Food, Predation, and Reproductive Ecology of the Dark-Eyed Junco in Northern Utah

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The Auk

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During 1976,1977, and 1978, 26, 29, and 19 nests, respectively, of the groundnesting, "Pink-sided" Dark-eyed Junco (Junco hyemalis mearnsi) were monitored in a sprucefir ecosystem in northern Utah. In 1976 and 1977, 4-egg clutches, found mainly in June through mid-July, produced more young than did 3-egg clutches, found after mid-July. In these two years, nestlings from 4-egg clutches in which all eggs hatched gained significantly more weight than did nestlings from 3-egg clutches. In 1978, only 5- and 4-egg clutches were found, and these nestlings were intermediate in weight between nestlings from the 4- and 3-egg clutches of the previous years but not significantly different from nestlings of either clutch size. Growth rates (K) of body weight were not significantly different among nestlings from all clutch sizes, so that the rate of weight gain seems independent of clutch size in the Pink-sided Junco. Final tarsal length upon leaving the nest also did not differ significantly among clutch sizes, but the rate of tarsal growth did. The length of tarsi was independent of clutch size, and the length upon leaving the nest was equal to that of adult juncos. We interpret this as support for the adaptive-growth hypothesis, as tarsal length should have high functional importance in passerines that run rather than fly from the nest. Populations of major food items and potential nest predators were monitored. Although climatic conditions varied dramatically during the three seasons we studied these juncos, the dry biomass of food items in junco diets peaked 1 week after the mean hatching date of 4-egg clutches in all three years. We suspect that nestlings from late 3-egg clutches experienced reduced food availability, which explains the difference in weight between nestlings from 3- and 4-egg clutches. Total rodent predation pressure remained at or above 10 individuals/ha, although populations of the species involved fluctuated between years. Increased predation was noted in 1977, when weasel populations were unusually high. Latespring snowstorms and late-summer thunderstorms also probably influence nesting success. We hypothesize that predation pressure and short periods of both food abundance and favorable weather influence the initiation of breeding, clutch size, and nestling-growth patterns and lead to a decline in clutch size as the breeding season progresses.


Originally published by the American Ornithologists' Union. Publisher's PDF available through remote link via the Searchable Ornithological Research Archive (SORA). Must click on corresponding link.
Note: This article appeared in The Auk.

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