Auditory processing of vocal sounds in birds

The avian auditory system has become a model system to investigate how vocalizations are memorized and processed by the brain in order to mediate behavioral discrimination and recognition. Recent studies have shown that most of the avian auditory system responds preferentially and efficiently to sounds that have natural spectro-temporal statistics. In addition, neurons in secondary auditory forebrain areas have plastic response properties and are the most active when processing behaviorally relevant vocalizations. Physiological measurements show differential responses for vocalizations that were recently learned in discrimination tasks, and for the tutor song, a longer-term auditory memory that is used to guide vocal learning in male songbirds.     

On the evolution of noise-dependent vocal plasticity in birds

Signal plasticity is considered an important step in the evolution of animal communication. In acoustic communication, signal transmission is often constrained by background noise. One adaptation to evade acoustic signal masking is the Lombard effect, in which an animal increases its vocal amplitude in response to an increase in background noise. This form of signal plasticity has been found in mammals, including humans, and some birds, but not frogs. However, the evolution of the Lombard effect is still unclear. Here we demonstrate for the first time the Lombard effect in a phylogentically basal bird species, the tinamou Eudromia elegans. By doing so, we take a step towards reconstructing the evolutionary history of noise-dependent vocal plasticity in birds. Similar to humans, the tinamous also raised their vocal pitch in noise, irrespective of any release from signal masking. The occurrence of the Lombard effect in a basal bird group suggests that this form of vocal plasticity was present in the common ancestor of all living birds and thus evolved at least as early as 119 Ma.     

Histologic study of the pineal organ in tinamid birds

The pineal organs of 10 species of tinamid birds belonging to the genera Crypturellus, Nothura, Rhynchotus and Tinamus were studied in serial microscopic sections of the entire medial region of the brain. All were located in a triangular space formed by the cerebral hemispheres and the cerebellum. They have the form of a club with the enlarged distal part enclosed in the dura-mater. In all species the pineal is formed of tubulofollicular structures, the walls of which are covered by a pseudo-stratified epithelium with elongated cells. In Rhynchotus and Nothura there is a predominance of small follicles with a reduced lumen and a regular outline, whereas in Tinamus and Crypturellus there is a predominance of large follicles with a wide lumen and an irregular outline. Our results show that morphological organization of the pineal organs in tinamids is similar to that observed for the majority of modern birds, in spite of the fact that they are generally regarded as very primitive.     

Mapping vocal communication pathways in birds with inducible gene expression

Expression mapping of activity-dependent genes has been very useful to reveal brain activation patterns associated with specific stimuli or behavioral contexts. In addition, activity-induced neuronal gene expression is likely associated with neuronal plasticity and may be part of the mechanism(s) involved in long-term memory formation. Analysis of the immediate-early gene zenk has been used to generate high-resolution maps of brain activation associated with perceptual and motor aspects of vocal communication in songbirds and other avian groups. This molecular approach has generated novel insights into the organization of perceptual and motor control pathways for vocal communication in birds. Its impact on the neurobiology of birdsong will be reviewed here. Emphasis will be given to the caudomedial neostriatum, the area that shows the most robust zenk induction upon presentation of song to songbirds. Another focal point will be the comparative analysis of vocally induced zenk expression patterns across the avian orders that evolved vocal learning (i.e., songbirds, parrots, and hummingbirds). New research directions indicated by this molecular analysis will be discussed throughout.     

Cardiovascular and organ weight adaptations as related to flight activity in birds

Pectoral muscle, heart and body weight, wing surface, blood volume, hemoglobin content and blood oxygen capacity were measured in three birds: pigeon, gull and hen. A relationship was found between flying activity and organ weight, blood volume and hematocrit. No significant differences were found in the O2 carrying capacity of hemoglobin in these birds.     

Sometimes slower is better: slow-exploring birds are more sensitive to changes in a vocal discrimination task

Animal personality, defined as consistent individual differences across context and time, has attracted much recent research interest in the study of animal behaviour. More recently, this field has begun to examine how such variation arose and is maintained within populations. The habitat-dependent selection hypothesis, which posits that animals with differing personality types may fare better (i.e. have a fitness advantage) in different habitats, suggests one possible mechanism. In the current experiment, we tested whether slow- and fast-exploring black-capped chickadees (Poecile atricapillus), determined by performance in a novel environment exploration task, perform differentially when the demands of an acoustic operant discrimination (cognitive) task were altered following successful task acquisition. We found that slow-exploring birds learn to reverse previously learned natural category rules more quickly than faster exploring conspecifics. In accordance with the habitat-dependent selection hypothesis, and previous work with great tits (Parus major), a close relative of the black-capped chickadee, our results suggest that fast-exploring birds may perform better in stable, predictable environments where forming a routine is advantageous, while slow-exploring birds are favoured in unstable, unpredictable environments, where task demands often change. Our results also support a hypothesis derived from previous work with great tits; slow-exploring birds may be generally more flexible (i.e. able to modify their behaviour in accordance with changes in environmental stimuli) in some learning tasks.     

Bird song and anthropogenic noise: vocal constraints may explain why birds sing higher-frequency songs in cities

When animals live in cities, they have to adjust their behaviour and life histories to novel environments. Noise pollution puts a severe constraint on vocal communication by interfering with the detection of acoustic signals. Recent studies show that city birds sing higher-frequency songs than their conspecifics in non-urban habitats. This has been interpreted as an adaptation to counteract masking by traffic noise. However, this notion is debated, for the observed frequency shifts seem to be less efficient at mitigating noise than singing louder, and it has been suggested that city birds might use particularly high-frequency song elements because they can be produced at higher amplitudes. Here, we present the first phonetogram for a songbird, which shows that frequency and amplitude are strongly positively correlated in the common blackbird (Turdus merula), a successful urban colonizer. Moreover, city blackbirds preferentially sang higher-frequency elements that can be produced at higher intensities and, at the same time, happen to be less masked in low-frequency traffic noise.     

