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Results for “The Open Ornithology Journal”

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23 records · Page 2Linked to original sources
USGS2018

Population estimates of the Endangered Hawaiʻi ʻĀkepa (Loxops coccineus) in different habitats on windward Mauna Loa

Endangered Hawai‘i ʻĀkepas ( Loxops coccineus ) are endemic to Hawai‘i island, where they occur in five spatially distinct populations. Data concerning the status and population trends of these unique Hawaiian honeycreepers are crucial for assessing the effectiveness of recovery and management actions. In 2016, we used point‐transect distance sampling to estimate the abundance of Hawai‘i ʻĀkepas in portions of Hawai‘i Volcanoes National Park (HAVO) and the Kaʻū Forest Reserve (KFR) on Mauna Loa volcano. We then compiled the survey data from four other populations to provide a global population estimate. In our HAVO and KFR study area, we mapped habitat classes to determine the population densities in each habitat. Densities were highest (1.03 birds/ha) in open‐canopy montane ʻōhiʻa ( Metrosideros polymorpha ) woodland. In contrast, densities of the largest ʻĀkepa population on Mauna Kea volcano were highest in closed‐canopy ʻōhiʻa and koa ( Acacia koa ) forest where the species is dependent on nest cavities in tall (> 15 m), large (> 50‐cm diameter at breast height) trees. We surveyed potential nesting habitat in HAVO and KFR and found only one cavity in the short‐stature montane ʻōhiʻa woodland and five cavities in the tall‐stature forest. Differences in densities between the Mauna Kea and Mauna Loa populations suggest that Hawai‘i ʻĀkepas may exhibit different foraging and nesting behaviors in the two habitats. The estimated overall population density in the HAVO and KFR study area was 0.52 birds/ha, which equates to 3663 (95% CI 1725–6961) birds in their 11,377‐ha population range. We calculated a global population of 16,428 (95% CI 10,065–25,198) birds, which is similar to an estimate of 13,892 (95% CI 10,315–17,469) birds made in 1986. Our results suggest that populations are stable to increasing in the two largest populations, but the three other populations are smaller (range = 77–1443 birds) and trends for those populations are unknown.

Hawai'i↗
USGS2014

Selection of forest canopy gaps by male Cerulean Warblers in West Virginia

Forest openings, or canopy gaps, are an important resource for many forest songbirds, such as Cerulean Warblers ( Setophaga cerulea ). We examined canopy gap selection by this declining species to determine if male Cerulean Warblers selected particular sizes, vegetative heights, or types of gaps. We tested whether these parameters differed among territories, territory core areas, and randomly-placed sample plots. We used enhanced territory mapping techniques (burst sampling) to define habitat use within the territory. Canopy gap densities were higher within core areas of territories than within territories or random plots, indicating that Cerulean Warblers selected habitat within their territories with the highest gap densities. Selection of regenerating gaps with woody vegetation >12 m within the gap, and canopy heights >24 m surrounding the gap, occurred within territory core areas. These findings differed between two sites indicating that gap selection may vary based on forest structure. Differences were also found regarding the placement of territories with respect to gaps. Larger gaps, such as wildlife food plots, were located on the periphery of territories more often than other types and sizes of gaps, while smaller gaps, such as treefalls, were located within territory boundaries more often than expected. The creations of smaller canopy gaps, <100 m 2 , within dense stands are likely compatible with forest management for this species.

