Leadership of Winter Mixed-Species Flocks by Tufted Titmice (Baeolophus bicolor): Are Titmice Passive Nuclear Species?

Thomas A Contreras,Kathryn E Sieving

doi:10.1155/2011/670548

Thomas A Contreras, Kathryn E Sieving

Open Access

https://doi.org/10.1155/2011/670548

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Abstract

The tufted titmouse (Baeolophus bicolor, TUTI) is a nuclear species in winter foraging flocks whose antipredator calls are used to manage predation risk by diverse heterospecifics. We hypothesized that satellite species in mixed flocks follow TUTI (not vice versa), thereby defining the role of TUTI as a “passive” nuclear species. We followed 20 winter mixed-species flocks in North-Central Florida and assessed angular-angular correlations between overall flock, TUTI, and satellite species movement directions. We observed significant correlations between overall flock movement directions and those of TUTI, confirming our central prediction. Within flocks, however, fine-scale movement directions of satellite species were often more highly correlated with those of other satellites than with TUTI movements. We conclude that TUTI are passive nuclear species whose movements define flock paths, but within flocks, TUTI movements may have less influence on satellite movements than do other factors.

Highlights

Multispecies bird flocks, comprising individuals that move together in organized association with each other as they forage during daylight hours, are a common phenomenon in forested ecosystems of the world [1]
There was a significant difference in flock movement rates in different cover types (F2,131 = 7.56, P = 0.0007, r2 = 0.10)
A total of 16 species were detected, and besides tufted titmouse (TUTI), only two others were present in the majority of flocks observed: Ruby-crowned Kinglets and Blue-gray Gnatcatchers which were present in 95% and 70% of the flocks, respectively (Table 1)

Summary

Introduction

Multispecies bird flocks, comprising individuals that move together in organized association with each other as they forage during daylight hours, are a common phenomenon in forested ecosystems of the world [1]. Flocking species are generally classified into “nuclear” and “satellite” roles [2,3,4]. Nuclear species are those flock participants whose conspicuous behaviors (distinctive alarm or other vocalizations and active movements) enhance flock cohesion and may stimulate flock formation. Passive nuclearity is hypothesized to come about when satellite species actively seek out and follow the nuclear species, thereby defining the nuclear species as the flock leader. Active nuclear species are hypothesized to seek out and join existing mixedspecies flocks and are just as likely to follow the satellites as to be followed by satellites, but active nuclear species are still able to maintain flock cohesion (see review in Farley et al [4]). A variety of fitness benefits can accrue to satellite species as a result of flocking with nuclear species, but benefits to nuclear species are less obvious [7,8,9,10,11,12]

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