Abstract:
Mixed-species flocks play a prominent role in the social organisation
of birds, especially in the tropics (Greenberg 2000, Sridhar et al.
2009) and it has long been observed that some ‘nuclear’ species
play essential roles in the formation, maintenance and leadership
of mixed-species flocks (Moynihan 1962, Goodale & Beauchamp
2010). Several authors have suggested that when nuclear species are
absent, flocks may break up, and this might make flocking species
vulnerable to human disturbance (Maldonado-Coelho & Marini 2004,
Sridhar & Sankar 2008, Zhang et al. 2013). This could be a particularly
interesting example of why non-trophic species interactions should
be taken into consideration when devising strategies for conservation
(Valiente-Banuet et al. 2015).
Here we report on flocks that persist in the absence of a typical
nuclear species. In the lowlands of the wet zone of Sri Lanka, Orangebilled
Babbler Turdoides rufescens demonstrates all the characteristics
of a nuclear species: it is noisy and active, highly gregarious and
leads most flocks (Kotagama & Goodale 2004, Jayarathna et al. 2013).
A secondary nuclear species might be the Greater Racket-tailed
Drongo Dicrurus paradiseus lophorhinus; this taxon—considered
by some authorities to be an endemic monotypic species, the Sri
Lanka Crested Drongo Dicrurus lophorhinus—makes loud alarm calls
(Goodale & Kotagama 2005a) and is as attractive to other species as
babblers in playback experiments (Goodale & Kotagama 2005b).
However, it is not gregarious and does not facilitate the foraging
of other birds, being a sallying species, and one that can also be
aggressive and kleptoparasitic (Satischandra et al. 2007). At the Hiyare
Forest Reserve study site there are no Orange-billed Babblers, but
the Sri Lanka Crested Drongo is present.
