Our results suggest that foraging efficiency is the primary driving force behind the formation of this unique association. There is a decline in foraging efficiency for the treeshrew which may have little ability to ward off attendant species (although they display aggression when there are more than two drongos and obvious food stealing by drongos). Drongos and sparrowhawks avail of insect and vertebrate prey, respectively, that are flushed by treeshrews; our limited data point to improved foraging rates for these species. There is considerable literature which supports the theory that drongos join mixed flocks for enhanced food benefits (Goodale & Kotagama 2006
; Satishchandra et al. 2007
). The role of raptors in mixed flocks in general also seems to point towards increased food benefits (Boinski & Scott 1988
; Sridhar 2007
Although foraging rates for treeshrews declined in the presence of drongos, these birds may reduce the predation risk for treeshrews from the sparrowhawk within the association or from other raptors or predators. In general, sallying birds are known to be extremely vigilant as a result of their habitual scanning for insects. Drongos are known to be adept at perceiving threats and to be especially vigilant during Accipiter
attacks (Goodale & Kotagama 2005
). Foliage gleaning species (like treeshrews), which are the most conspicuous as well as the least vigilant foragers, may be the most likely to benefit from flocks (Thiollay 1999
). In our study, when drongos were absent, treeshrews maintained greater distances from the sparrowhawk and displayed greater alertness, indicating that they did consider the raptor as a potential predator. Thus, the high degree of association between drongos and treeshrews may be owing to both foraging efficiency for the former and predator avoidance for the latter.
Sparrowhawk presence in mixed groups was strongly correlated to that of drongos. The pair-singleton dichotomy of the treeshrews plays a role in detection as breeding pairs, unlike singletons, exchange frequent contact calls even when foraging close by. Drongos may depend either on mimicry to access solitary individuals or track down the pairs on the basis of their contact calls. For the sparrowhawks, the drongos may serve as a signal for finding the treeshrews. Our occurrence data suggest that either sparrowhawks cannot trace singletons without the help of drongos or singletons do not tolerate sparrowhawks in the absence of drongos. In limited observations on flock formation, sparrowhawks were always found to join treeshrew breeding pairs or groups with drongos, suggesting support for the former hypothesis. Out of 310 scans, only four records were of a sparrowhawk with a singleton, and these were remnants of groups abandoned by drongos.
The sparrowhawk diverges from typical hunting techniques displayed by accipiters and exhibits a local, context-dependent foraging preference. Accipiter
species in general are known to supplement about 10 per cent of their diet with small mammals (Gotmark & Post 1996
) and to attack the most vulnerable group of mammals that are present in the area (Cresswell & Quinn 2004
). However, the lowered chance of predation success in the presence of drongos may have led to the choice of the predator to join the group as a commensal. Other mammal species that are known to be followed by raptors are much larger and at lower risk of predation (e.g. ungulates, primates, armadillos) compared with treeshrews.
Whereas most studies on mixed foraging flocks have examined benefits in terms of foraging efficiency, reduced predation risk or vigilance as fixed states for species or flocks, our results suggest that strategies driving flock formation are more complex and context dependent with varying benefits for different actors. What makes this association unusual is the cohesiveness and stability of the group as a whole, with the drongos and sparrowhawks actively seeking out these small mammals and the different group members receiving varied benefits. Unlike opportunistic associations that are reported in the literature, these associations are consistent, last longer periods of time and have predictable outcomes (e.g. obvious kleptoparasitism when there are more drongos; treeshrews feeding further away from sparrowhawks when drongos are absent). To conclude, the complexity of interspecific relationships in the context of mixed foraging is increased by new observations where predators join potential prey as foraging commensals and need to be examined further in the light of evolution of context-dependent strategies. At a broader scale, the dynamics of associations such as these has the potential to shed light on both ecological resource states and ethological adaptations of species.