Asymmetric interspecific aggression and elevational distributions

white-eyed robins -- very common, but only in the lower foothills (On mt. karimui, from ~1200 to 1700 meters). white-rumped robins are found further down, and slaty robins higher up. 

white-eyed robins -- very common, but only in the lower foothills (On mt. karimui, from ~1200 to 1700 meters). white-rumped robins are found further down, and slaty robins higher up. 

There are many cases where closely related bird species live at different elevations. So why doesn't the lower elevation species live higher on the mountain, or the higher elevation species lower? After all, they have wings...

It is tempting to think that interspecific competition is responsible for this pattern. But there are several other factors that can generate parapatric distributions, and inferring interspecific competition in the absence of any further information is dubious. We decided to measure interspecific aggression in five species-pairs of elevational replacements of New Guinean songbirds. We could use playback experiments to measure interspecific aggression, and aggression is a likely mechanism of interspecific competition. It took a lot of hard work traversing steep muddy slopes to find territories of our target species and conduct playbacks. In the end, we found interspecific aggression in three of the five species-pairs; in each case, the interspecific aggression was asymmetric, with the lower elevation species more aggressive to the upper than vice versa. In sum: some evidence that interspecific aggression (and presumably interspecific competition) is one important factor shaping the elevational distributions of New Guinean songbirds. Read the paper for a more nuanced view, or, better yet, check out this blog post describing our study: http://www.bou.org.uk/freeman-elevation-aggression/.