Demographic studies of many bird species are challenging because their nests are cryptic, resulting in few nests being found. To maximize statistical power, methods are needed that minimize disturbance while yielding as much information per nest as possible. One way to meet these objectives is to use miniature thermal data loggers to precisely date nest fates. Our objectives, therefore, were to (1) examine the possible effect of thermal data loggers on nest success through hatching by grass- and shrub-nesting songbirds that differed in their parasite egg-accepting and -rejecting behavior, (2) examine the effect of using daily temperature data versus less frequent nest-visit data on statistical power, bias, and precision when estimating the daily survival rate (DSR) for nests, and (3) compare these two approaches using a simulation study and field data. We monitored the survival of nests located in agricultural landscapes and used a binomial logistic regression with main effects for data-loggers and parasite-accepting or -rejecting status and their interaction. We also compared maximum likelihood–derived DSR for differences in estimated rates, precision, and sample sizes with both data collected in the field and simulated with varying sample sizes and visit frequencies. We found no evidence that thermal data loggers had any effect on hatching rates either for all species or for parasite egg-accepting and -rejecting species, separately. Both our simulation and analysis of real nest data indicated that use of data loggers increased the statistical power from each nest studied by increasing effective sample sizes and precision of DSR estimates compared to in-person visits. We also found a negative bias in DSR estimates with longer visit intervals, which use of data-loggers removed. Both the results of simulated- and field-data analyses suggest that future studies of nest survival can be improved by automated nest monitoring by removing a source of bias and providing more time to find additional nests.
Few investigators have examined in vivo bite-force of birds, likely due, at least in part, to the difficulty in accessing suitable force transducers. We describe a low-cost, easy-to-build, and portable force transducer with the goal of encouraging ornithologists to improve our knowledge of in vivo bite forces of birds. We used a commercial piezo-resistive force sensor (Tekscan©) to construct our transducer, and measure the bite forces of captive birds with different beak morphologies, including the following species: Greater Rhea (Rhea americana), Black-chested Buzzard-Eagle (Geranoaetus melanoleucus), King Vulture (Sarcoramphus papa), Toco Toucan (Ramphastos toco), and Graylag Goose (Anser anser). Bite forces ranged from 25.1 N for the Toco Toucan to 109.9 N for the King Vulture. By using the bite forces obtained in our study and those reported for other birds in the literature, a regression analysis was performed, revealing that half of the variation in bite force is explained by allometry, with other factors such as beak morphology likely explaining the remaining variation. Our bite-force transducer should allow researchers to obtain low-cost, reliable bite-force data, which will improve our understanding of the relationship between form and function of the avian cranio-mandibular complex.
The Guineo-Congolian “rain” forest (G-C forest) in West and Central Africa is threatened by deforestation. From 1975 to 2013, the extent of the G-C forest decreased by 37%, from about 131,000 to 83,000 km2. Overall, 46% of bird species in the G-C forest (123 of 268) have declining populations, and about 31 species (12%) are categorized as endangered, near threatened, or vulnerable. Impacts of harvesting for “bushmeat” and the cage bird industry are largely unknown, but, of 60 species of birds in the G-C forest known to be hunted or trapped, six are categorized as vulnerable, one as near threatened, and one as endangered. In addition, 35 of the 60 species are estimated to have decreasing populations, 18 species have stable populations, and three are increasing in number. The impacts of clearing or disturbing G-C forest to cultivate cash crops are not fully known, except that avian diversity is markedly reduced in such areas. Traditional “sacred groves,” mostly small patches of forest, are not formally designated as conservation areas, but may serve as protected sites for some species of birds. Temperatures have increased and rainfall has decreased over the last five decades in West Africa. These changes will likely contribute to a further loss of suitable habitat for range-restricted species of birds. In addition, species currently found in lowland and montane habitats may be forced to move to higher elevations. Of 53 species of birds found in lowland habitat, five are endangered, seven are near threatened, 11 are vulnerable, and one is data deficient, suggesting that ~44% of lowland species may have an increased risk of extinction. Countries with G-C forest all have large human populations with high incidences of poverty, resources harvested at unsustainable rates, and increasing rates of deforestation. Networks of large protected areas in West and Central Africa, with much tighter controls over unsustainable harvesting, are urgently needed to ensure conservation of the birds and, more generally, the biodiversity of the G-C forest.
As the availability of natural nesting habitat for Bank Swallows (Riparia riparia) continues to decline, operational aggregate (sand and gravel) pits have begun to provide alternative nesting habitat. With best management practices being developed and revised for the aggregate industry, an understanding of site factors affecting burrow and bank occupancy by Bank Swallows is needed. We compared patterns of burrow occupancy of breeding Bank Swallows in lakeshore and aggregate habitats and documented other potential factors that could influence occupancy levels using data collected over a two-year period. We also used a larger sample of data from other studies collected over longer time periods to compare the persistence of colonies in lakeshore, riverbank, and aggregate habitats. In both habitats, rates of burrow occupancy were impacted by colony size and bank-face height. At lakeshore sites, burrow occupancy decreased in low burrows with increasing colony size whereas, in medium/high burrows, there was no relationship between colony size and burrow occupancy. In aggregate pits, burrow occupancy in low burrows increased rapidly with colony size, whereas in medium/high burrows, occupancy increased only slightly as colony size increased. Colonies of Bank Swallows in lakeshore bluffs were the most persistent, whereas those in aggregate pits had the lowest year-to-year persistence, likely due to annual removal of targeted resources. Colonies in aggregate pits were more likely to persist from one year to the next if the previous year’s colony was large, and faces > 200 m2. Although colonies at natural sites were the most persistent over time, the aggregate pit industry can play an active role in the conservation of this threatened species by encouraging pit operators to maintain large, tall, vertical faces on stable, unused banks to increase the probability of Bank Swallows recolonizing these sites in future years.
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