Christopher R. Olson and Carol M. Vleck Iowa State University
Birds incubating eggs must allocate time between maintaining egg temperatures required for embryonic growth and spending time foraging for food. It has long been known that the range of thermal conditions for eggs to hatch successfully is narrow. However, the eggs of many small songbirds frequently undergo wide thermal fluctuations when the adults leave their nests to forage, and yet the eggs hatch successfully. Birds vary in their incubation behaviors. In some species both parents incubate and eggs seldom cool. In other species only the female incubates, resulting in episodic cooling of the eggs when the female leaves the nest to forage. This suggests that some avian embryos have evolved thermal tolerance, that is, they are able to survive episodes of cooling and still maintain normal growth. This study's general goal is to describe the relationship between the thermal profile of the eggs and the incubation strategy of the incubating adult. It will incorporate two experiments carried out in the lab on the physiological and developmental responses of embryos to episodic cooling and one field experiment. 1. Metabolism of zebra finch (Taeniopygia guttata) eggs will be examined during simulated cooling and rewarming bouts to define the embryonic metabolic response to different degrees of episodic cooling. 2. The phenotypic and survival consequences of episodic cooling throughout development will be examined. This knowledge will then be extended to the field to study constraints and limitations that both adult and embryonic birds endure. 3. Cooling rates of house wren (Troglodytes aedon) eggs will be increased or decreased and adult response monitored. When cooling rates are increased, adults are predicted to compensate by shifting to behaviors that benefit the eggs, mainly spending increased time on the nest. However, when cooling rates are decreased, adults should spend more searching for food. Combining the results of these three experiments will aid in understanding how egg thermal physiology places limits on adult incubation behavior and breeding biology.
