Mixed-species bird flocks in a mountain rainforest, edge habitat and cattle fields in the Ecuadorian Chocó Maartje Albertine Musschenga Mixed-species bird flocks in a mountain rain forest, edge habitat and cattle fields in the Ecuadorian Chocó region Minor project for the master Environmental Biology, track Behavioural Ecology, Utrecht University. Period: August 2011-July 2012 Author: Maartje Albertine Musschenga, BSc. Student number: 3116913 Supervisors: Nicole Büttner, biological field station ‘Un poco del Chocó’, Pichincha province, Ecuador. Dr. Marie José Duchateau, Behavioural Biology, Utrecht University. Photo cover: the author and another student watching birds in biological reserve ‘Un poco del Chocó’, Pichincha, Ecuador. Contents Abstract 1 Introduction 2 Materials and Methods 7 Results 16 Discussion 37 References 48 Appendices 52 Acknowledgements 57 Abstract Mixed-species flocks are groups consisting of different bird species which travel and forage together. Species in a flock can be divided in nuclear species, which act as a leader or sentinel and attendant species, which follow the nuclear species. In the tropics, their main habitat, flocks are all year round. Tropical forests are threatened by many problems such as invasive settlements, roads and conversion into pastures/agricultural land. These disturbances can affect flocks in various ways. The aim of the present study was to investigate the effects of disturbance on mixed- species flocks. It was investigated how flocks in three habitats (mountain rain forest, forest edge, cattle fields) in North-west Ecuador differ from each other with respect to composition in niches, number of individuals and species, stability, nuclear and core species. The forest lies in the Chocó eco-region, ranging from Panama to northwest Peru, which is known for its exceptional high biodiversity and endemism. Trails in forest, edge and in cattle fields were walked on working days. When a flock was encountered, all species and number of individuals per species were recorded. Further it was recorded if a bird was calling and which species was going first (to identify the nuclear species). Also a description of the vegetation was made in the three areas. Vegetation in the areas differed in composition of tree type (primary forest tree, secondary forest tree, primary and secondary tree, planted). Density of trees differed significantly between the forest and the cattle fields. Flocks in the forest and in cattle fields did not differ in diversity, but their composition in niches was altered: significantly more fruit eaters, seed eaters and omnivores were seen in cattle fields, but warblers and vireos were less diverse and abundant. Further, flocks in the forest seemed to be more stable and cohesive than in the cattle fields, reflected in a higher number of core species (species that are present in 50% of flocks or more), a much higher flocking propensity (percentage of individuals of a species which are seen in a flock foraging) and more calling. Edge flocks had a significant higher average number of species and individuals per flock than forest and cattle field flocks. Nuclear species were difficult to identify, as no species fitted all the characteristics. However, the One-colored Becard was at the edge and in the cattle fields the most frequently seen species and in the forest only exceeded by Wedge-billed Woodcreeper. It was also seen leaving first more than others. Overall, the flocking system in this area was highly dynamic with much turnover of species over the days and on one day. Number of species and individuals per flock were highly correlated in all habitats. 1 Introduction Animals can form groups consisting of different species. These groups can consist of closely related species to species of different orders. Mixed species groups vary a lot in their duration, frequency, activity and structure. In mammals these groups are mainly investigated in primates, ungulates on the African savannas and cetaceans in the ocean. For example two species of tamarin monkeys (genus Saguinus) form very stable mixed-species groups in Brazilian Amazonia (Peres 1992), striped dolphins, short-beaked common dolphins and Risso’s dolphins join each other in the Mediterranean Sea (Frantzis and Herzing 2002) and Thomson’s and Grant’s gazelles mix in Serengeti National Park (Fitzgibbon 1990). In North America, coyotes and badgers hunt together in groups on ground squirrels (Stensland et al. 2003). Mixed-species groups occur even in fish, for example groups of parrotfish and surgeonfish (Wolf 1984). By forming groups of different species, each with their specialization, animals have the advantage of a large group without the costs of competition with conspecifics (Stensland et al. 2003): in