Patterns of avian life history variation across an elevational gradient

Background/Question/Methods

The importance of environmental gradients in producing geographic variation in life history attributes has been long recognized, for e.g. avian life history variation along latitudinal gradients. Likewise, elevational gradients capture environmental variation similar to latitudinal gradients, over much shorter spatial scales. Compared to lower ones, higher elevations have colder temperatures, shorter breeding seasons, less predictable weather and higher seasonality similar to the differences between lower and higher latitudes. Thus extrapolating latitudinal patterns to altitudinal ones, we expect birds breeding at higher elevations to show decreased adult survival, higher fecundity and shorter developmental period. However, higher elevations are energetically expensive environments to breed in, and few studies have shown the opposite trends of reduced fecundity and longer developmental periods as organisms cope with the harsh environment. Here, we aim to examine patterns of life history variation present in a community of birds breeding along an elevational gradient, and environmental factors associated with them. We applied 4^thcorner analysis to assess the relationship between ecological and life history traits and environmental factors  in a breeding bird community in the White Mountains, CA, spanning an elevational range of 2,200-3,600m, from Pinyon-Juniper (low elevation) to Bristlecone Pine (high elevation) woodlands

Results/Conclusions

At the community level we found a significant positive relationship between length of the nestling period and elevation (P<0.0001). The proportion of female-only incubation also increased as elevation increased (P<0.0011). Elevation was negatively related to max and min temperature; thus ,higher elevations with colder temperatures contained more species with longer nestling periods and a more female-biased incubation.  As higher elevations pose greater energetic demands, we expected nestling periods to get longer. However, contrary to expectations, bi-parental incubation was not higher and we found increased female-only incubation.  Detecting similar trends across an entire community of birds allows us to understand similar selective pressures on species that are favored across these different environments. Further research can help elucidate the generality of these patterns and potentially identify traits that may change as the environment changes.