High proportion of non breeding individuals in an isolated red-billed chough population on an oceanic island (La Palma, Canary Islands)
Published: Volume 56(2), December 2009. Pages 229-239.
Isolated bird populations on oceanic islands may be good study models for the investigation of the interrelationships between social fractions of populations, especially due to their lack of long-range dispersal as a major mechanism influencing the dynamics and persistence of populations. We examined whether the ratio of breeders to non-breeders varies in a completely isolated red-billed chough (Pyrrhocorax pyrrhocorax) population on an oceanic island (La Palma, Canary Islands) as compared to continental and other island populations, and assessed whether limited breeding opportunities may influence population crowding with non-breeding birds. The chough population in La Palma was composed of a proportion of non-breeders (about 60 %) representing twice the values reported in other populations. Most communal roosts were used during the breeding season by floaters sufficient in numbers to replace any loss among breeders. The average number of pairs nesting at several roosts did not differ between consecutive years despite much higher numbers of floaters using these sites throughout the year. The high proportion of non-breeding choughs suggests that nesting areas were saturated with non-breeding floaters due to some kind of limitation on breeding opportunities. Under conditions of isolation, limited breeding opportunities of floaters can not be eased by dispersing to other nuclei or vacant geographical areas outside the island, leading to a crowded non-breeding fraction. The dense chough population mostly composed of gregarious floaters in La Palma may be considered a guarantee of persistence and even future numerical increase. However, a low contribution of floaters to the effective population size compared with their contribution to the total population density may enhave repercussions in conservation by reducing the availability of essential resources for breeding pairs, which requires further research.