a large group, the chance that an animal gets eaten by a predator is smaller. This is due to different mechanisms like the selfish-herd effect (group members try to ‘hide’ themselves in the middle of the group), the dilution effect, the encounter effect, the confusion effect and the many-eyes effect (increased chance that a predator is being detected because there are a lot of individuals). Further, the different species may be different in the way they scan predators, thereby increasing detection efficiency (Stensland et al. 2003). Mixing also has the advantages that the species can cooperate in finding food or that they can utilize the food resources more efficiently and further it may have social and reproductive benefits (Stensland et al. 2003). Not all the participating species may receive the same amount of benefits. Birds also form groups of different species; these are called mixed-species flocks or simply flocks. Mixed-species flocks are defined by Hutto (1987) as ‘groups of two or more bird species occurring within 25 meter of each other and moving in same direction for minimal 5 minutes’. Morse (1970) describes a flock as ‘any group of two or more birds, whose formation depends upon positive responses by individuals to members of their own or other species’. A flock can be distinguished from an aggregation by the fact that the birds travel together and that the advantages the birds have of joining the flock are dependent from the other flock members and not from a food source (Greenberg 2000). Members in flocks tend to use different food sources (Greenberg 2000), feeding heights and techniques (Wiley 1980). Flocks exist in all parts of the world. In temperate regions they can be found mainly outside the breeding season. For example in the USA, chickadees, titmice, wood warblers, woodpeckers, nuthatches, creepers and kinglets form mixed- species flocks (Morse 1970). However, in tropical regions, flocks are bigger and all-year round and they are an important component of community structure (Powell 1989). Scientists have long tried to define the roles of species in a flock and terminology is 2 sometimes very confusing (Greenberg 2000). Species in flocks can be divided in at least two different types: leaders (‘nuclear’) and followers (‘attendant species’). By analyzing more than hundred flocks from all over the world, Sridhar et al. (2009) discovered that nuclear species are often gregarious and cooperative breeding birds. Because of this, they have evolved an efficient alarm system and this may be a benefit for other bird species in the flock. Nuclear species can also be divided in different types and a flock can contain more than one (type of) nuclear species (Greenberg 2000): according to Hutto (1994) nuclear species ‘tend to be relatively numerous, averaging a greater number of individuals per species than any of the other flock participants, are almost always seen in flocks (high ‘flocking propensity’), most flocks contain this species, they are conspicuous by their plumage coloration, the near-continuous nature of their calls and/or their active behavior’. According to Jullien and Clobert (2000), nuclear species may also be sit-and-wait foragers, which act as sentinels for more actively foraging birds. So there are flocks were one nuclear species is of the sentinel kind and the other one acts as leader and is important for cohesion of the flock (Greenberg 2000). Greenberg also points out that nuclear species not necessarily are conspicuous birds and also their size differs. Another terminology used sometimes is that of ‘core species’: sometimes it is used for nuclear species, but according to Greenberg (2000) it is ‘the set of species found in most individual flocks in a particular habitat or stratum’. As in mixed species groups of other animals, flocks must have advantages, in order to explain their existence. The two most important hypotheses explaining mixed-species flocks are improved feeding efficiency and reduced risk of predation. Birds could improve their feeding efficiency because of insects that are flushed by other birds. They may also use flock members to copy foraging behaviour, exploit new food sources or obtain information about the location of food resources. Reducing predation risk could be caused by the mechanisms mentioned above. The two hypotheses are not mutually exclusive: birds may benefit from an increased feeding efficiency because they can decrease their vigilance (Sridhar et al. 2009, Greenberg 2000). Birds may also get advantages of flocks because they can forage in a larger territory than just in their own territory (Harrison and Whitehouse 2011). Sridhar et al. (2009) performed an analysis to discover the benefits of flock participation. They found out that follower