Blackbird.
The percentage of otherwise successful nests containing ≥ 1 unhatched egg in Cliff Swallows (Petrochelidon pyrrhonota) varied inversely with colony size in southwestern Nebraska. Colony-site characteristics other than colony size had no significant effect on egg hatchability. The incidence of unhatched eggs at a colony site did not vary significantly with year, mean date of first egg laying, mean incubation period, extent of ectoparasitism, or mean body mass of nestlings or adults. This suggests that increased hatching failure in smaller colonies was unrelated to ectoparasitism by fleas and cimicid bugs, foraging success, or egg inattendance. Furthermore, because reproductive interference and incidence of brood parasitism do not increase in smaller colonies, these variables seem unlikely to account for the relationship between egg hatchability and colony size. More frequent matings between genetically similar individuals and reduced opportunities for females to seek extrapair fertilizations with outbred males or as fertility insurance might explain reduced egg hatchability in small colonies. Increased hatching success in larger colonies may be a benefit of coloniality for Cliff Swallows.
House Sparrow (Passer domesticus) eggs are useful in artificial nest experiments because they are approximately the same size and shell thickness as those of many forest passerines. House Sparrow eggs can be readily collected in quantity by providing nest boxes in active livestock barns. We collected over 1200 eggs in three years (320–567 per year) from a colony of about 24 breeding pairs by providing 60 nest boxes. Eggs dry-refrigerated at 8–9°C lost mass after 2 weeks, whereas eggs submerged in sodium silicate solution at 8–9°C remained fresh for 2 months until deployment. Eggs stored in sodium silicate solution should be rinsed with clean water before use.
Knowledge of the survival of the Pileated Woodpecker (Dryocopus pileatus) is essential in managing viable populations of the species. In eight study areas in northeastern Oregon, survivorship of adult Pileated Woodpeckers was 0.60 after 6 mo, 0.47 after 12 mo, and 0.35 after 18 mo. Of three juveniles radio-tagged in late summer or fall, two survived to breed the next year. Of 13 juveniles radio-tagged as nestlings, 23–54% survived 3.5 mo. In these populations adult mortality exceeded recruitment of young into the breeding population. Survival may have been influenced by transmitter attachments, habitat quality, or annual variability in predation.
Creation of forested edges in some landscapes can increase the risk of nest predation and consequently lower the reproductive success of forest passerines. This edge effect has not been well studied in western coniferous forests, particularly in southeast Alaska. In a series of artificial nest predation experiments, conducted in southeast Alaska between 1994 and 1997, we tested the risk of nest predation among open, edge, and interior forest habitats associated with natural (wetland) and anthropogenically created (clearcuts and suburbs) openings in coniferous north-temperate rainforest. We also censused known (Steller's Jays [Cyanocitta stelleri] and red squirrels [Tamiasciurus hudsonicus]) and probable nest predators (Common Ravens [Corvus corax] and Northwestern Crows [C. caurinus]). In general, higher nest predation was seen in habitats with the highest abundance of nest predators. Nests in wetland forest edges, where both jays and squirrels were detected frequently, were depredated more often than those in wetland openings or forest interior, where predators were less common. High nest predation was seen on the edges of suburbs where jays and crows were abundant, and in clearcut openings, edges, and interior forest, where squirrels were a common nest predator throughout. Type and abundance of predators differed among habitats and possibly with degree of forest fragmentation, edge type, and forest matrix.
In the Fraser River Delta, scavenging of poisoned waterfowl by raptors during winter has led to secondary anticholinesterase poisoning. During the winters of 1996 and 1998, we used still and video photography to examine scavenging activity on waterfowl carcasses in agricultural fields. Carcass discovery was rapid for both study years; all but two of 54 carcasses were found within 72 h, and 77.8% were found within 24 h (primarily in the first 12 h after sunrise). Single duck carcasses attracted a mean of 16.6 individual scavengers (range 0–79). Seven different species fed on a carcass at least once during 1998, with Bald Eagles (Haliaeetus leucocephalus) and Northwestern Crows (Corvus caurinus) being the most common scavengers. In 1996, Bald Eagles arrived first in 26.7% of the trials. In 1998, Northwestern Crows arrived first in 50.0% of the trials with Bald Eagles and Northern Harriers (Circus cyaneus) each arriving first in 16.7% of the trials. Results demonstrate that individual poisoned duck carcasses can place numerous scavengers of a variety of species at risk for secondary poisoning due to high incidence of discovery and rapid exchange of information among birds.
Killdeers (Charadrius vociferus) are considered a common species that inhabits a wide range of wetland and upland habitats throughout much of North America, yet recent information suggests that they may be declining regionally, if not throughout much of their range. To address this issue, we examined population trends of this species at multiple spatial and temporal scales using data from two major avian survey efforts, the Breeding Bird Survey (BBS) and Christmas Bird Count (CBC). A summary of BBS trends indicates significant long-term (1966–1996) declines in breeding populations across North America. Geographic regions driving this decline were Canada, western survey regions of the continent, and select southeastern states. In contrast, over the same time period, Killdeer populations increased in some midwestern states, particularly those in the Great Lakes region. Recent BBS trends (1986–1996) indicate highly significant declines across most areas of North America surveyed. Trends from CBC data (1959–1988) indicate declines at a smaller spatial scale. While the ability of current major avian survey efforts to detect population trends for Killdeer and other shorebird species warrants further examination, significant negative trends in Killdeer populations indicates the need to further investigate the status of this species.
We radio-tagged Black Rails (Laterallus jamaicensis) and located active nests at two sites in Florida from May 1992 to August 1996. We collected telemetry locations from 1–6 h daily during the egg-laying and incubation period. Home range estimates differed significantly between sexes; males used 1.3 ha and females used 0.62 ha. Nests were found at all stages of egg-laying and incubation using radio telemetry and visual nest searching. Nests were located during each month from May through August. Nests were constructed over moist soil in low (≤1 m) dense herbaceous vegetation. Black Rails preferred nest sites near hyper-saline patches of bare sand. The Mayfield estimate of nesting success was 43%, nest failures were caused by flooding and predation. Imported red fire ants (Solenopsis invicta) were observed constructing mounds under three nests and killed one hatchling before it emerged from the egg. Water level and hydrology may have the greatest impact on nest-site selection and nesting success of Black Rails.