Abstract
P eople have long marveled at the striking mimicry abilities of parrots. Mimicry occurs when birds raised in captivity learn vocalizations and other sounds used by their human companions and incorporate these learned sounds into their large and varied communication repertoire. Many parrot species display this advanced learning ability and are prized as pets for this reason. Yet despite their popularity as pets, little is known about how these parrot species use their learning ability in the wild. Even the most basic information, such as the size and composition of the vocal repertoire, is known for only a few species, and there is almost no information on how repertoires might vary between different individuals or different geographic locations.
One species that is particularly well known for its mimicry abilities is the Yellow-naped Amazon Amazona auropalliata. For the last five years my assistants and I have studied wild populations of this species in its natural habitat in Costa Rica. The goal of this study has been to describe the communication repertoire of this species and examine how different vocalizations in the repertoire vary geographically. Here I describe some of the most interesting vocalizations and document their distinctive pattern of regional variation, termed vocal dialects.
Several aspects of the biology of the Yellow-naped Amazon facilitate the study of its vocal repertoire. It is found
in the tropical dry forest habitat of the Pacific slope of Central America, a habitat that is very conducive to studying parrots because its low forest canopy and long dry season during which many trees lose their leaves. Parrots are much easier to observe and follow in this habitat than in the high canopy of the tropical rain forest.
Two important aspects of the Yellow-naped Amazon's social system, nest territories, and communal night roosts, further simplify the study of its vocalizations. Mated pairs will use the same nest trees, and even the same cavities for several consecutive years. High-quality cavities appear to be an important resource for pairs, and a pair will vigorously defend their nest tree and the area around it against intruding pairs. This defense continues throughout the year, and pairs can often be found vocalizing near their nest areas long after nesting season has ended.
These same pairs will gather each night into large communal night roosts that number from 20 to over 300 birds. These night roosts occur in highly traditional locations that are used throughout the year and in some cases over several years and even decades.
The appearance of birds in these traditional nest and roosts sites on a daily basis makes it relatively simple to get repeated recordings of· vocalizations and behaviors from the same birds and to compare these vocalizations to those recorded from birds at
other nests and roosts. We record calling birds on video and audio tapes and take notes on the social behaviors and visual displays that accompany these calls. These tapes are then analyzed using computer programs that transform vocalizations into visual graphs called spectrograms (see Figure 2). Different aspects of calls can be measured from these spectrograms and compared to measurements from the same calls made by members of different pairs and individuals from different roosts. We also conduct playback experiments, in which we broadcast specific calls from speakers placed in trees and note the vocal and behavioral responses of nearby birds to these controlled stimuli.
Using these methods, we have discovered several striking features of the species vocal communication repertoire. One is that wild Yellow-naped Amazons do not appear to mimic other species, nor do they have a particularly large vocal repertoire. In fact, the repertoire is not substantially larger than that of most other bird species, with roughly eight to 10 functional classes of calls.
A single call type, the contact call, composes 40-60 percent of all calls used in any one location. The contact call is a short, loud, relatively stereotyped call that is used in a wide variety of situations, including at roosts, in flight, and near the nest. It appears to function as a marker of social identity that is used to maintain contact with other members of a social group such as a mate or offspring. The acoustic form of this call is fairly consistent among individuals in an area, but as discussed below, shows marked differences among different areas. Figure 2 is a spectrogram of contact calls from three different regions in Costa Rica.
The relative stereotypy of the contact call contrasts strongly with a different functional class of calls, the pair duet. The pair duet is a loud, complex, and highly conspicuous call produced in tandem by the two members of a mated pair. It is used primarily by pairs around their nests, where it appears to function as a marker of territorial ownership.