Wilson Journal of Ornithology↗
USGS2015

The nest-concealment hypothesis: New insights from a comparative analysis

Selection of a breeding site is critical for many animals, especially for birds whose offspring are stationary during development. Thus, birds are often assumed to prefer concealed nest sites. However, 74% of studies ( n = 106) that have evaluated this relationship for open-cup nesting songbirds in North America failed to support the nest-concealment hypothesis. We conducted a comparative analysis to identify factors that contribute to variation in the ability of researchers to find support for the nest-concealment hypothesis. We found that some of the discrepancy among studies can be explained by interspecific differences in morphological and extrinsic factors that affect nest predation. Moreover, methods that investigators used to estimate concealment affected whether studies found support for the nest-concealment hypothesis; 33% of the studies that used quantitative estimates found support for the nest-concealment hypothesis whereas only 10% of the studies that used qualitative estimates found support. The timing of measurements also explained some of the ambiguity; studies that provided little information regarding the timing of their foliage density estimates were less likely to support the nest-concealment hypothesis. Species with more conspicuous male plumage were less likely to support the nest-concealment hypothesis when we analyzed studies that used visual estimates. Whereas species with more conspicuous female plumage were more likely to support the nest-concealment hypothesis when we analyzed studies that used quantitative measures. Our results demonstrate that support for the nest-concealment hypothesis has been equivocal, but that some of the ambiguity can be explained by morphological traits and methods used to measure concealment.

Wilson Journal of Ornithology↗
USGS2014

Age-Ratios and Condition of En Route Migrant Blackpoll Warblers in the British Virgin Islands

The en route migration ecology of Blackpoll Warblers ( Setophaga striata ) is poorly understood, yet intriguing. Blackpoll Warblers undertake the longest open water migration of any wood warbler species, traveling from northeastern North America to South America, with the first potential landfall being the West Indies. This migration requires substantial energy reserves and subjects Blackpoll Warblers to unpredictable weather events, which may influence survival. Few studies have examined age ratios or condition of Blackpoll Warblers while the warblers are en route through the Caribbean region. I captured and banded Blackpoll Warblers in the British Virgin Islands over 10 consecutive autumn migrations. Ratios of hatch-year to adult Blackpoll Warblers were variable but averaged lower than the ratios reported at continental departure locations. Average mass of Blackpoll Warblers was less than that reported at continental departure locations, with 26% of adults and 40% of hatch-year birds below the estimated fat free mass; hatch-year birds were consistently in poorer condition than adults. Blackpoll Warblers captured in the British Virgin Islands were also in poorer condition than those reported from the Dominican Republic and Barbados; this may be because of the British Virgin Islands being the first landfall after the transatlantic crossing, whereas Blackpoll Warblers arriving at the other Caribbean study locations may have had opportunities for stopover prior to arrival or have departed from farther south on the continent. However, this suggests that the British Virgin Islands likely provide important stopover habitat as a first landfall location for Blackpoll Warblers arriving from the transatlantic migration route.

Guana Island↗
USGS2022

Demography and site fidelity of a grassland bird, the Henslow’s Sparrow, in powerline right-of-way habitat

Grassland birds are among the fastest declining avian species in North America, primarily due to habitat loss. In the southeastern U.S., much grassland and open savanna habitat has been converted to timber production or agriculture, neither of which typically provides habitat for breeding or wintering grassland birds. Powerline right-of-ways could provide suitable habitat for many grassland species as these areas are maintained to be treeless. We studied the population dynamics of Henslow’s Sparrows ( Centronyx henslowii ) wintering in powerline right-of-ways in southeastern Georgia through an 11-year mark-recapture study. We used a robust design Cormack-Jolly-Seber model to estimate probability of detection and apparent survival. Abundance varied substantially among years at each site, with density varying from 1.7 to 8.5 birds/ha. Within-year detection probability was moderately high at 28% (24-33%, 95% credible interval [CI]), but apparent survival was very low at 13% (9-17%, 95% CI). This low apparent survival was likely due to low return rates (and not necessarily low survival). However, birds that did return to the study sites had extremely high site fidelity, with 82% of across-year recaptures < 200 m apart. This apparent incongruity between low apparent survival rates (likely due to emigration from the study sites) and high site fidelity for returning individuals could be explained by the dependability of the right-of-way habitat, which differs from typically patchy and temporally variable grassland and savanna wintering habitats. Dependable habitat may allow for higher site fidelity than this species would otherwise have, potentially resulting in the high densities we observed. Thousands of miles of right-of-ways in Georgia, and other southeastern states, could be managed to maximize potential habitat for declining grassland bird species.

Georgia↗