Racial Differences in Nests of White-Crowned Sparrows

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Abstract
Comparisons were made of the nests of eastern white-crowned sparrows (Zonotrichia leucophrys leucophrys) from a subartic area in Manitoba, Canada, mountain white-crowned sparrows (Z. l. oriantha) from subalpine meadows in California and Colorado, and Nuttal''s white-crowned sparrows (Z l. nuttalli) from the coast of California [USA]. The nests were open bowls lined with fine grasses. They had a more or less bulky frame of twigs, the woody stems of shrubs, leaves, bark, and/or coarse grasses. Small amounts of many other items were buried in them. The adults of these races are about the same size, but their nests are not. Nests of Z. l. oriantha were the largest, those of Z. l. leucophrys the smallest, and those of Z. l. nuttalli intermediate in size. Mountain white-crowned sparrows build on and above the ground: their elevated nests were much larger than their ground nests. All nests blocked out 96-99% of air currents to which they were exposed. This characteristic is particularly well-suited to the windy conditions that prevail on the breeding grounds. The nest''s overall thermal conductance was significantly higher in Z. l. leucophrys than in the other races, even though Z. l. leucophrys nest under colder and wetter ambient conditions that the other two. They build their nests in the ground, however, which probably provides additional insulation and certainly reduces exposure to drafts. Thermal conductance was significantly related to the nest''s mass and several of its dimensions, but was not simply a function of nest size. For example, the thermal conductance of the nest wall depended on the wall''s porosity, but not on its thickness. Thermal conductance is not only a quantitative measure of the nest''s insulation, but also is can be used to estimate the energetic cost of incubation.