Temperature in the nest is recorded by a data logger, hidden in the nest cup and held in place by a thin wire. Another data logger, mounted on the nest-box wall, records ambient temperatures. John Schmitt

From the tropics to the polar regions, only a few millimeters of eggshell separate a delicate bird embryo from the fluctuating temperatures of the external world. Birds are remarkable for their ability to regulate their eggs, heating or cooling them to a narrow range of optimal temperatures. While incubating eggs, birds use a variety of strategies to meet their energy needs (see The Strategy of Sitting on Eggs). But until recently, exploring this variation was hampered by the lack of suitable techniques for monitoring the microclimate of the nest.

Now in a pilot study from The Birdhouse Network (TBN), participants are using new technologies to help scientists understand the variation in incubation behavior of female Eastern Bluebirds. In 130 nest boxes throughout the eastern United States, female Eastern Bluebirds are sitting not only on their eggs, but on tiny electronic data loggers that record the time and temperature. Preliminary data already show considerable variation in incubation rhythms among different females.

Traditionally, nest temperatures were monitored by using small thermocouples placed inside the nest and attached to a large and expensive externally mounted data logger. The expense and size of the equipment limited the number of nests that could be monitored during a breeding season.

Participants of TBN's pilot study are monitoring nests with new devices, each one the size of a stack of four dimes, programmed to record temperatures automatically. One device is placed in the nest cup, where it records temperatures every two minutes (see illustration above). Two others are affixed to the inside and the outside of the nest box, where they log ambient temperatures every two hours. After the participants return the data loggers, we download the data for analysis.

Figure 1. Temperatures in an Eastern Bluebird nest cup in Tennessee on March 22, 2002. This nest contained three eggs. Arrows indicate an example of the female departing and returning to the nest. When the female left the nest at 1005, the temperature dropped from 34 °C to 9 °C in 10 minutes. When she returned at 1015, the clutch warmed to 34 °C in 22 minutes. The female departed the nest again 10 minutes later (1047), thus starting a new cycle of temperature declines and increases. These patterns varied throughout the day, with the clutch rewarming more quickly in the afternoon. (Note: A graduate student at Cornell will be validating the technique by videotaping nests that are monitored with data loggers.)

The preliminary data show considerable variation in temperature fluctuations and incubation rhythms among different females (Figures 1 and 2). For example, a female bluebird in Tennessee left her nest twice as often between sunrise and sunset as a female in Texas. When she was off the nest, the temperature in the nest cup dropped as low as 5 °C in Tennessee, compared with the nest in Texas, which always stayed above 17 °C. These temperature fluctuations are useful as relative measures, permitting inferences about incubation patterns, but do not necessarily reflect the exact egg temperatures.

In general, the bluebird in Tennessee spent shorter periods away from the nest than the bluebird in Texas did (10-minute periods compared to 30-minute periods). She also spent less time sitting on the eggs before leaving, presumably to forage (30-minute periods, compared to 1-hour periods). The bluebird in Tennessee also had to spend proportionately more time rewarming the clutch to an optimal temperature - about two-thirds of each incubation bout compared with half of the incubation bout for the Texas bluebird.

Figure 2. Temperatures in an Eastern Bluebird nest cup in Texas on March 31, 2002. This nest contained six eggs. Arrows indicate an example of the female departing and returning to the nest. The first departure was at 0620 (4 minutes after sunrise), at which point the temperature dropped from 27 °C to 17 °C in 34 minutes. At 0645, the female returned to the nest and began rewarming the clutch, which reached 27 °C after 46 minutes (0740). The nest temperature continued to rise until the female departed the nest again 42 minutes later (0822). These patterns varied throughout the day. The midafternoon rhythms involved shorter on- and off-bouts, while the late-afternoon rhythms were similar to the morning rhythms.

These differences could be a result of differences in clutch sizes, ambient temperatures, the female's condition or experience, or food supply. We will have a better idea of the causes of this variation at the end of the breeding season, when participants report clutch sizes and return the remaining data loggers, which record both ambient and nest-box temperatures all season.

We'd like to thank the dedicated participants who helped us develop this pilot study for future protocols, and the North American Bluebird Society for contributing funding.

	Right on the Button
	Size: 16 mm Memory: 2,048 times-temperature recordings Battery Life: 10 years Sensitivity: -20 °C to +85 °C in .5 " increments Price: $10 Function: Records temperature and relative time