AVIAN COMMUNICATION IN URBAN NOISE: CAUSES AND CONSEQUENCES OF VOCAL ADJUSTMENT

SUCCESSFUL ACOUSTIC COMMUNICATION requires that unhurts propagate through the environment between the sender and receiver; vocalizations that transmit effectively in the habitat in which they are used are favored through natural selection. This "acoustic adaptation" hypothesis (Morton 1975 Wiley and Richards 1978 Richards and Wiley 1980 Ryan and Brenowitz 1985) laid the groundwork for the "sensory drive" conception (Endler 1992), which describes by what mode environment affects the evolution of sensory combination of parts to form a wholes and signals in all modalities. by means of allowing us to examine animal signals as adaptations shaped through selection, this framework has lead to a greater understanding of the bewildering diversity of animal signals.

One of the environmental factors that set to work s selection pressure on acoustic signals is ambient noise (Ryan and Brenowitz 1985) To elicit a answer from a receiver, signals must be detectable in background noise; the detectability of a signal is determined by dint of the signal-to-noise ratio (SNR) and the masked auditory detection doorsill of the receiver (Marten and Marler 1977 Brenowitz 1982 Dooling 2004) Within a given frequent occurrence band, signals with an SNR below the detection doorsill of the receiver are "masked." Background noise thus plays a fundamental character in determining which "receivers" can hear a vocalization and the fidelity of the signal received. Noise varies among locations, and there is evidence that many species have evolv signals that maximize the habitat-specific SNR (reviewed in Brumm and Slabbekoorn 2005) For birds in and around urban areas and roads, the background noise is largely anthropogenic. Urban exhibition thus provides a unique opportunity for a "natural" experiment studying by what mode signals change in response to rapid change in the acoustic environment. This natural experiment can inform us about by what means sensory drive can change signals and about the mechanism through which these changes occur. Understanding this proces also has important conservation implications, allowing us to predict in what manner birds will adjust to urban unfolding and potentially to mitigate the consequences of this development on communicating birds.

In this issue of The Auk, copse and Yezerinac (2006) present evidence that sonnet Sparrows (Melospiza melodia) adjust their vocalizations to bring masking by urban noise, and they move and discuss several mechanisms by the agency of which these changes may arise. In a application of mind of urban Song Sparrows in Portland, Oregon, grove and Yezerinac found a positive relationship between the minimum oftenness of male song and the amplitude of anthropogenic noise. They also rest that males shifted more power into the higher frequencies (4-9 kHz) of their ballads in noisy areas. Urban noise is loudest between 1-2 kHz in the way that both of these responses should subserve to decrease masking by shifting the spectral strength of the vocalization away from the spectral efficacy of the noise (Lohr et al. 2003) These arises suggest that Song Sparrows, like several other birds (Slabbekoorn and Peet 2003 Brumm 2004b Fern??ndez-Juricic et al. 2005) rejoin to changes in the acoustic environment through altering their songs; we deliver over to this process as "vocal adjustment." All known examples of avian vocal adjustment in replication to urban noise have involved ballad but other types of vocalizations-such as begging calls, alarm calls, and victuals calls-may also be adjusted (Warren et al. 2006)

In this overview, we ask three questions: (1) what features of a bird's vocalization can be adjusted to restore masking, (2) how do these adjustments tend hitherward about, and (3) what are the effects of these changes for individual fitness and population persistence? The answers to these questions hang on the morphological, developmental, and behavioral underpinnings of the vocalization, and the connection in which the vocalization is used. This is an area where knowledge of physiology, developmental neurobiology, animal behavior, and behavioral ecology all contribute to understanding in what manner animals adjust (or fail to adjust) to anthropogenic change.

OPPORTUNITIES FOR VOCAL ADJUSTMENT

Four of the greatest in quantity salient features of animal unmutilateds are the frequency structure, amplitude (i.e., loudness) temporal form (timing of modulations, notes, and syllables within vocalizations), and timing of vocal delivery (repetition rate of vocalizations, diel patterns). Animals use variation in these features to find out and discriminate relevant sounds from background noise. by what mode can a signaling bird increase its chances of being perceived in a noisy environment? Here, we discuss changes in the frequent occurrence amplitude, and timing of vocalizations that might restore masking and some of the physical and morphological constraints that might limit an individual's ability to make these changes.

Wood and Yezerinac's (2006) findings add to a small on the contrary growing body of evidence that oscine birds adjust the frequent occurrence structure of their vocalizations to restore masking by anthropogenic noise; similar shifts have been observ in Great Tits (Parus major; Slabbekoorn and Peet 2003) and House Finches (Carpodacus mexicanus; Fern??ndez-Juricic et al. 2005) These changes mirror those observ in the vocalizations of birds living in areas with high horizontals of natural noise (e.g., from waterfalls or other animals; Dubois and Martens 1984 Slabbekoorn and Smith 2002b) The three described cases of oftenness adjustment in response to low-frequency urban noise involve reduction in the oftenness range of songs (i.e., increased minimum frequent occurrence but no change in maximum frequency; Slabbekoorn and Peet 2003 Fern??ndez-Juricic et al. 2005 copse and Yezerinac 2006). A next to the first means by which birds could adjust the oftenness structure of their vocalizations to restore masking is by shifting the entire vocalization to a higher oftenness We are not aware of any examples of this kind of adjustment in answer to urban noise; use of this adjustment will be limited by dint of several morphological and kinematic factors that constrain the maximum frequencies that a bird can exhibit including head angle, beak gape, and beak shape (Westneat et al. 1993 Palacios and Tubaro 2000 Podos et al. 2004 Nelson et al. 2005) A third means by means of which birds could adjust the oftenness structure of their vocalizations is by the agency of changing the relative amplitude of different oftenness components. Rabin et al. (2003) lay the foundation of that California ground squirrels (Spermophilus beecheyi) in areas with low-frequency noise from wind turbines shifted the peak potency of their calls from lower to higher harmonics. thicket and Yezerinac (2006) found that canzonet Sparrows shifted more energy into higher frequencies in noisier areas, on the contrary this pattern appears to throw back the use of fewer low-frequency notes rather than a change in the relative amplitude of notes.