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Box 1 Is singing low important to great tits?There is a general relationship in birds between body size and frequency of pitch: larger species typically sing lower. Small birds with small vocal organs may be lacking the power, the essential size of the vibrating source, or the suitable resonance features of the vocal tract, that allow the production of low-pitched sounds. This creates the possibility of using low frequencies as a way to signal condition or size to competitors and potential mates (†). However, we do not know whether intraspecific variation in male body measures is related to the minimum frequency in great tits, nor do we know if female great tits prefer males singing low-pitched songs. If it were hard for great tits with a territory to sing low, you may expect that they sing songs relatively high in pitch when the motivation to sing is low. Such a situation occurs at the end of the breeding season, in both urban and forest habitat. Males still sing their stereotypic song types but the resources at stake in their territory have decreased in value until the start of the next breeding season. A decline in motivation to sing is clear from a decline in number of repeated songs (phrases) in a strophe, independent of habitat: in the city of Rotterdam from six to three phrases, and in the forest called the Liesbos from six to four phrases on average (anova: F2,100 = 27.4, P < 0.001). Interestingly, controlling for habitat differences, we found that the minimum frequency increases dramatically when recordings from relatively early in the season (early April: ‘Early’) are compared to recordings from late in the season (late May: ‘Late’, anova: F2,100 = 7.8, P < 0.01). We therefore hypothesize that it may be an important signal of quality to have low-pitched notes in the song type repertoire, which may cause a trade-off especially for the urban birds in noisy territories. Low-frequency noise may constrain the capacity to signal individual qualities: urban birds may need to choose between being heard well or being rated high as a competitor or mate. High noise levels may increase the number of intruding competitors needing physical combat and may affect the number and quality of potential mates that are attracted. Consequently, although great tits are successful in cities, and even though they show a remarkable flexibility in spectral adjustment to local noise conditions, this does not mean that traffic noise interference is not harmful to individu
al birds.
