OLSON, C. R.; VLECK, C. M.; VLECK, D.; Iowa State University; Iowa State University; Iowa State University: Transient Thermal Fluctuations in Avian Eggs and their Implications for Growth and Ontogeny of Metabolism

For bird embryos, episodic cooling that occurs when the incubating adult leaves the nest to forage is a fact of life for many species. These cooling events occur frequently throughout incubation and may result in extended incubation periods. We conducted artificial incubation experiments on eggs of zebra finch (Taeniopygia guttata) and house wren (Troglodytes aedon) where the magnitude of cooling during simulated cooling events was varied. Eggs were allowed to develop at a constant temperature suitable for the species (ca. 37�), or were placed in treatments where eggs were kept at a suitable temperature but forced at hourly intervals to cool briefly to predetermined values (e.g. 25�C) and then rewarm. These treatments were designed to expose eggs to quantifiable thermal fluctuations, but do not exactly mimic what occurs in the wild. Eggs experiencing lower minimum temperatures also experienced lower mean temperatures over the incubation period. Egg metabolism at 37.5�C was measured at 2-day intervals until hatch or death, and ontogeny of metabolism was used as a proxy to non-invasively measure the trajectory of development of eggs throughout incubation. Metabolism increased in an approximate exponential pattern with time into the incubation period, but the rate of increase was reduced in treatments with greater temperature fluctuations. Zebra finch eggs were more severely affected by thermal fluctuations than house wren eggs, and these differences in embryo thermal tolerance coincide with different incubation behaviors in these two species. Zebra finches are biparental incubators who alternate time on the eggs, thus limiting natural cooling events. House wrens are uniparental female incubators who frequently leave the nest unattended, and whose eggs may regularly experience thermal fluctuations.