However, collaborators from Leiden and Groningen Universities have recently published a paper in which they say that there are two even more important questions that have, until now, been ignored. The first is whether low-pitched songs are inherently valued in avian mate attraction. This is important because most noise-exposed male birds shift their song frequencies upwards in order to avoid being "masked" by sounds in the environment. This behavior may not be that detrimental if those lower frequencies were not fundamentally important in mate-mate communication. The second question is whether song adjustments actually alter communication efficiency in the field. Although previous studies have looked at differences in the transmission of higher- and lower-pitched signals, little work has focused on documenting variations in behavioral responses to different signals played under quiet and noisy conditions. Cumulatively, the answers to these questions could allow researchers to understand whether anthropogenic noise pollution really could influence avian breeding success and population health, as has been suggested.
(Great tit, Parus major)
In order to investigate these issues, the Dutch researchers utilized a network of nest boxes in the Nationaal Park Dwingelderveld in The Netherlands. The boxes were inhabited by great tits (Parus major), which have previously been studied extensively in the context of anthropogenic noise. Microphones were placed both within and outside active nest boxes so the researchers could collect recordings of interactions between males and females. Paternity tests were conducted for all of the chicks within each nest in order to see whether vocal traits were related to a male's ability to keep his female faithful. Finally, the researchers performed a playback experiment in which they broadcast recordings of higher- and lower-pitched male songs to their mates. This allowed them to examine whether the females responded differently depending on the pitch of the vocalization and how well it stood out against the background noise.
The results of the study were striking and consistent with the idea that anthropogenic noise could have serious impacts on the health of avian populations. Individual males varied with respect to how low they sang and how often the used low-pitched song types. One thing that was similar across all males was a concentration of lowest-pitched songs in the period just before egg-laying, when their females were most fertile. This pattern suggests that males are using these low-frequency vocalizations to indicate their attractiveness and quality to their mates, thus discouraging the females from casting a wandering eye on neighboring males. During this same period of the nesting cycle, female calling activity peaked, indicating a strong pair bond between the two birds.
(Nationaal Park Dwingelderveld, The Netherlands)
Results of the DNA tests also support the theory that low-pitched songs are used to broadcast male quality and reduce the likelihood that females will "cheat" on their mates. Males that spent more time singing lower-frequency songs were less likely to be cuckolded than males who used higher-frequency songs. Females who did dally with the neighbors left their nest boxes earlier each day than faithful females, and, in general, this behavior was more common during the peak of fertility. In other words, the pre-egg-laying period is a critical time during which it is vital for a male to make himself heard and to sound particularly sexy.
The playback experiment indicated that female response to vocalizations is strongly influenced by both environmental condition and song pitch. Under normal ambient noise levels, females responded to both low-frequency and high-frequency songs in similar ways. However, under noisy conditions, females emerged significantly less to low-frequency song types, and significantly more to high-frequency song types. In this case, emergence does not indicate sneaking off to visit another male, but rather an active response to her own partner's voice. Therefore, it appears that noise can interfere with mate-mate communication--a problem that can be rectified by the males' use of higher frequencies.
(Great tit at a nest box)
These results indicate that male great tits in noisy areas may face a tradeoff during the pre-egg-laying period of the breeding season--what the authors describe as a "modern tradeoff with crucial fitness consequences." If they want to ensure their paternity, male tits need to sing at a lower pitch, but if they need to communicate effectively with their mates--for instance, to alert them to an approaching predator--they will need to sing at a higher pitch. The current study shows that the noise pollution generated by anthropogenic activities is likely altering the balance between the various pressures that have shaped male bird song over evolutionary history. This will likely lead to the evolution of new song properties or song-production behaviors; there is already some preliminary evidence that this has begun to happen in some habitats. Alternatively, in birds that are unable to display much vocal flexibility, breeding success may decrease, or, as different types of males become more successful fathers, the contents of the local gene pool may shift. Thus, in addition to providing some fascinating insights into the variety of forces driving the use of particular bird song characteristics, this study underlines the powerful impacts that anthropogenic change may have on wildlife.
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Halfwerk, W., Bot, S., Buikx, J., van der Velde, M., Komdeur, J., ten Cate, C., Slabbekoorn, H. 2011. Low-frequency songs lose their potency in noisy urban conditions. Proceedings of the National Academy of Sciences, online early publication.
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