Multi-species territoriality and dynamic of neotropical forest understorey bird flocks

1. Eleven contiguous mixed-species bird flocks, with colour-banded individuals, were monitored continuously during 3 years in a 132-ha study area of primary rainforest in French Guiana.
2. Flock members were divided into six categories according to their flocking propensity and occurrence: 10 core or permanent species and 56 regular, occasional or incidental species. Each core species was represented by a single breeding pair with their fledglings and extra 'floaters' (unmated subadults and adults).
3. Flock home ranges overlapped slightly, but were communally defended by all core species in areas of overlap. Their size varied from 3·2 to 14·3 ha and was inversely correlated with vegetation density, but not flock size or species composition.
4. Flock number, size and composition, as well as boundaries were highly stable between seasons and years. Each flock had a single permanent gathering site and bathing site in late afternoon, the latter sometimes shared by 2–3 flocks.
5. Core species produced 0·18–0·73 fledglings per pair per year, which stayed in their natal flock for 200 to over 421 days. Then, these individuals usually moved between two and six different flocks, sometimes for up to 3 years, before finding a mate and a flock where they could settle and breed. Once breeding, they probably remained for life in the same flock. The mean annual survival rate was at least 0·75.
6. This highly evolved and stable organization, associated with a low breeding success and high survival rate was a critical factor maintaining low species density, delayed reproduction and a proportion of floating individuals buffering population fluctuations.
7. These social groups with their multi-species territoriality and co-evolved roles of flock members were similar to those described elsewhere in South America. They seem to be a general phenomenon in neotropical lowland rainforests.