Summary
1.Elevational gradients create environmental variation that is hypothesized to promote variation in life-history strategies. We tested whether differences in life-history strategies were associated with elevation in a songbird, the dark-eyed junco ( Junco hyemalis; Aves; A.O.U. 1998). 2. We monitored birds in four replicated sites per elevation, at 2000 m a.s.l. (high elevation) and 1000 m a.s.l. (low elevation), in the Rocky Mountains of Canada. 3. Over 5 years, we measured the following traits and vital rates: egg-laying schedules, morphological indicators of reproductive stage, seasonal reproductive success, indicators of competitive class (age, size, arrival time), and survival rates. 4. We found two main patterns: with an increase in breeding elevation, dark-eyed juncos delayed the development of structures necessary for reproduction (e.g. cloacal protuberance in males) and reduced the duration of their reproductive period to less than half of the time used by low-elevation birds; and 5. Juncos at high-elevation sites had 55-61% lower annual reproductive success and 15 to 20% higher survival rates. While adult juncos at high elevations produced fewer offspring, those offspring were in better condition. Proportions of age and size classes in high-compared to low-elevation populations were similar, suggesting that a life-history trade-off is present, rather than competition forcing inferior competitors to breed in a peripheral habitat. The apparent trade-off between reproduction and survival corresponded to a shorter period of favourable weather and available food in high-compared to low-elevation habitats. 6. Thus, elevation had a strong influence on life-history characteristics of a single species over a short spatial distance, suggesting a shift in life history from a high reproductive strategy at lower elevations to a high survivor strategy at high elevations. 7. This is the first paper to show a shift in avian life-history strategies along an elevational gradient (in both genders, of multiple age classes) when region (latitude) and phylogenetic histories are controlled for.
Morphologies of bird species often vary along elevation gradients, yet causes of the variation have not been examined experimentally. We investigated variation in morphological traits of the dark-eyed junco Junco hyemalis, breeding at 1,000 m a.s.l. (low-elevation; i.e. low) and 2,000 m asl (high-elevation; i.e. high) in the Rocky Mountains, Canada. Eight morphological traits were measured in free-living birds. We found two consistent differences in populations between elevations: at high-elevation sites, females had longer wings and males had longer tails than birds from low-elevation sites. Other age-and gender-specific results were observed in free-living birds between elevations: tarsi were shorter in high-elevation second year (SY) females and after second year (ASY) males, beak lengths were slightly longer in lowelevation SY females, and high-elevation ASY females tended to have lower fat than low-elevation ASY females. Morphological differences may result from genetic differences between elevations, or phenotypic flexibility resulting from exposure to the different environmental conditions. To identify which mechanism caused the difference in morphometrics, hand-reared birds from low-and high-elevation habitats were raised in identical conditions with unlimited access to high quality food until they had replaced all feathers. The traits measured in the lab (wing and rectrix length, weight and fat score) tended to increase in magnitude compared to field values. Juncos from high-and lowelevations had similar responses to the aviary environment, with one exception: males from high-elevation sites had greater weight gain relative to free-living juncos than males from low-elevation sites. Thus, morphological traits in darkeyed juncos were phenotypically flexible, capable of growing larger in the laboratory environment. However, there were also persistent genetic or perinatal/maternal differences underlying population responses that prevented traits from converging under aviary conditions. As a result, trait size differences between high-and low-elevation populations were maintained or exacerbated in the common aviary environment.
Blood samples were taken to identify parasite genera, and to test for differences in parasite prevalence among elevations. The most common parasites at either elevation were Haemoproteus spp., Leucocytozoon spp., and Trypanosoma spp. A significantly higher proportion of low-compared to high-elevation birds was infected by at least one of these, supporting the prediction that high-elevation habitats may be refuges from parasites.
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