Alyssa Maxwell

Vocal behaviour can be dramatically changed by both neural circuit development and postnatal maturation of the body. During song learning in songbirds, both the song system and syringeal muscles are functionally changing, but it is unknown if maturation of sound generators within the syrinx contributes to vocal development. Here we densely sample the respiratory pressure control space of the zebra finch syrinx in vitro . We show that the syrinx produces sound very efficiently and that key acoustic parameters, minimal fundamental frequency, entropy and source level, do not change over development in both sexes. Thus, our data suggests that the observed acoustic changes in vocal development must be attributed to changes in the motor control pathway, from song system circuitry to muscle force, and not by material property changes in the avian analog of the vocal folds. We propose that in songbirds, muscle use and training driven by the sexually dimorphic song system are the crucial drivers that lead to sexual dimorphism of the syringeal skeleton and musculature. The size and properties of the instrument are thus not changing, while its player is.

Both mental and physiological conditions determine the well-being state in an animal. Enrichment is a way to increase an animals’ well-being and may require problem solving through thinking, tolerance of ambiguity, openness, and intrinsic motivation. It is unclear if it is enriching when an animal participates in different types of research. Therefore, it is important to answer the question of whether research can be used as an enrichment tool in zoological facilities. Here, we examine if participation in psychophysical research affected the mental stimulation of three grey seals under human care. The effects varied amongst the three individuals that took part in the research, and indicated that their participation in the research task was dependent on their individual personalities and life history. Two seals indicated that their involvement in the research was positive and motivating, and therefore can be considered enriching. In comparison, the third seal displayed a tendency for frustration and low motivation. Our results indicate that research can be a powerful enrichment tool with animals that find research motivating.

Summary: The motor control resolution of any animal behavior is limited to the minimal force step available when activating muscles, which is set by the number and size distribution of motor units (MUs) and muscle specific force [1, 2]. Birdsong is an excellent model system for understanding sequence learning of complex fine motor skills [3], but we know surprisingly little how the motor pool controlling the syrinx is organized [4] and how MU recruitment drives changes in vocal output [5]. Here we combine measurements of syringeal muscle innervation ratios with muscle stress and an in vitro syrinx preparation to estimate MU size distribution and the control resolution of fundamental frequency ( f _o ), a key vocal parameter, in zebra finches. We show that syringeal muscles have extremely small MUs, with 50% innervating ≤ 3, and 13 – 17% innervating a single muscle fiber. Combined with the lowest specific stress (5 mN/mm ² ) known to skeletal vertebrate muscle, small force steps by the major f _o controlling muscle provide control of 50 mHz to 4.2 Hz steps per MU. We show that the song system has the highest motor control resolution possible in the vertebrate nervous system and suggest this evolved due to strong selection on fine gradation of vocal output. Furthermore, we propose that high-resolution motor control was a key feature contributing to the radiation of songbirds that allowed diversification of song and speciation by vocal space expansion.

The problem-solving capabilities of four small parrots (peach-fronted conures, Eupsittula aurea) were investigated using string-pulling tests. In seven different tasks, one string was baited following a randomized order. The parrots could retrieve the food reward after a wrong choice as the choice was not forced. Additionally, we applied a non-intuitive pulley task with the strings arranged in front of, instead of below the birds. All four parrots performed very well in the multiple, slanted, and broken string tasks, but all failed in the crossed-string task. Only two parrots solved the single pulley task. All four parrots performed successfully in the multiple pulley task but all failed in the broken pulley condition. Our results suggest that peach-fronted conures solve string-pulling tasks without relying on simple proximity based rules, but that they have evolved cognitive abilities enabling goal-directedness, the understanding of functionality, and a concept of connectedness between two objects.

In-air hearing in birds has been thoroughly investigated. Sound provides birds with auditory information for species and individual recognition from their complex vocalizations, as well as cues while foraging and for avoiding predators. Some 10% of existing species of birds obtain their food under the water surface. Whether some of these birds make use of acoustic cues while underwater is unknown. An interesting species in this respect is the great cormorant (Phalacrocorax carbo), being one of the most effective marine predators and relying on the aquatic environment for food year round. Here, its underwater hearing abilities were investigated using psychophysics, where the bird learned to detect the presence or absence of a tone while submerged. The greatest sensitivity was found at 2 kHz, with an underwater hearing threshold of 71 dB re 1 μPa rms. The great cormorant is better at hearing underwater than expected, and the hearing thresholds are comparable to seals and toothed whales in the frequency band 1–4 kHz. This opens up the possibility of cormorants and other aquatic birds having special adaptations for underwater hearing and making use of underwater acoustic cues from, e.g., conspecifics, their surroundings, as well as prey and predators.

Little is known about underwater hearing abilities of diving birds. To help fill this gap we measured audiograms of cormorants (Phalacrocorax carbo) using two different methods. Wild-caught cormorant fledglings were anesthetized and their auditory brainstem responses (ABR) to clicks and tone bursts were measured; first in an anechoic box in air and then in a large water-filled tank with their head and neck submerged. In addition, audiograms were obtained from two adult cormorants using the psychophysical method of constant stimuli both in air and underwater. The shape of audiograms obtained from in-air ABR-recordings was similar to that reported for birds of similar size. The highest sensitivity in air was found at about 2 kHz, while the most sensitive response was at about 1 kHz underwater. In general, the audiograms obtained from psychophysical measurements were similar to the ABR audiograms but had much higher sensitivity than the ABR audiograms. The results from both methods suggest that cormorants have rather poor in-air hearing compared to similar-sized birds. Their underwater hearing sensitivity, however, is higher than what would have been expected for purely air-adapted ears, and it is likely that cormorants use their underwater hearing abilities during foraging dives.

Many aquatic birds use sounds extensively for in-air communication. Regardless of this, we know very little about their hearing abilities. The in-air audiogram of a male adult great cormorant (Phalacrocorax carbo) was determined using psychophysical methods (method of constants). Hearing thresholds were derived using pure tones of five different frequencies. The lowest threshold was at 2 kHz: 18 dB re 20 µPa rms. Thresholds derived using signal detection theory were within 2 dB of the ones derived using classical psychophysics. The great cormorant is more sensitive to in-air sounds than previously believed and its hearing abilities are comparable to several other species of birds of similar size. This knowledge is important for our understanding of the hearing abilities of other species of sea birds. It can also be used to develop cormorant deterrent devices for fisheries, as well as to assess the impact of increasing in-air anthropogenic noise levels on cormorants and other aquatic birds.

Psychoacoustic and electrophysiological methods were used to measure the in-air hearing sensitivity of the great cormorant (Phalacrocorax carbo sinensis). One individual was used to determine the behavioral thresholds, which was then compared to previously collected data on the auditory brainstem response (ABR) thresholds from 11 individuals. The behavioral hearing sensitivities were measured at 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 6 kHz, while the ABR hearing sensitivities were measured at 500 Hz, 1 kHz, 2 kHz, and 6 kHz. For both methods the sensitivities were found to be lowest at 2 kHz. The ABR hearing thresholds were found to be 23-53 dB higher than the hearing thresholds determined from the psychoacoustic results, but the audiograms derived with the two methods shared similarities in shape. The results from this study show that the hearing sensitivities of the great cormorant are more sensitive in the behavioral results than from the ABR measurements.