Appendix A. A figure of metabolically corrected biomass/100 ha of the antbird clinger guild during census years on Barro Colorado Island and in Soberania National Park, Panama.

A figure of metabolically corrected biomass/100 ha of the antbird clinger guild during census years on Barro Colorado Island and in Soberania National Park, Panama

Effect of captive rearing on song structure in <i>Hylophylax naevioides</i>, where song is defined by PC1, PC2 and PC3 (mean ± SD).

<p>*Statistics derive from a Kruskal Wallis test; N₁ = 5 captive-reared birds, N₂ = 32 wild birds (sexes pooled).</p

Representative song spectrograms from individuals in each treatment group

<p>. Broadband spectrograms show (<b>A, B</b>) captive <i>H. naevioides</i> reared in silence with no tutor, (<b>C–E</b>) captive <i>H. naevioides</i> reared with <i>H. naevius</i> tutor, (<b>F</b>) wild male <i>H. naevioides</i>, (<b>G</b>) wild female <i>H. naevioides</i>, (<b>H</b>) wild male <i>H. naevius</i>.</p

Null distributions showing range of acoustic variation in wild birds.

<p>Arrows indicate where the songs of captive-reared individuals fall within the sampling distribution generated from the songs of wild males (<b>A</b>) and females (<b>B</b>), where songs are described by PC1 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0095746#pone.0095746.s001" target="_blank">Table S1</a>). Null distributions were generated with 10,000 bootstrap replicates.</p

Comparison of the structure of the songs of experimental versus wild individuals.

<p>Songs by male (closed symbols) and female (open symbols) wild (circles) and captive-reared (red triangles) <i>H. naevioides</i> grouped together while those produced by <i>H. naevius</i> (closed diamonds) grouped separately. Plot produced according to three principal components generated from acoustic data extracted from spectrograms; PC3 is represented by depth and is not labelled (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0095746#pone.0095746.s001" target="_blank">Table S1</a> for factor loadings).</p

Acoustic analysis of <i>Hylophylax naevioides</i> song.

<p>Broadband spectrogram illustrates an example of one song produced by a wild adult male. Boxes denote the acoustic selections used in this study to calculate acoustic parameters using robust statistical estimators in Raven 1.4. Parameters were calculated as averages across five different subsets of notes separately: the full song, the first half of the song by the nearest note to the middle time, the second half of the song by the nearest note to the middle time, the long notes and the short notes (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0095746#pone-0095746-t001" target="_blank">Table 1</a>).</p

Description of acoustic parameters extracted from songs (see Figure 3).

<p>*For all parameters except 1–3, mean was calculated for notes within a song.</p>†<p>Power values in short-time spectra and frequency bands that compose the spectrogram are summed to generate aggregate power envelopes and spectra, resulting in a power versus time envelope and power versus frequency spectrum, respectively.</p><p>The aggregates are normalized and treated as probability density functions with time or frequency being the variate, and density the fraction of the total signal energy. From the distribution function, various measures of central tendency and dispersion are then used to characterize the signal energy distribution in time and frequency. (See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0095746#pone.0095746-Cortopassi1" target="_blank">[39]</a>).</p

Discriminant function analysis grouping (proportion) of song profiles by captive-reared no tutor and heterospecific treatment individuals with respect to song profiles by wild conspecific <i>H. navioides</i> and heterospecific wild <i>H. naevius</i>.

<p>Discriminant function analysis grouping (proportion) of song profiles by captive-reared no tutor and heterospecific treatment individuals with respect to song profiles by wild conspecific <i>H. navioides</i> and heterospecific wild <i>H. naevius</i>.</p

Spectrograms of playback vocalizations from Heterospecific eavesdropping in ant-following birds of the Neotropics is a learned behaviour

Animals eavesdrop on other species to obtain information about their environments. Heterospecific eavesdropping can yield tangible fitness benefits by providing valuable information about food resources and predator presence. The ability to eavesdrop may, therefore, be under strong selection, although extensive research on alarm-calling in avian mixed-species flocks has found only limited evidence that close association with another species could select for innate signal recognition. Nevertheless, very little is known about the evolution of eavesdropping behaviour and the mechanism of heterospecific signal recognition, particularly in other ecological contexts, such as foraging. To understand whether heterospecific signal recognition was an innate or learned behaviour in a foraging context, we studied heterospecific signal recognition in ant-following birds of the Neotropics, which eavesdrop on vocalizations of obligate ant-following species to locate and recruit to swarms of the army ant <i>Eciton burchellii</i>, a profitable food resource. We used a playback experiment to compare recruitment of ant-following birds to vocalizations of two obligate species at a mainland site (where both species are present) and a nearby island site (where one species remains whereas the other went extinct approx. 40 years ago). We found that ant-following birds recruited strongly to playbacks of the obligate species present at both island and mainland sites, but the island birds did not recruit to playbacks of the absent obligate species. Our results strongly suggest that (i) ant-following birds learn to recognize heterospecific vocalizations from ecological experience and (ii) island birds no longer recognize the locally extinct obligate species after eight generations of absence from the island. Although learning appears to be the mechanism of heterospecific signal recognition in ant-following birds, more experimental tests are needed to fully understand the evolution of eavesdropping behaviour