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At least 55 records · Page 3Linked to original sources
USGS2010

Impact of harvest on survival of a heavily hunted game bird population

Context . Despite their economic importance and intensive management, many game bird species, including the northern bobwhite Colinus virginianus , are in decline. Declines may be explained, at least in part, by low survival due perhaps to poor habitat quality, high predation or excessive hunting pressure. Aims . This study sought to estimate and model annual/seasonal survival probabilities, to evaluate factors influencing them and to determine the cause-specific mortality rates for northern bobwhites subject to varying levels of harvest on the Babcock–Webb Wildlife Management Area (BW area), south Florida, USA. Methods . We applied Cox’s proportional hazard models to data collected from 2066 radio-tagged bobwhites during 2002–2008 to test for intrinsic and extrinsic factors affecting survival and the non-parametric cumulative incidence function estimator to estimate cause-specific mortality rates. Key results . Mean annual survival (0.091 ± 0.006) in the BW area was lower than most estimates reported for other bobwhite populations. Annual survival differed between adults (0.111 ± 0.008) and juveniles (0.052 ± 0.008), and varied among years. Survival in winter (October–March; 0.295 ± 0.014) was similar to that in summer (April–September; 0.307 ± 0.013). Density of food strips (i.e. long and narrow food plots) did not influence survival, but hunting effort (number of hunters per day per km 2 ) had a substantial negative impact on survival. In the lightly hunted field trial zone, winter (October–March) survival was significantly higher (0.414 ± 0.035) than in the other more heavily hunted management zones (0.319 ± 0.016). Cause-specific mortality analyses revealed that bobwhite mortality during summer (April–September) was mainly due to raptor (39.7%) and mammalian predation (35.6%), whereas hunting was the primary cause of mortality during winter (47.1%). Conclusions . Our results highlight the potential role of harvest as an important cause of the northern bobwhite population declines in south Florida. High mortality during winter may reduce recruitment of juveniles to the reproductive segment of the population, and ultimately the population growth. Implications . Our results suggest that reduction in hunting pressure may be necessary to reverse the declining population trends in heavily hunted game species in public lands, such as the northern bobwhites in the BW area.

Florida↗
USGS1999

Predatory bird populations in the east Mojave Desert, California

We surveyed 7 species of predatory birds weekly during a 12-month period (December 1992 through November 1993) in the east Mojave Desert, California. The Common Raven (Corvus corax) was the most frequently observed species with an average of 6.9 sightings per 100 km. Turkey Vultures (Cathartes aura), Red-tailed Hawks (Buteo jamaicensis), Loggerhead Shrikes (Lanius ludovicianus), American Kestrels (Falco sparverius), Golden Eagles (Aquila chrysaetos), and Prairie Falcons (Falco mexicanus) were seen in decreasing order of frequency of observation through the study period. Ravens, Red-tailed Hawks, Loggerhead Shrikes, American Kestrels, and Prairie Falcons were seen throughout the year. Turkey Vultures were not present during winter months, while Golden Eagles were seen only during November and December. Turkey Vultures, Red-tailed Hawks, and ravens were most numerous on agricultural lands, while Loggerhead Shrikes were most Common at urban areas. Raven numbers increased with increasing number of linear rights-of-way parallel to the survey route. Perching was the most common behavior type, although Turkey Vultures and ravens were often observed soaring, flying, or standing on the ground near highways. Transmission powerline towers and telephone poles were used as perch sites disproportionately to availability.

Great Basin Naturalist↗
USGS2025

Status and trends of forest bird populations at Hakalau Forest National Wildlife Refuge, 1987–2024

