Territorial animals face challenges in recognizing and responding to the threat posed by competitors. Particularly interesting for studies of territoriality are circumstances where two closely related animals in strong competition for resources use dramatically different signals to deter intruders. Such a situation occurs at the narrow boundary between the recently diverged Moreau's and Fuelleborn's sunbirds, which differ dramatically in territorial song and appear to show highly disparate levels of territoriality towards one another. Intriguingly, the geographical distribution of these two species suggests that one is expanding its range at the expense of the other across the 'sky island' forest isolates of the Udzungwa Mountains, Tanzania. This study integrates experimental and molecular approaches to examine the causes and consequences of aggressive asymmetries between populations. To examine support for alternate hypotheses that explain apparent aggressive asymmetry, the study will employ a series of playback experiments in which territorial subjects incur a simulated intrusion via the broadcast of a competitor's songs. Using DNA sequence data, the study will examine: 1) whether one species has recently invaded the Udzungwa Mountains at the expense of the other, and 2) the extent of gene flow between species.
This study takes advantage of an unusual 'sky island' contact zone between species to test how signal evolution and territoriality interact, and how such interactions may influence the long-term outcomes of competition among ecologically similar species. It is rarely possible to examine how behavioral differences might scale up to impact extinction-recolonization dynamics in island-like distributions.
This study couples field studies of territorial aggressive response with molecular genetics to understand past and present interactions between two closely related bird species, Moreau’s and Fuelleborn’s Sunbirds, whose ranges abut. These two species have extremely similar ecological requirements and similar morphologies, but have strikingly different territorial advertisement songs. The two species mutually exclude one another from suitable breeding habitat, at high elevations (>1400m) in the Udzungwa Mountains, southern Tanzania. The study’s main objectives are: 1) to examine whether one of these two species responds more aggressively toward the other where they come into contact; 2) to compare, using genetics, the ancestral demography of the two species to test whether one species has more recently entered the Udzungwa Mountains and ‘pushed back’ the range of the other; 3) to compare asymmetries in aggression (1st objective) with ancestral demography (2nd objective); and 4) to test hypotheses regarding the evolutionary ecology of aggressive response to a closely related species’ song when songs are highly divergent. While analyses of collected data for the listed objectives are on-going, we can report here that the field playback tests proposed to study territorial aggressive response were successful. Responses from each individual subject were elicited using playback from three different stimuli – own species’ song, the strikingly different song of the closely related species, and the song of a distantly related sunbird that is found in the same areas. The results from these experiments will be presented at the Evolution 2011 meeting in Norman, Oklahoma, in June. Data collection for the molecular genetic component of this study has also been successful, with DNA sequences from six different regions (called loci) produced for over 100 individuals. Analyses for these data tend to be time-consuming, and are on-going. A surprising finding from this study was the discovery of a small cluster of territorial Moreau’s Sunbird males on the ‘wrong’ side of the geographical boundary between the two species. These individuals, who are otherwise surrounded by Fuelleborn’s Sunbirds, exhibit song that resembles neither species’ song. This result demonstrates the power that interactions with closely related species can have on the development of a phenotype, made especially evident in this case because of the plasticity of learned song in songbirds.
