We found consistent detectable differences between the songs of male great tits occupying territories of high and low background noise across the UK. This is consistent with other studies of the same species in continental Europe (Slabbekoorn & Peet 2003
; Slabbekoorn & den Boer-Visser 2006
), as well as in house finches Carpodacus mexicanus
in California (Fernández-Juricic et al. 2005
) and song sparrows Melospiza melodia
in Oregon (Wood & Yezerinac 2006
). However, we found this distinction present over just a few kilometres, between the city centre and the quieter outskirts, within the dispersal radius for young of this species (Greenwood et al. 1979
; Verhulst et al. 1997
). Furthermore, we found that territorial males responded at a significantly lower level when hearing song from a territory where background noise differed from their own.
The most prominent difference between rural and urban song was the minimum frequency. This was significantly lower in great tit song from quieter territories than noisy ones, and was significantly positively correlated with territory background noise.
Variation in noise levels at the spatial scales found here may pose problems for locally dispersing birds seeking to establish territories. Traffic noise in cities is generally low in frequency, and therefore creates direct competition in that acoustic space (an example of which can be seen in ). In a multi-species study, Rheindt (2003)
found a correlation between motorway noise and the dominant frequency of a species inhabiting the area around the road, suggesting that traffic noise causes declines in species that sing with a lower frequency song. Yearling great tits may avoid this situation by preferentially dispersing to areas with noise levels similar to their natal territory. However, this may not be possible: suitable habitat may be limited or already occupied and therefore, a dispersing male may have to occupy a territory where the noise is at a level different from their natal territory. The potential fitness detriment of decreased signal efficacy imposed upon males moving into noisier territories may be reduced if they were to avoid low frequencies.
Figure 5. (a–c) Sonograms of recordings taken in Cambridge and (i–iii) their associated power spectra. (a) Great tit at rural site (power spectrum (i)). (b) The background noise at the urban site during rush hour from exactly the same location as (more ...)
Previously it has been found that urban great tits shorten the first note of their song (Slabbekoorn & den Boer-Visser 2006)
. However, neither this, nor any other temporal measure, was found to correlate with background noise, although we found a non-significant trend suggesting that great tits in noisy territories sang shorter notes overall.
Not only were there differences in signal characteristics in relation to background noise, but our analysis also clearly shows that male great tits respond significantly more strongly to songs from territories with noise levels similar to their own. This suggests that great tits dispersing to areas with different background noise levels from that which they have experienced previously will be at a competitive disadvantage in territorial disputes. A weaker response to songs may allow neighbouring males to encroach on a new arrival's territory. Alternatively, a reduced response to intruders may decrease the incidence of territorial disputes and permit larger, or overlapping, territories.
Birds from quieter territories show sufficient distinction in their behaviour towards the homo- and heterotypic songs, that the model correctly identified 85 per cent of them as rural individuals. Similarly, males from noisier territories showed a significantly lower response to songs from quieter territories. In both cases, great tits were slower to overlap their song with the playback song, did not approach the speaker as closely, spent less time within a 5 m radius of the playback song and spent less time reacting to the playback song throughout when that song was of a male with different level of background noise from their own.
However, the model was only able to identify 61 per cent of urban birds correctly from their behaviour during playback. This could be because of a difference in the behavioural plasticity of both rural and urban birds. A varying response to differing playback frequency has been shown in field sparrows Spizella pusilla
). In controlled laboratory conditions, field sparrows were found to use a combination of five-song parameters in song recognition, although song frequency was given more weight than other temporal measures such as inter-note interval and phrase number (Nelson 1988
). The great tits in our study may be using a similar weighting system, where frequency features strongly. Other song characteristics unidentified by us, possibly involving temporal measures, may be causing some ambiguity in the signal receiver's interpretation of the song, accounting for the shortfall of differentiation in the urban sample.
There are clear conservation implications of these adaptive variations in song characteristics, including population divergence (for a review, see Slabbekoorn & Ripmeester 2007
). However, it is not yet known whether the difference in song between the two environments is because of a permanent change in the repertoire, or the individual's ability to detect masking of their signal and use this auditory feedback to select or alter their songs in order to increase detection (Patricelli & Blickley 2006
). It is also possible that male great tits are seeking territories that optimize perception of their song. Until recently, great tits were thought to exhibit close-ended learning and so crystallize their repertoire at the end of a sensitive learning period in late autumn of their first year (McGregor & Krebs 1989
). This would allow them to fit their repertoire to their environment and would select for songs that yearlings can hear from their neighbours (tutors). Presuming that young birds learn throughout the day, including quiet times, it is likely that the masking of songs is not sufficient to exclude learning of low-frequency notes completely, but instead produces a bias towards songs that are audible for the most amount of time. This would explain why, with a crystallized repertoire, birds with a higher minimum frequency still recognize lower frequency songs, albeit exhibiting a lower behavioural response. Nonetheless, recent work has suggested that adult great tits are able to assimilate new songs into their repertoire, suggesting lifelong plasticity controlled by social circumstances (Franco & Slabbekoorn 2009
). A laboratory study on Bengalese finches has also shown that these song birds may adjust their song pitch on a short temporal scale (Tumer & Brainard 2007
). Great tits may therefore be able to make small adaptations to their song post-dispersal. However, the extent to which they are able to adapt their songs in this manner, and thereby adapt to a new acoustic environment, remains unknown.
While background noise clearly has an effect on male song, we cannot rule out other factors completely. For example, air pollution poses a known threat to both a great tit's physiological condition (Isaksson et al. 2005
) and song (Gorissen et al. 2005
). The latter study found a significantly lower total amount of song during the dawn chorus from males close to the pollution sources than from the males 4 km away. However, there is no evidence that air pollution affects the frequency of the song. It is also unlikely that the difference in playback response to rural song is a result of reduced energy levels in urban birds, as their reaction to homotypic song was comparable to the rural bird's response to rural song.
In summary, our data show a clear difference in song characteristics between areas of high and low background noise. These variations in song elicit a significantly reduced behavioural response in territory-holding males when the level of background noise differs between the territories of the signaller and receiver. The adaptation of great tit song to overcome the increased noise levels associated with urban areas appears, therefore, to have occurred at a cost to their ability to respond appropriately to conspecifics from rural areas. Further cues as to the evolutionary significance of these behavioural alterations may be obtained from investigating how female great tits respond to these changes and the extent of cross-rural/urban great tit dispersal.