Since 1985, the Hakalau Forest Unit of the Big Island National Wildlife Refuge Complex (hereafter, Hakalau) has protected the largest endemic forest bird diversity in the State of Hawaii. This includes three endangered and one threatened species and their habitats. Hakalau’s vast area (155 km2), mostly high elevation (>1500 m) montane forest, provides refuge from avian malaria (Plasmodium relictum) vectored by introduced southern house mosquitoes (Culex quinquefasciatus). However, increases in the seasonal temperatures optimal for disease carrying mosquitoes associated with climate change have coincided with recent downward trends of native species in the closed forest area of the refuge (1450–1750 m), where disease is most likely to occur. Therefore, to inform refuge management with the most updated information on these populations and their trends, we analyzed forest bird survey data collected using point transect distance sampling with new survey data from 2021–2024. We stratified our analysis across management units, including the open forest and pasture since 1987, and the closed forest since 1999, and across four elevation ranges (<1500, 1500–1700, 1700–1900, and >1900 m) since 1999. We used distance sampling to estimate species- and strata-specific abundances and applied log-linear regression to detect trends across the timeseries. We found a continuation of previous trends, wherein most native forest birds declined in the closed forest strata (mostly below 1700 m) and increased in the highest elevation and pasture strata. Patterns were highly species-specific for the three lower elevations and open forest strata. ‘Apapane (Himatione sanguinea) and warbling white-eye (Zosterops japonicus), the most abundant native and introduced bird species, respectively, increased in nearly all strata. ‘I‘iwi (Drepanis coccinea) and Hawai‘i ‘amakihi (Chlorodrepanis virens virens), also common, decreased in closed forest, were stable in open forest, and increased in pasture. Both species were generally stable or increasing across all elevation bands, except Hawaiʻi ʻamakihi decreased in the below 1500 m and the 1500–1700 m elevation bands. All three of the federally endangered forest bird species declined in closed forest. Hawai‘i ‘ākepa (Loxops coccineus) also declined in the open forest and 1700–1900 m band and was the only species to decline overall. ‘Akiapōlā‘au (Hemignathus wilsoni) declined in closed forest but increased overall. ‘Alawī (Loxops mana, also known as Hawai‘i creeper) also decreased in the closed forest, but was stable to increasing in most strata and remained stable overall. Hawaiʻi ʻelepaio (Chasiempis sandwichensis) declined in closed forest, was stable in open forest and most elevation strata, and increased in pasture. ʻŌmaʻo (Myadestes obscurus) was stable in open and closed forest and at middle elevations and increased in the pasture. Introduced red-billedleiothrix (Leiothrix lutea) declined in closed forest, was stable in the open forest and at most elevations, and increased in the pasture. Coinciding with these changes, seasonal conditions for vector occurrence have continued to lengthen in the lower elevation strata through 2024, while Hakalau’s outplanting efforts continued to increase forest cover in the pasture, suggesting that disease-free habitat may have decreased in the closed forest and increased in the pasture. The declines in the closed forest and mixture of trends in the open forest and middle elevations bands suggest emigration into the higher elevation strata from lower elevation strata, and that possible threats have suppressed forest birds even at elevations >1500 m.

Hawaii↗
USGS1999

A test of the variable circular-plot method where exact density of a bird population was known

Variable circular-plot (VCP) counts are statistically more sound than point counts because they are adjusted for the probability of detecting birds at different distances and under different conditions. However, many ornithologists use point counts rather than VCP counts because they believe that assumptions of the VCP method are almost always violated, leading to poor results, and because earlier field tests using ad hoc analysis methods gave variable and relatively poor results. We conducted the first field test of the VCP method where the exact density of a forest bird was known as part of re-establishing the 'Oma'o Myadestes obscurus in former range. All 'Oma'o in the new population were intensively monitored by radio telemetry so that the number present during four VCP censuses was known. Excluding the first census, when three of the four detections were of the same individual, differences in VCP density estimates ranged from -34% to +24% (mean 0%) even though =18 'Oma'o were detected per survey. We review critical assumptions of the VCP method and make recommendations for data analysis based on our experience with the method on Pacific islands.

Pacific Conservation Biology↗
USGS2025

Forest bird population status on Saipan, a small oceanic island

Tropical oceanic islands are critical biodiversity hotspots where population monitoring can help to determine the status and trends of rare and endangered species. Saipan is the second largest island in the Mariana Islands and contains many endemic and range-restricted bird species. Surveys of forest birds were conducted on Saipan using point-transect distance sampling, starting in 1982. To determine population status for Saipan avifauna, we generated abundance estimates based on surveys conducted in 1982, 1997, 2007, and 2018. We also assessed community composition by exploring if there were changes in assemblage diversity, total forest bird abundance, and ratio of native to non-native populations over time. For the 2018 survey, there were 5791 detections of 18 species during 242 counts, with six of 18 species detected at > 50 % of the counts and with relatively high abundance, while 12 species were detected at < 50 % of the counts and were uncommon or rare. The most abundant species was Bridled White eye (nosa', Zosterops conspicillatus saypani ) at > 40 birds/ha and the least common species was the Saipan Reed Warbler (gå'ga' karisu, Acrocephalus hiwae) at < 1 bird/ha. We found that the community composition is becoming slightly more even over time and the overall community abundance is stable (around 375,000 birds), but non-native species have become more common while native species have remained stable. Results from this study can be used to inform conservation management of forest birds in Saipan and contribute to a broader understanding of forest bird status in the Marianas.

Saipan↗