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Nest Box Network Efforts Pay Off
CNBN 1997 preliminary results The Cornell Nest Box Network (CNBN), which asks citizens across the continent to team up with Lab scientists to study cavity-nesting birds, began in early 1997 with a humble first-year goal of 500 participants. But, by the time breeding season started, we found the project had sparked the interest of 645 participants from 43 U.S. states and 9 participants from 6 Canadian provinces. Last September, scores of envelopes and computer disks began arriving at the Lab, containing data on birds’ nesting activities at more than 3,000 nest boxes across North America. These data represent the efforts of 190 CNBN participants. Although we’re expecting the tally to grow as more people send their data, we decided to share a few preliminary results from the project. CNBN participants submitted data on 27 species of cavity-nesting birds—the most common of which are listed in Table 1. In addition to these species, they also monitored the nests of Hooded Mergansers, Eastern Screech-Owls, Western Screech-Owls, Northern Flickers, Great Crested Flycatchers, Mountain Chickadees, Chestnut-backed Chickadees, Oak Titmice, Red-breasted Nuthatches, White-breasted Nuthatches, Pygmy Nuthatches, Brown-headed Nut- hatches, and Carolina Wrens. Clutch Size and Fate of Nesting Attempt The average clutch sizes reported by CNBN participants for 14 cavity-nesting species are shown in Table 1, and the numbers don’t differ from those typically reported for these species. By tracking clutch sizes in years to come, we will be able to identify annual and longer-term fluctuations. The fate of the nesting attempt—whether eggs hatch and chicks fledge—is another variable that CNBN participants monitor. Our data show that for most of the species studied, if their eggs hatched, chances were good that some or all of their young fledged (Table 1). We received data on nest fate for 28 species, 10 of which had fledging success rates of at least 50 percent. In fact, 83 percent of the nests reported for Wood Ducks fledged some or all of their young. Tree Swallows, Eastern Bluebirds, Mountain Bluebirds, Carolina Chickadees, and American Kestrels successfully fledged young at least 70 percent of the time. In contrast, only 11 percent of House Sparrow nests and 23 percent of European Starling nests fledged young. These low percentages, combined with the high percentages seen for these species where eggs were laid but none hatched, most likely reflect intervention by nest-box monitors to keep these non-native species from nesting in their boxes.
Clutch Size and Latitude Clutch size is influenced by many factors, one of which is latitude. The breeding ecology of birds nesting at northern latitudes tends to differ from those breeding farther south. For example, many birds in the north begin breeding later and finish earlier, producing fewer clutches each season than birds breeding in the south. The number of eggs in a clutch can also vary between birds of the same species nesting in these different regions. Because most cavity-nesting birds nest readily in boxes, they are great subjects for investigating how latitude affects clutch size. Participants can easily find nests, count eggs, and report the nest-box location to us, providing the essential data we need to examine the clutch-size question in detail. Our preliminary data, based on just one season’s research, suggest that Tree Swallow clutch sizes do vary at different latitudes. Participants reported 539 Tree Swallow nesting attempts this past breeding season, and the swallows nesting at northern latitudes laid more eggs per clutch on average than those nesting farther south. Although participants reported 364 nesting attempts for Eastern Bluebirds, we did not find a relationship between clutch size and latitude for this species. In other words, whether these bluebirds nested in Dryden, New York, or Live Oak, Florida, they laid an average of 4.4 eggs per clutch. We also did not find a relationship between clutch size and latitude for the other 17 species, but it’s possible that we may find evidence of such a relationship in these species as the number of CNBN participants rises and more data from different latitudes are amassed.
Breeding Season Profiles
For each species studied, we used our continentwide data to examine the variation in the dates that first eggs were laid. The profile for Eastern Bluebirds shows two peaks of effort during their breeding season (Figure 1a). Most female Eastern Bluebirds began their first clutches by late April, and after their broods fledged, they laid a second clutch sometime in mid-June. We expected to find this pattern with bluebirds—this is why they are referred to as "multiple-brooded" birds. In contrast, Tree Swallows show only one peak of effort during the breeding season; females begin clutches in late May, and they breed during a narrow window of time (Figure 1b). These data illustrate perfectly that Tree Swallows are "single-brooded" birds that breed in a single "pulse." As our data set grows, we will be able to compare breeding patterns of cavity-nesting birds in different regions of North America. For example, does the breeding profile of Eastern Bluebirds living in southern latitudes show three peaks, and do they successfully raise three broods? These results are only the beginning. In future issues of Birdscope, we will report more results from the four CNBN protocols: the Clutch Size Study, the Feather Study, the Nest Selection Study, and the Calcium Study. Until now, cavity nesters had been studied only on a small scale, but with CNBN participants spanning North America, we will now be able to study them on a huge geographic scale, adding immensely to our understanding of the breeding biology of these birds. If you participated in CNBN in 1997, we thank you and invite you to join us again in 1998. Your continued help is crucial to the success of this important project. And if you did not take part in CNBN’s inaugural season, please consider signing up now. Just fill out the CNBN coupon on page 6 and mail it today. |
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