Most bird species around the world produce some sort of “song” that provides information important to mate attraction, territoriality, foraging, and predator avoidance. Often times, those individuals that sing the best (have the most complex songs with the greatest diversity of syllables) are also the most successful at holding territories, attracting the most females, and thus have greater reproductive success than their less vocally accomplished neighbors.
For most orders of birds, songs are primarily passed from parents to offspring genetically. However, for the oscine passerines, a group that comprises almost half the bird species worldwide, song is acquired through social learning from parents and neighbors much like human language. But what happens to bird song as their habitat becomes increasingly fragmented and their populations decline? Under the cultural erosion hypothesis, the decreasing number of tutors that song-learning juveniles are exposed to may lead to a reduction in the number of learned vocal elements or “memes” within a population and less-complex song, which may have further negative effects on the bird populations.
Our lab at the University of Hawaii at Hilo has conducted two recent studies that appeared in the journal Animal Behaviour to address these ideas. In Changes in vocal repertoire of the Hawaiian Crow, Corvus hawaiiensis, from past wild to current captive populations, graduate student Ann Tanimoto examined how severe population decline and extinction in the wild may have affected the vocal repertoire of the ‘alalā, or native Hawaiian Crow.
Hundreds of hours of old cassette tapes from the last few wild individuals in the early 1990’s were digitized, uploaded to sound analysis software, and compared with modern recordings of the few remaining ‘alalā, all in captivity at the Keauhou Bird Conservation Center, managed by the San Diego Zoo on the island of Hawaii. All the vocalizations of the last wild individuals were placed into five categories based on the types of social behavior the individuals were engaged in, including alarm, territorial broadcast, aggression, submission, and courtship. Over 52 different vocalization types recorded different vocalization types were recorded.
There was little difference in the overall number of call types between wild and captive birds, however the repertoire was significantly different. For example, there was a total loss of territorial broadcast calls in the captive population, and wild birds had twice the number of alarm calls as the captive birds, demonstrating how socially learned behaviors may change over a relatively short period of time for an entire species and providing potentially valuable information for ongoing efforts to reintroduce this species back into the wild.
A collection of calls from past-wild ‘alalā that appear to have been lost from the current captive population
Similar results but in a completely different system were reported in Birdsong characteristics are related to fragment size in a neotropical forest. In that study, the effect of habitat fragmentation and decreasing population size was compared between the song-learning Orange-billed Sparrow (Arremon aurantiirostris) and the non song-learning Scale Crested Pygmy Tyrant (Lophotriccus.). Recordings were taken from individuals in twelve premontane wet forest fragments ranging in size from approximately 1.4ha to 350ha in southern Costa Rica.
As predicted under the “cultural erosion” hypothesis, we found that acoustic characteristics associated with song complexity such as the number of syllables per song, the frequency bandwidth of each song, and song duration decreased with decreasing fragment size for the oscine but not the sub-oscine species. Like the study on Hawaiian crows, this study supports the idea that the richness and complexity of bird song are sensitive to fragment and population size. Learned behaviors like song are a form of cultural diversity in animals and should be considered along with other forms of biodiversity in the conservation of social learning species.
These findings are described in the article entitled Changes in vocal repertoire of the Hawaiian Crow, Corvus hawaiiensis, from past wild to current captive populations, recently published in the journal Animal Behaviour. This work was conducted by Ann M. Tanimoto, Patrick J. Hart, Adam A. Pack, and Esther Sebastián-González from the University of Hilo, Richard Switzer and Lisa Komarczyk from the San Diego Zoological Society, Paul C. Banko from the U.S. Geological Survey, Donna L. Ball from the U.S. Fish and Wildlife Service, and Miyako H. Warrington from St George’s University.
These findings are also described in the article entitled Birdsong characteristics are related to fragment size in a neotropical forest, recently published in the journal Animal Behaviour. This work was conducted by Patrick J. Hart, Esther Sebastián-González, Ann Tanimoto, and Tawn Speetjens from the University of Hawaii at Hilo, Alia Thompson from the University of Hawaii at Mānoa, Madolyn Hopkins from Chaminade University, and Michael Atencio-Picado from Las Cruces Biological Station.