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Insights from 60,000 Nests

A decade of results from The Birdhouse Network

Scientists use participants? data to learn more about the challenges birds face in raising young, such as the Eastern Bluebird chicks above.

Jonathan Morgan

For 10 years, participants of The Birdhouse Network have reported a tremendous amount of information to the Cornell Lab of Ornithology: details of more than 60,000 nest attempts for 40 bird species that breed in nest boxes. This robust and powerful data set includes localities from across the United States and Canada through time, enabling scientists to study how environmental factors affect birds' success in raising their young. Scientific journals have published 10 articles using data collected by The Birdhouse Network's citizen-science participants, some of which are described below.

How many eggs in the nest?

The Clutch Size Study, The Birdhouse Network's longest running and most widely published study, examines factors that affect the number of eggs a bird lays in her nest. Using more than 20,000 records for Eastern Bluebirds, researchers found that birds that initiated nests later in the season tended to lay fewer eggs. In addition, bluebirds laid more eggs per nest in northern latitudes than in southern areas (Dhondt et al., Ibis 144: 646–651).

This might lead to the inaccurate conclusion that birds are more productive in northern latitudes. In fact, we found that birds breeding in the North and South achieve the same overall productivity in different ways. Southern birds experience longer nesting seasons, so they squeeze in more nest attempts each year, but on average each complete clutch is smaller than those in the North (Cooper et al., Journal of Avian Biology 36: 31–39). This study provides important evidence of latitudinal variation in nesting strategies of Eastern Bluebirds.

Eggs, hatching, and climate

With recent emphasis on global climate change, researchers are also studying how climate affects when birds lay their first egg of the season and how many eggs they lay per clutch. Using data collected by citizen-science participants, researchers found that Tree Swallows were laying their eggs earlier as spring temperatures increased during 1959–1991. At the end of this period, they were laying their eggs about 9 days earlier. (Dunn and Winkler, Proceedings of the Royal Society of London B 266: 2487–2490.)

Hatching failure may also be linked to climate. Birds typically lay just one egg per day. Often, they wait until all the eggs have been laid before regulating the eggs' temperature by incubating or shading them. Extreme prolonged heat can cause abnormal embryonic development before incubation begins, increasing the risk of hatching failure in the first few eggs laid.

In a recent analysis of data from The Birdhouse Network, we found that Eastern Bluebirds had the highest rates of hatching failure in situations with warmer conditions: at lower latitudes in the South, and late in the season, regardless of latitude. We also found high hatching failure for both small (three-egg) and large (six-egg) clutches, the latter of which may also be correlated with higher temperatures, since the first-laid eggs sit unattended for longer periods of time in larger clutches. (Cooper et al., Ibis 148: 221–230). Nesting patterns of Eastern Bluebirds across a wide geographic area suggest that when females lay large clutches in warmer climates, they begin incubating before the clutch is complete, possibly to minimize hatching failure (Cooper et al., Ecology 86: 2018–2031).

During a three-decade period, researchers found that Tree Swallows laid their eggs earlier in the season as spring temperatures increased.

Jonathan Morgan

Exploratory analysis of additional data from The Birdhouse Network suggests that other birds are also laying their eggs earlier now than they have in recent decades. Researchers will continue to use long-term data from The Birdhouse Network to examine the timing of nesting of birds in response to global climate change and other environmental factors.

Incubation rhythms

The geographic differences in hatching failure prompted us to investigate incubation patterns in greater depth. We were particularly interested in how female birds divided their time between looking for food to meet their own energy requirements and staying at the nest to regulate the temperature of their eggs.

Beginning in 2002, nest-box monitors inserted data loggers into Eastern Bluebird nests at 40 sites to record time and temperature. This allowed us to deduce when and for how long the females were incubating the eggs, and when they left the nest to look for food.

The preliminary analysis shows that birds in northern and southern latitudes have different daily incubation rhythms. Southern bluebirds take it slow, spending longer periods away from their nests each day, whereas northern bluebirds are more attentive to their clutch. We reported these results last year at the International Ornithological Congress in Germany and are now refining our understanding of these patterns and the mechanisms that may explain them.

No place like close to home

Participants of The Birdhouse Network have also helped researchers document the movements of Tree Swallows from their birth sites to the places where they breed. These participants were granted permits to put uniquely numbered leg bands on nestling Tree Swallows in New York and Pennsylvania so that the swallows could be identified when recaptured.

The study found that, on average, female swallows chose to breed about 5.2 miles from where they had hatched; males nested within 1.5 miles of their natal site. Considering that Tree Swallows travel long distances to spend the winter in the southern United States, Caribbean, or Central America, the results showed that the birds were returning to an area remarkably close to their natal homes (Winkler et al., Journal of Animal Ecology 74: 1080–1090).

Nest box characteristics

Participants have also helped researchers investigate whether physical characteristics of nest boxes affect the success of breeding birds. For example, one study found that there was no relationship between a bird's choice of a clean or dirty nest box and the incidence of blowfly infestation. Because blowfly maggots suck the blood from nestlings, nest-box monitors often wonder whether they should remove old nests at the end of the breeding season to discourage blowfly infestation the following year. The results from 24 cavity-nesting species suggest that removing old nests will not significantly discourage new infestations. (Kast, BirdScope 13(4): 13–14).

Birds and pesticides

Because pesticides may affect the health of birds, The Birdhouse Network also conducted a study that compared nesting success in areas with and without pesticide application. Participants reported on the type of habitat and the type, frequency, and duration of pesticide application, if any.

When we compared several habitats with and without pesticides, we were surprised to confirm what many nest-box monitors had asserted for years: birds did very well at fledging their young on golf courses. In fact, our results suggested that nesting success on golf courses is higher than in any other habitat that the study considered, despite the higher prevalence of pesticides on golf courses (Phillips and Cooper, BirdScope 18(2): 14–15). These findings should not be misconstrued to suggest that pesticides are harmless. Nesting occurs during the span of just a few weeks, and it is possible that pesticides have effects that can only be measured over the course of a bird's lifetime.

More data needed

We have learned a lot about birds during the past decade and we'll continue to learn more, thanks to the dedicated participants of The Birdhouse Network. The ability to collect large amounts of data over time and space is the single greatest power of the citizen-science method of data collection. This spring, a new data interface will enable participants to enter data about each visit they make to a nest, providing a chronological record that can be used to estimate daily nest survival.

Please consider putting up a nest box this year and joining The Birdhouse Network. You will become a collaborator in a continentwide effort to better understand the impacts of environmental change on bird reproduction. The data you contribute will be part of a permanent historical record and a legacy of your important nest-monitoring efforts that researchers will use for many more decades to come.

For more information,visit

Tina Phillips is project leader of The Birdhouse Network. Caren Cooper is a research associate in the Lab's Citizen Science program and Bird Population Studies. Megan Whitman is project assistant for The Birdhouse Network.


For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email:

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