Table 2.

Number of trials that led to typical mobbing behaviours (response strength = 3), with number of total trials in parentheses. Treatments were playbacks of heterospecific calls that do or do not incorporate a copy of the focal species' call. p-values are from Fisher's exact tests comparing a species' responses to playbacks with the copy and those without. Combined across all trials: p < 0.001. Phylloscopus trochiloides trochiloides call recordings targeting P. pulcher individuals were played back twice each. We include here the responses to the first replicate. The responses to the second replicate were similar: P. pulcher individuals never responded (response strength = 3) to P. trochiloides calls without a copy and responded on four occasions to calls with a copy (p = 0.06).

target species	P. trochiloides calls			P. humei calls
	with copy	without copy	p	with copy	without copy	p-value
P. pulchera	5 (6)	0 (6)	0.015	—	—	—
P. occipitalisb	4 (5)	0 (5)	0.048	3 (5)	0 (5)	0.167

^aPhylloscopus trochiloides trochiloides calls incorporating the call of P. pulcher were natural variants (figure 2).

^bPlaybacks to P. occipitalis were digitally manipulated to incorporate P. occipitalis calls (see the electronic supplementary material, figure S3).