Mixed species flocks of insectivorous birds are a common feature of the understory in amazonian rainforests. The groups often follow a "leader" species, whose frequent vocalizations are thought to communicate information about movement and predator threats. Other species, like the Black-faced Antbird, join flocks when possible. Since the flock territories are 2-4 times larger than antbird territories, antbirds cannot permanently join the flocks. 
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Antbird calls are very similar to antshrike (flock leader) calls, and antbirds may use the similarity to confuse flock species into responding. This would allow antbirds to easily find flock species in the dense rainforest understory. To test this hypothesis, we used two methods. First, we compared the calls of antbirds whose range overlaps with antshrikes to populations of antbirds whose range does not overlap with antshrikes. Second, we performed a playback experiment where we directly tested the behavioral response of flock species to the mimic call of Black-faced Antbirds.

Figure 1 The flock leader (top left) produces vocalizations that several species follow. The Black-faced Antbird (Male, female, bottom) generates mimetic vocalizations that the flock responds to. We evaluate the mimetic call over large geographic areas.

When comparing the calls of antbirds populations, we found a pattern consistent with evolutionary convergence. Populations of antbirds that share their range with antshrikes (flock leaders) also have similar calls to antshrikes. Populations of antbirds that do not share their range with antshrikes differ in their calls. Further, these patterns are not reflected in the songs of antbirds, supporting the potentially different evolutionary pressures in the signal design of the mimetic call vs the non-mimetic song.

Figure 2. Map and PCA. The range of mimicking Antbirds (map) is represented by solid color, and the different colors represent subspecies. The range of Antbirds (flock leaders) is represented by the distribution of small dots. The two species do not overlap over their entire range, allowing us to compare subspecies populations across large areas.  In the PCA analysis (graph, right) the calls of overlapping antbird and antshrike populations are similar (yellow diamonds, blue triangles, orange circles), and the non-overlapping populations have non-similar calls (green squares, blue dots)

The results of our playback experiments further support the hypothesis. Antbird calls resulted in behavioral response from both flock leading antshrikes and other flocking birds. These behavioral responses were similar to antshrike responses to their own species. This suggests the call is confused by both flock leaders and flocking species, allowing antbirds to generate responses from them in order to easily locate and join flocks.

Figure 3. Flock leading antshrikes responded similarly to their own species calls and the mimetic antbird calls (graph: mimic, leader). The flock leading antshrikes did not respond to the song of antbirds or to controls (graph: mimic control, forest control).