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Thermal adaptation in North American Sturnidae Johnson, Stephen Robert

Abstract

Differential colonization success by the European starling (Sturnus vulgaris) and the tropical crested myna (Sturnus cristatellus), both introduced to North America in the late 1890's was examined in the Lower Mainland of British Columbia. Evidence existed which indicated mynas might be less well adapted than the European starling to the thermal regime in the Lower Mainland, thus, the following hypothesis was formulated: "an important reason for the observed difference in colonization success in North America by starlings and crested mynas is the relative difference in thermal adaptation". To test the above hypothesis a two part study was designed. (1) Field measurements indicated nesting season, clutch size, hatching success, growth, fledging success and ontogeny of thermoregulation for both species. Also, simple field experiments were designed to measure incubation temperatures and determine results of between species cross-fostering studies. (2) Laboratory investigations were conducted on wild caught, captive adults. Energy input, outgo and metabolic response to temperature fluctuations were measured under both laboratory and outside Vancouver conditions for both species. Finally, plumage quality was assessed in a series of cooling experiments using feathered and unfeathered carcasses of both species. The most important factors supporting my hypothesis were the relatively low nest attentiveness and consequent poor incubation success exhibited by crested mynas, compared with both the European starling living in the same environment and the common myna (Sturnus tristis) living in West Bengal (India). Basal resting' metabolism in both the crested myna and the European starling did not deviate significantly from the predicted values for birds of similar weight, however, at the extremes in the environmental temperature spectrum, both adult and nestling mynas were not as efficient in conserving energy as the starlings. Nestling myna growth (weight gain and plumage development) was slower than starling growth, however, nestling response to fluctuations were not different if comparisons are made on the basis of percent total plumage development. The results of cooling experiments indicated that adult crested myna plumage was an inferior insulator against cold compared with starling plumage. Results of bioenergetics investigations suggested mynas required more energy at colder temperatures to maintain a daily caged existence. Poor correlations were observed between energy intake, energy metabolizeable and energy excreted by caged birds and environmental temperature fluctuations in an outdoor situation. Only in the colder than usual early half of 1969 was there significant correlations between metabolizeable energy and environmental temperature. Factors not investigated in this study were: (1) inter and intra specific competition, (2) differential response to. interference by humans,, resulting in mortality, and (3) food limitations throughout the year which could limit sturnid populations in the Lower Mainland.

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