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Nest-Box
Data Reveal Surprising Results
A summary of 1999 data from The Birdhouse Network The Birdhouse Network (TBN), a citizen-science project that studies cavity-nesting birds, is pleased to report preliminary results from 1999 data collected by TBN nest-box monitors. TBN participants took part in any of four studies examining different aspects of the breeding biology of cavity-nesting birds. The studies look at variation in clutch size, nest-site selection and ectoparasites, feather use in nest construction by swallows, and calcium intake. This report summarizes results from all but the last-mentioned study. Data compiled from 1999 reflect 5,565 nest boxes and 5,912 nest attempts as reported by TBN participants. Unless otherwise noted, the summaries pertain to the 10 most-reported species. A more complete summary of results can be found on the web site at http://birds.cornell.edu/birdhouse. Clutch Size Study As part of the Clutch Size Study, participants count the number of eggs laid inside each nest box. For the most common species, mean clutch sizes ranged from 4.32 eggs for Eastern Bluebirds to 6.12 eggs for Black-capped Chickadees (Table 1, page 14).
Carolina Chickadees showed the highest average number of fledglings (3.66/box), followed closely by Western Bluebirds (3.65/box) and Mountain Bluebirds (3.63/box); the lowest average number of fledglings was reported from Eastern Bluebirds, averaging 2.94 fledglings per box. Eastern Bluebirds, a species capable of nesting more than once per season, also had the longest breeding season with earliest and latest clutch initiation dates spanning 149 days. On the other hand, the time span between earliest and latest first-egg dates for Carolina Chickadees was only 54 days. Excluding House Sparrows, the remaining top-10-reported species fledged at least one young in at least half of their nest attempts (Figure 1, page 14). In addition, Mountain, Western, and Eastern bluebirds, along with Carolina Chickadees and Ash-throated Flycatchers, had one or more young fledge in 70 percent of their nest attempts. The House Sparrow fledged young in only 7 percent of its nest attempts, presumably due to human intervention.
In addition to counting the number of eggs laid, participants report the number of unhatched eggs in each clutch. Last year, nest-box monitors reported unusually high numbers of nests with at least one unhatched egg. In 1999, 7 of the 10-most-common cavity-nesting birds (excluding House Sparrows) had at least one unhatched egg in more than 20 percent of their nests. According to André Dhondt, a principal investigator of The Birdhouse Network, this is an unusually high number and a possible cause for concern. Among bluebirds, unhatched or infertile eggs occur in approximately 10 to 15 percent of nests. It’s surprising that TBN participants reported unhatched eggs in‰ more than 25 percent of their nest boxes for all three bluebird species. Mountain Bluebirds showed the highest percentage of boxes with one or more unhatched eggs (28.7 percent), whereas House Wren nests with one or more unhatched eggs totaled 17.38 percent. Typically, eggs don’t hatch because one parent is infertile, the eggs are poorly incubated, or the shell is weak and doesn’t allow the embryo to develop properly. Most often, inadequate incubation occurs because of inclement weather, disturbances to the nest, or because the female is young and has had little incubation experience. As nest-box monitors, there is nothing we can do about infertile parents. We can, however, monitor boxes sparingly (once or twice weekly), avoid early morning checks (especially during incubation), and always avoid checking boxes in bad weather. These precautions will minimize disturbances to a nesting pair (especially an incubating female). Low hatching rates because of weak shells may indicate environmental hazards such as acid rain—precipitation polluted by sulphur dioxide and nitrogen oxide. The primary culprits in sulphur and nitrogen emissions are cars and power plants burning fossil fuels such as coal and oil. Many factors can affect a habitat’s ability to combat acid rain. Most notable is the amount of calcium and limestone in soil, which act as acid neutralizers. Acid rain has more impact on soils that lack calcium and limestone, stripping away essential plant nutrients. The acidic precipitation also causes toxic elements such as aluminum to become more soluble, and thus easier for plants to absorb. Insects feeding on plants (especially spruce trees) ingest unhealthy levels of aluminum and decreased amounts of calcium, and birds feeding on the insects ingest toxic chemicals while receiving fewer of the essential nutrients they need. Scientists believe that high aluminum levels and low calcium levels interfere with a bird’s ability to produce healthy eggshells. Areas such as the Adirondack Mountains of New York, the Appalachians, the Green Mountains of Vermont, and the Canadian Shield are most vulnerable to acid rain because of their high elevations, small watersheds, and naturally acidic soils. The Birdhouse Network would like to determine whether some bird species are more susceptible to acid rain than others, and whether acid rain causes the high number of