Date of Award:

5-1-1986

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Biology

Committee Chair(s)

James A. Gessaman

Committee

James A. Gessaman

Committee

Ivan Palmblad

Committee

Hugh Stanley

Committee

Martyn Caldwell

Committee

Keith Dixon

Abstract

Black-billed Magpie eggs that lost 9-21% of their initial egg mass as water during natural incubation had a high hatching success (80-93%). Fractional water losses of 21-27% resulted in lower hatching success (67%), and only one of four eggs that lost more than 27% of their initial mass as water hatched. Although many embryos tolerated relatively high rates of water loss throughout incubation, embryonic mortality increased if the rate of water loss during the first five days of incubation exceeded 3.00 mg cm-2 d-1. Black-billed Magpie eggs that were incubated at 37.5°C and at 4-8%, 28-32%, or 56-60% relative humidity in the laboratory had similar hatching success despite large differences in the rates of egg water loss. The wet weight and water content of hatchlings incubated at 56-60% relative humidity was greater than that of hatchlings incubated at 4-8% relative humidity or 28-32% relative humidity, but the wet weights and water contents of these latter two groups were not different. Embryos that experience high rates of egg water loss can maintain tissue water content at levels equal to that of embryos that experience rates of water loss that are 17.5% lower. The ability of embryos to tolerate a wide range of egg water loss rates precludes the necessity for regulation of nest humidity by the incubating adult; field measurements revealed that the humidity in the nest was not maintained at constant levels, nor was it related in any predictable way to the humidity of the air outside the nest. The average temperature of Black-billed Magpie eggs during natural incubation was 36.5°C. Early embryos (4 or 5 days old) and mid-stage embryos (9 or 10 days old) survived heating to 43°C, and late embryos (14 or 15 days old) survived heating to 44°C. A predictive model of heat exchange by eggs in a nest during the absence of the incubating adult revealed that eggs in roofed nests never heat to lethal temperatures even when air temperature and solar irradiance are high. However, eggs in roofless nests will heat to lethal temperatures when air temperature exceeds 19°C, solar irradiance is high, and wind speed in the nest is low. Embryos of all ages can survive cooling to 2-5°C for 30 minutes. Unattended eggs are exposed to environmental thermal conditions that result in cooling to blow 2°C only at night or on cold, overcast days during the early part of the breeding season.

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