Data from: Heterospecific eavesdropping in ant-following birds of the Neotropics is a learned behavior

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 behavior 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 behavior 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 Eciton burchellii, 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 ~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 (1) ant-following birds learn to recognize heterospecific vocalizations from ecological experience, and (2) island birds no longer recognize the locally extinct obligate species after eight generations of absence from the island. Although learning is the mechanism of heterospecific signal recognition in ant-following birds, more experimental tests are needed to fully understand the evolution of eavesdropping behavior.

动物会通过窃听其他物种来获取自身生存环境的相关信息。异种窃听（heterospecific eavesdropping）可通过提供关于食物资源与捕食者存在情况的宝贵信息，带来实实在在的适合度收益（fitness benefits）。因此，窃听能力可能处于强烈的选择压力之下；不过针对鸟类混合种群报警鸣叫的大量研究仅发现了有限证据，表明与另一物种的紧密关联会筛选出先天的信号识别能力。
尽管如此，学界对窃听行为的演化以及异种信号识别的机制仍知之甚少，尤其是在觅食这类其他生态场景中。
为了探明觅食场景下的异种信号识别究竟是先天行为还是后天习得行为，我们对新热带区的蚁伴鸟类（ant-following birds）展开了研究：这类鸟类会通过窃听专性蚁伴物种（obligate ant-following species）的鸣叫声，来定位并召集同类前往布氏行军蚁（Eciton burchellii）的蚁群——这是一种极具价值的食物资源。
我们采用回放实验法，比较了两处样地中蚁伴鸟类对两种专性蚁伴物种鸣叫声回放的招引反应：一处是大陆样地（两种专性物种均存活），另一处是邻近的岛屿样地（其中一种专性物种仍存活，另一种已于约40年前灭绝）。
我们发现，蚁伴鸟类会强烈响应在岛屿与大陆样地均存活的专性蚁伴物种的鸣叫声回放，但岛屿种群不会响应已在当地消失的专性蚁伴物种的鸣叫声回放。
我们的研究结果强有力地表明：（1）蚁伴鸟类可通过生态经验习得识别异种鸣叫声的能力；（2）在当地专性蚁伴物种消失约8代后，岛屿种群已无法再识别该物种。
尽管后天习得是蚁伴鸟类异种信号识别的机制，但仍需开展更多实验验证，才能全面阐明窃听行为的演化历程。