Temporal trophic segregation between Lincoln's sparrows and White-crowned sparrows in a high elevation habitat

1. Background/Question/Methods: Resource partitioning is at the basis of the coexistence of sympatric species. However the underlying factors of this partitioning have been poorly investigated in free-ranging animals. A central parameter affecting resource partitioning is resource availability and interspecific segregation is intuitively expected to decrease as resource availability increases. In the present study, we investigated temporal trophic segregation between syntopic Lincoln’s sparrows (Melospiza lincolnii) and white-crowned sparrows (Zonotrichia leucophris) by repeatedly examining their plasma isotopic signature as resource availability increases from late winter to summer in a high elevation meadow in the Rocky Mountains.
2. Results/Conclusions: We found that trophic segregation was low when resource availability was low, as both species occupied the same isotopic niche in late winter. But interestingly, as resource availability increased in spring, Lincoln’s sparrows shifted to another niche characterized by lower plasma δ¹⁵N values. This may be explained (1) by the appearance of new niche opportunities that were only available to Lincoln’s sparrows and (2) by the density of competitors that increased faster than resources in the meadow, resulting in higher interspecific competition for common resources. This shows that resource partitioning between sympatric species is a dynamic equilibrium that needs to be constantly adjusted to environmental conditions. Variations in seasonal and inter-annual conditions are therefore fundamental to consider when examining resource partitioning between co-existing species.