unhatched eggs reported to TBN. Are birds in one region, such as the Adirondacks, more likely to be affected by acid rain than birds in the Rocky Mountains? How large a role does diet play in determining the fertility of eggs? Could this high number of unhatched eggs be part of a natural cycle helping to keep populations in check? To continue to study this phenomenon, The Birdhouse Network needs data from all over North America. Lab of Ornithology scientists need participants’ data from a vast geographic area to help them first discover such phenomena and then investigate the underlying causes. Another surprise revealed by the Clutch Size Study is that Tree Swallows have taken a back seat to Eastern Bluebirds as the species most commonly reported by Birdhouse Network participants. Since 1997, the number of Tree Swallows reported has decreased by 17 percent, while the number of Eastern Bluebirds increased by 5 percent (Figure 2). This change could be due to various factors, including a change in Tree Swallow and Eastern Bluebird populations, an increase in the number of participants actively attracting bluebirds to specific habitats, or a change in the geographic distribution of Birdhouse Network participants. No evidence suggests that Tree Swallow populations are declining or that the distribution of TBN participants changed greatly, so the cause may be a predisposition by some participants to attract or report bluebirds over Tree Swallows. While it is true that bluebirds receive a lot of attention, we would like to emphasize that data on all cavity-nesting birds are important. Pairing boxes 10 to 15 feet apart is a great way to enjoy and attract bluebirds as well as Tree Swallows. We will continue to monitor this situation closely to determine why the number of Tree Swallow nests in our study has declined.
Feather Study The Feather Study looks at the use of feathers in nest construction by swallows. During nest building, the majority of nests (61 percent) contained zero to five feathers. Within three days of clutch completion and within three days of end of incubation, many nests (55 percent and 47 percent, respectively) contained between 6 and 20 feathers. According to David Winkler, a co–principal investigator for The Birdhouse Network, Tree Swallows line their nests with feathers because feathers help to reduce the cooling rates of nests, “allowing nestlings to spend less energy maintaining body heat and more energy on growth. If nestlings develop quicker, they have a better chance of survival outside the nest.” This is especially true during cold, wet conditions when their metabolic requirements are high but the flying insects they feed on are stationary and therefore harder to capture. The data collected in 1999 are consistent with feather-use trends reported in 1997 and 1998. Nest Site Selection The Nest Site Selection Study examines the effects of blowfly parasitism on cavity-nesting birds using nest boxes. Data from this study showed evidence of blowfly parasitism in 23 percent of boxes. Blowfly larvae were present in 35 percent of nests and pupal cases in 55 percent of nests, while 10 percent of boxes showed some other evidence of blowfly infestation. Dozens of nest-box monitors have sent their used nests to Terry Whitworth, a researcher collaborating with TBN, for further examination. In his estimation, larval populations exceeding 10 actively feeding larvae per nestling cause nestlings to become anemic and more susceptible to starvation, hypothermia, and other parasites, such as mites, fleas, and lice. Whitworth found that 53 percent of the nests he received were were infested with blowflies, and of these infested nests, 24 percent had enough blowflies to cause anemia in chicks. Our thanks to Terry Whitworth for his informative analysis of all the individual nests he received. I would like to thank all of the participants who not only monitored boxes, but submitted their data for this summary. Without them, none of these analyses could take place. As we head further into cyberspace, I hope to see not only an increase in the number of participants, but more importantly, an increase in the number of people submitting data. Collecting data on this large scale is especially important for monitoring less-common birds such as the Violet-green Swallow, Wood Duck, Purple Martin, and Bewick’s Wren, all of which are underrepresented in our database. As most of you know, data for the 2000 nesting season will be accepted only over our web site at http://birds.cornell.edu/birdhouse. We are certain the new web-based data entry will prove to be easier than paper forms and electronic software, and we hope this prompts more of our participants to tell us what is happening inside their nest boxes. If you are signed up for 2000, please remember to enter your data by September 30. Your data are important to us—and to the birds! To see this summary in its entirety, as well as a more complete analysis of Terry Whitworth’s research, visit the News and Results section of TBN’s web site. If you are not signed up for TBN but would like to add to our growing database of nest boxes, please join The Birdhouse Network—one box is all you need. We look forward to your participation and continued dedication to our native cavity-nesting birds. |
