Cold, Not Warm Temperatures Influence Onset Of Incubation And Hatching Failure In House Wrens (Troglodytes Aedon)
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Understanding the patterns and mechanisms underlying expression of avian life history traits, including seasonal variation in the onset of incubation and clutch size, will shed light on the plasticity of avian responses to environmental change generally, and global climate change in particular. It is commonly believed that hyperthermia is more injurious to the developing embryo than hypothermia. However, most studies evaluate effects of high but not low temperatures on egg viability, and do not examine how exposure to both warm and cold temperatures influence individual laying and incubation behavior. In my study, I evaluated the behavioral responses, specifically onset of incubation and clutch size determination, of female House Wrens, Troglodytes aedon, to warm and cold ambient temperatures. I then assessed hatching failure as a function of egg exposure to these high and low temperatures. To quantify within species variation in onset, I used a continuous record of incubation from clutch initiation through hatching. Females were not significantly more likely to initiate incubation prior to clutch completion with increased duration of pre-incubation temperatures above physiological zero (greater than or equal to 24 degrees Centigrade); however, the probability of a female initiating incubation early decreased with longer exposure to cold (less than or equal to 16 degrees Centigrade) weather. Females did not tend to initiate incubation early with larger clutches, and clutch size did not decrease or increase significantly with increasing exposure to warm (greater than or equal to 24degrees Centigrade) or cold (less than or equal to 16degrees Centigrade) pre-incubation temperatures, respectively. The likelihood of partial hatching failure within a clutch and the per-egg probability of not hatching did not decrease with increased exposure to warm preincubation temperatures, but did increase with the time pre-incubation temperatures were hypothermic (less than or equal to 16degrees Centigrade). Partial hatching failure was not significantly higher in larger clutches, and was not more likely in earlier laid eggs. These results suggest that, contrary to common belief, in temperate climates, egg exposure to colder rather than warmer temperatures may be a more influential factor affecting decreases in egg viability: in House Wrens in Ithaca, New York, hatchability declined with exposure to cold, but not warm temperatures. In cold weather, females were likely constrained by the environmental conditions, and due to an increased need to spend more energy on self-maintenance, did not initiate incubation early enough to counteract the negative effects of cold temperature on egg viability.
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