"Does Egg Viability Constrain the Onset of Incubation in Birds? An Experimental Analysis using a Tropical Climate Gradient"

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Birds are unique among vertebrates because they can control the hatching intervals of their eggs by determining when to start incubating them. However, many avian parents choose an incubation strategy that causes their eggs to hatch asynchronously, which often leads to the death of younger chicks. Researchers have rarely considered how air temperature affects the survival of eggs before they are incubated (the Egg Viability Hypothesis). Eggs do not develop below 24 degrees C and must be warmed by a parent to 36 degrees to ensure proper development. Abnormal development of embryos leading to death is caused by prolonged exposure of eggs to intermediate temperatures (27 degrees-35 degreesC).This 3-year study employs a natural climate gradient along the slope of a tropical mountain in Puerto Rico to test the Egg Viability Hypothesis using eggs of the Pearly-eyed Thrasher (Margarops fuscatus), a species that nests in cavities (and nest boxes) throughout the mountain. The PI will move unincubated eggs up and down the mountain to expose them to different temperature regimes, and measure their subsequent hatching success. The PI will also place thrasher eggs in incubators to expose them to controlled changes in temperature that parallel temperature changes along the mountain. Finally, the PI will use video cameras and temperature recorders to examine whether thrashers vary the onset of incubation across this gradient. This will be the first study to examine the factors that affect the hatching success of bird eggs by experimentally exposing them to a critical set of temperatures using a natural thermal regime. A decline in the hatching success of unincubated eggs may have implications for understanding why tropical birds lay few eggs than temperate counterparts and what might happen to the hatching success of their eggs from global warming.