Parrotlets in Venezuela
Vocal imitation is a complex social behavior that is required for spoken language, but also found in select groups of mammals and birds. Parrots are among the most prolific of vocal linguists, but have been aloof in nature. How does their bizarre ability to mimic human speech patterns in captivity benefit individuals in nature?
Current work addresses such questions in a wild parrot population that has been meticulously marked and studied for decades. Masaguaral Biological Station, in the State of Guarico, Venezuela is home to the longest ongoing study of any wild parrot population.
Figure 1. Seasonally-flooded tropical savanna at Masaguaral Biological Station, Guarico, Venezuela, is home to numerous long-term demographic studies of tropical organisms over the last fifty years.
In 1988 collaborator Steve Beissinger established a study population of green-rumped parrotlets (Forpus passerinus) at Masaguaral using a unique nest box design and successful marking of individual parrotlets with unique color leg-band combinations. This study has continued uninterrupted for 32 years and counting, and enables our field team to identify individual parrotlets with spotting scopes from day-to-day and year-to-year. Over many years this enables us to determine their survival and reproductive output and to compare to long-term trends in the environment (Radchuk et al. 2019).
Wild Green-rumped parrotlets have nested in specially designed nest tubes since 1987 at Masaguaral Biological Station.
In 2004 we began studying their communication. The nest boxes located a few inches off the ground, sexual dimorphism in plumage and the fact that all birds were color-banded with detailed histories provided an excellent opportunity to document their repertoire and individual variation. It took us several years to develop an understanding of their functional vocal repertoire, though we continue to learn about it today.
In 2006-07 we conducted a series of playback experiments demonstrating that individual males had unique contact calls and that females incubating inside cavities and out of visual sight could distinguish their mates from males from other boxes. (Berg et al. 2011) This provided us with the first experimental evidence for how one component of their vocal repertoire functions – the contact call appears to be used in individual identification.
Spectrogram of adult parrotlet contact call
With support from NSF’s Office of International Science and Engineering, in 2007 we began filming inside nest cavities to understand vocal ontogeny and the origins of contact calls. Contact calls emerged late in nestling development and were characterized by a strong upward inflection of frequency and a shortening of duration (Berg et al. 2013). Given the evidence in other species for contact call learning, we hypothesized that they might be learned. We swapped eggs between unrelated pairs in order to compare to natural, unmanipulated nests. We then compared the acoustic structure of contact calls between parents and the nestlings they raised and found that the adopted nestlings fashioned their calls after the adults that raised them, regardless of genetic affinity (Berg et al. 2012). This provided us with experimental evidence for contact call learning in nestlings and a starting point for understanding the developmental origins of vocal learning in this species.
With support from National Geographic Society, in 2011 we began filming more intensively inside nest cavities to learn more about nestling development. With continued support from National Geographic and the National Science Foundation, this work has continued uninterrupted for the past nine years resulting in over 10,000 hours of audio-video recordings of nestling development in over 500 nestlings. Because all nestlings are permanently color-banded, we are able to follow these individuals throughout their lives, allowing us to retrospectively compare to early behavioral and endocrinological foundations.
Nestcam view of brood of wild nestling parrotlets inside specially designed nest cavity at Masaguaral Biological Station.
Spectrogram of signature contact calls from four nestling parrotlet siblings.
Because stress hormones are often involved in early developmental changes we also began quantifying normal and acute stress levels of circulating corticosterone in nestlings and adults. Collaborating with U.S. and Venezuelan endocrinologists, we found that nestlings became physiologically responsive to their environment several weeks before contact calls emerged raising the question of whether environmental stressors might be somehow be involved in learning processes, a focus of current work.
Nestlings parrotlets showing physiological responsiveness to the environment earlier than predicted for altricial species (Berg et al. J. Exp. Zool. 2019).