Seasonal variation of vocal behaviour in a temperate songbird: assessing the effects of laboratory housing on wild-caught, seasonally breeding birds

Many laboratories are conducting research using songbirds as their animal model. In particular, songbirds are widely used for studying the behavioural and neural mechanisms underlying vocal learning. Many researchers use wild-caught birds to conduct this research, although few studies of behaviour have been conducted to determine the effects of captive housing on these species. We investigated the vocal production pattern of wild-caught black-capped chickadees (Poecile atricapillus) over an entire season in laboratory housing. We documented the frequency of production of four vocalizations (fee-bee song, chick-a-dee calls, dee calls, and gargle calls) across seasons and diurnal pattern and compared the observed pattern of laboratory vocalizations to those previously observed and reported in the wild. Laboratory-housed chickadees had seasonal and diurnal vocal production shifts that were related to both photoperiodic changes (season) and diurnal pattern. For instance, there was significantly more fee-bee song in the spring than summer, autumn, and winter with the most fee-bee song occurring at spring dawn as seen in the wild. Our results also confirmed that the general pattern of vocalizations was consistent between wild and laboratory populations, with no significant differences for either the seasonal or diurnal pattern of fee-bee song production between populations. Differences between settings were observed in the pattern of chick-a-dee calls at dawn and sunset between field and laboratory populations. However, differences in the quantity of vocalization types between laboratory and wild populations suggest that housing conditions are influencing the normal vocal behavioural patterns.     

Prosthetic avian vocal organ controlled by a freely behaving bird based on a low dimensional model of the biomechanical periphery

Because of the parallels found with human language production and acquisition, birdsong is an ideal animal model to study general mechanisms underlying complex, learned motor behavior. The rich and diverse vocalizations of songbirds emerge as a result of the interaction between a pattern generator in the brain and a highly nontrivial nonlinear periphery. Much of the complexity of this vocal behavior has been understood by studying the physics of the avian vocal organ, particularly the syrinx. A mathematical model describing the complex periphery as a nonlinear dynamical system leads to the conclusion that nontrivial behavior emerges even when the organ is commanded by simple motor instructions: smooth paths in a low dimensional parameter space. An analysis of the model provides insight into which parameters are responsible for generating a rich variety of diverse vocalizations, and what the physiological meaning of these parameters is. By recording the physiological motor instructions elicited by a spontaneously singing muted bird and computing the model on a Digital Signal Processor in real-time, we produce realistic synthetic vocalizations that replace the bird's own auditory feedback. In this way, we build a bio-prosthetic avian vocal organ driven by a freely behaving bird via its physiologically coded motor commands. Since it is based on a low-dimensional nonlinear mathematical model of the peripheral effector, the emulation of the motor behavior requires light computation, in such a way that our bio-prosthetic device can be implemented on a portable platform.     

Tracking the gaze of birds

Accurate measurements of what birds are looking at are valuable to most studies of vision and visually guided behaviour in avian species. Compared to human visual research, studies of gaze behaviour in birds are limited. A new laser-based method that can be used to quantify several aspects of a given sequence of gaze shifts and fixations is presented. Using near infrared light to estimate gaze direction is a relatively simple and cheap method to study visually guided behaviour. It has the potential to contribute significantly to the ornithological research by allowing the quantification of several behavioural traits of birds. Other methods are discussed and compared to the use of infrared light, and possible fields of application are suggested.     

Fossil birds of the Kibish Formation

The Kibish Formation has yielded a small collection of bird fossils, which are identified here as belonging to five species in four different families: Pelecanidae (pelicans), Anhingidae (darters), Ardeidae (herons) and Phasianidae (gamefowl). Two species of pelicans are identified: Pelecanus cf. P. onocrotalus, and P. aff. P. rufescens. The darter is referrable to Anhinga melanogaster. The heron is identifiable as Ardea sp., and the gamefowl as Numidinae indet. (guineafowl). Pelecanus cf. P. onocrotalus is represented by, among other remains, a well-preserved partial skull. Four of the birds are thus referrable to extant taxa that provide some paleoenvironmental clues for Member I of the Kibish Formation. The two species of pelican, the darter, and the heron indicate the presence of local freshwater bodies, a lake or a slow river, supporting resources of fish. The guineafowl is poorly informative ecologically, but probably excludes the notion that the local terrestrial landscape was treeless.     

Cities change the songs of birds

Worldwide urbanization and the ongoing rise of urban noise levels form a major threat to living conditions in and around cities. Urban environments typically homogenize animal communities, and this results, for example, in the same few bird species' being found everywhere. Insight into the behavioral strategies of the urban survivors may explain the sensitivity of other species to urban selection pressures. Here, we show that songs that are important to mate attraction and territory defense have significantly diverged in great tits (Parus major), a very successful urban species. Urban songs were shorter and sung faster than songs in forests, and often concerned atypical song types. Furthermore, we found consistently higher minimum frequencies in ten out of ten city-forest comparisons from London to Prague and from Amsterdam to Paris. Anthropogenic noise is most likely a dominant factor driving these dramatic changes. These data provide the most consistent evidence supporting the acoustic-adaptation hypothesis since it was postulated in the early seventies. At the same time, they reveal a behavioral plasticity that may be key to urban success and the lack of which may explain detrimental effects on bird communities that live in noisy urbanized areas or along highways.