|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.
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
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
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
Memory: 2,048 times-temperature recordings
Battery Life: 10 years
Sensitivity: -20 °C to +85 °C
in .5 " increments
Function: Records temperature and relative time