The pink pigeon is the lone survivor of all the columbids – pigeons and doves – native to Mauritius. In 1990 the species was down to just nine individuals, but thanks to the work of the Mauritian Wildlife Foundation, there were some 400 individuals flying the skies of the island by 2013. In the year 2000, the IUCN downgraded the species from “critically endangered” to “endangered.” They’re not out of the woods yet, but their recovery remains an impressive and rare example of good news in conservation.

Still, pink pigeons have yet to recolonize certain parts of the island and nobody is quite sure why. Some researchers have considered disease, inbreeding, and habitat loss to explain it, all important factors to be sure, but Manchester Metropolitan University researcher Andrew Wolfenden thinks that no explanation can be complete without considering the role of the Madagascan turtle dove, a sister species to the pink pigeon (they’re both members of the same taxonomic genus). It’s thought that the Madagascan turtle dove was first introduced from Mauritius from its native Madagascar around 1770 following the extinction of a small dove species native to Mauritius.

Despite the fact that the two species – pink pigeons and Madagascan turtle doves – are not similar either in appearance or in ecology, their calls are nearly indistinguishable, at least to human ears. The researchers hypothesized that similarities in the calls (called “coos”) of the two species may be confusing to the pink pigeons and could explain, at least in part, the puzzle. To find out, the researchers conducted a playback experiment to see whether wild pink pigeons would respond to the coos of the Madagascan turtle doves. In all, they tested 42 pink pigeons.

Wolfenden and his colleagues found that the pink pigeons responded to the coos of Madagascan turtle doves, but not to the coos of a more distantly related species, the spotted dove, native to India and Southeast Asia. In addition, the pink pigeons’ responses to the sounds of Madagascan turtle doves were indistinguishable from their responses to the coos of their own species. At minimum, this means that pink pigeons perceive the calls of the turtle doves as relevant. It’s possible that they perceive them as the sounds of their own species. The two species’ calls aren’t only similar to human listeners, but also to the pink pigeons themselves.

What’s interesting is that the two species are not competitors. The natives forage in tree canopies and the invasives forage on the ground. “After 20 years of intensive observations of nearly all [pink pigeon] individuals in the population, there is little evidence for direct physical contact between the two species,” writes Wolfenden. Instead, he thinks that the two species compete through signal jamming. Signal jamming is when two calls are so similar that an individual mistakes the call of an unrelated species for that of a conspecific. Other research has shown that signal jamming can impact mate finding, territory defense, courtship behaviors, and even egg fertility. Even in sexually incompatible species – as in this case – the costs of signal jamming can therefore be quite high.

For example, male pink pigeons who respond both to their own species and to the turtle doves will use up more time and more energy on territory defense, to the possible detriment of their own breeding success. And since Madagascan turtle doves are far more abundant on Mauritius, a pink pigeon looking to establish a territory may think that a new area is already packed with possible competitors, even if there’s not a pink pigeon to be found. If Wolfenden is right, then that could explain the pink pigeons’ confusing pattern of recolonization on the island. The researchers conclude that conservation plans for pink pigeons will have to be reformulated to account for the signal jammers. – Jason G. Goldman | 19 December 2014

Source: Andrew Wolfenden, Carl G. Jones, Vikash Tatayah, Nicolas Züel & Selvino R. de Kort (2015). Endangered pink pigeons treat calls of the ubiquitous Madagascan turtle dove as conspecific, Animal Behaviour, 99 83-88. DOI: http://dx.doi.org/10.1016/j.anbehav.2014.10.023

Header image: Left, pink pigeon via Dick Daniels/CarolinaBirds.org/Wikimedia Commons; right Madagascan turtle dove via Roland zh/Wikimedia Commons