Conflict between biotic and climatic selective pressures acting on an extended phenotype in a subarctic, but not temperate, environment

Abstract

Climatic selective pressures are thought to dominate biotic selective pressures at higher latitudes. However, few studies have experimentally tested how these selective pressures differentially act on traits across latitudes because traits can rarely be manipulated independently of the organism in nature. We overcame this challenge by using an extended phenotype-active bird nests-and conducted reciprocal transplant experiments between a subarctic and temperate site, separated by 14° of latitude. At the subarctic site, biotic selective pressures (nest predation) favoured smaller, non-local temperate nests, whereas climatic selective pressures (temperature) favoured larger local nests, particularly at colder temperatures. By contrast, at the temperate site, climatic and biotic selective pressures acted similarly on temperate and subarctic nests. Our results illustrate a functional trade-off in the subarctic between nest morphologies favoured by biotic versus climatic selective pressures, with climate favouring local nest morphologies. At our temperate site, however, allocative trade-offs in the time and effort devoted to nest construction favour smaller, local nests. Our findings illustrate a conflict between biotic and climatic selective pressures at the northern extremes of a species geographical range, and suggest that trade-offs between trait function and trait elaboration act differentially across latitude to create broad geographic variation in traits.

Keywords: bird nest; climate; geographical variation; latitude; predation; trade-offs.

Figure 1.

Differences between subarctic and temperate breeding sites. (a) Subarctic and temperate sites are separated by approximately 2000 km, and 14° latitude; dark grey area on the map indicates yellow warbler breeding distribution. (b) Typical nests from the subarctic (left) and temperate (right) sites; subarctic nests are larger and better insulated than temperate nests [23]. (c,d) Differences (mean ± 1 s.e.) in (c) biotic and (d) climatic challenges confronting breeding yellow warblers between subarctic (sub) and temperate (tem) sites: the proportion of nests parasitized by arthropod ectoparasites, proportion of nests that failed because of nest predation (c), and wind speeds (maximum gusts), and average minimum temperature during the breeding season (d). Proportion data are from control nests at each location and standard errors were estimated from 1000 bootstrapped simulations; climate data are from [24]. Climatic factors were more challenging at the subarctic site compared to the temperate site, while biotic factors were relatively more challenging at the temperate site (p < 0.05, all comparisons). (Online version in colour.)

Figure 2.

Biotic costs for transplanted local and non-local nests at the subarctic and temperate sites. Costs of nest predation illustrated by Kaplan–Meier survival curves (±95% CI) and average (±1 s.e.) number of young fledged per nest for transplanted local and non-local nests at the (a) subarctic and (b) temperate site. At the subarctic site, local nests suffered higher predation (p = 0.0035) and fledged on average one nestling fewer compared to non-local nests (p = 0.014). At the temperate site, local and non-local nests suffered similar levels of predation (log rank test: χ² = 0.0, p = 0.88) and fledged similar numbers of young (p = 0.99). (Online version in colour.)

Figure 3.

Climatic costs for transplanted local and non-local nests at the subarctic and temperate sites. (a) Changes in average and minimum nest temperatures after nest transplants (post-transplant–pre-transplant temperature values), between local and non-local nests at subarctic (left) and temperate (right) sites; points are means ± 1 s.e. At the subarctic site, non-local temperate nests had colder average (p = 0.01) and colder minimum (p < 0.002) temperatures compared to local subarctic nests, while at the temperate site, local and non-local nests had similar average (p = 0.90) and minimum (p = 0.37) temperatures. Local and non-local nests did not differ in maximum nest temperature or measures of humidity at subarctic and temperate sites (electronic supplementary material). (b) Average growth rates (±95% CI) of nestlings in transplanted local and non-local nests at the subarctic site. Nestlings in non-local nests grew more slowly (p < 0.008, local n = 21, non-local n = 29), especially during cold temperatures (p = 0.029), compared with nestlings in local nests; categories ‘cold’ and ‘warm’ correspond to minimum ambient temperatures of 5.4–9.1°C and 9.2–13.1°C, respectively, recorded during the period that we measured nestling growth rate. (Online version in colour.)

Figure 4.

Time and effort to construct subarctic and temperate nests. Plots show (a) the average (±1 s.e.) number of trips a female made to construct her nest, (b) the average (±1 s.e.) amount of time a female spent at her nest during construction and (c) the average (±1 s.e.) number of days required to construct the nest for n = 5 females at both the subarctic (sub) and temperate (tem) sites. Females constructing subarctic nests made more trips (p < 0.004) and spent more time at the nest site during construction (p = 0.015), but took similar numbers of days to construct their nests (p = 0.23), compared with females constructing temperate nests. (Online version in colour.)