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.

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.

One-to-one innervation of vocal muscles allows precise control of birdsong Highlights d Motor neurons in the zebra finch vocal motor pool innervate single muscle fibers d Zebra finch vocal muscles have the lowest measured isometric stress d Small motor unit size and low muscle stress provide subhertz pitch control resolution d High-resolution control is key to vocal space expansion and songbird radiation

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.

15SummaryThe 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 20 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 (fo), a key vocal parameter, in zebra finches. We show that syringeal muscles have extremely small MUs, with 50% 25innervating ≤ 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 fo 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. 30Furthermore, 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. Results & DiscussionVocal communication is of paramount importance for reproduction and survival of songbirds and even drives speciation. This is clearly exemplified by sympatric species that are 40 morphologically indistinguishable, but nevertheless fully separated solely by song [6, 7]. The potential to acoustically separate depends on the number of distinct sounds -or vocal space -that can be produced and perceived. The vocal space is set both by the range available to vary an acoustic feature, and in what steps, or resolution, the feature can be controlled within this range. Thus, both resolution and range expand the vocal space and may form a rich 45 substrate for species diversification [8,9]. While the range of an acoustic feature, for example fo, is typically limited by intrinsic constraints of the vocal organ [10-12], the ability 65 ( Fig 1A, B, Table S1, See Methods). The average total number of muscle fibers was 6995 ± 789 (n = 4), corroborating earlier reported ~6730 [17]. The left side had significantly less muscle fibers (3234 ± 232, range: 2992 -3503, n = 4) than right (3794 ± 334, range 3586 -4286, n = 4) (Welch t-test: t = -2.8, df = 5.4, p-value = 0.04). The number of axons in the tracheosyringeal branch of the hypoglossal nerve (NXIIts), assumed to represent the 70 number of MUs (See Methods), was not significantly different between left (820 ± 187, range: 602 -1162, n = 8) and right (790 ± 148, range: 677 -1045, n = 5) (Welch t-test: t = 0.32, df = 10.2, p-value = 0.76) and lower but within range of the 1026 ± 126 (n = 6) reported earlier [18]. The ...