Male song stability shows cross-year repeatability but does not affect reproductive success in a wild passerine bird.

Predictable behaviour (or 'behavioural stability') might be favoured in certain ecological contexts, for example when representing a quality signal. Costs associated with producing stable phenotypes imply selection should favour plasticity in stability when beneficial. Repeatable among-individual differences in degree of stability are simultaneously expected if individuals differ in ability to pay these costs, or in how they resolve cost-benefit trade-offs. Bird song represents a prime example, where stability may be costly yet beneficial when stable singing is a quality signal favoured by sexual selection. Assuming energetic costs, ecological variation (e.g. in food availability) should result in both within- and among-individual variation in stability. If song stability represents a quality signal, we expect directional selection favouring stable singers. For a 3-year period, we monitored 12 nest box plots of great tits Parus major during breeding. We recorded male songs during simulated territory intrusions, twice during their mate's laying stage and twice during incubation. Each preceding winter, we manipulated food availability. Assuming that stability is costly, we expected food-supplemented males to sing more stable songs. We also expected males to sing more stable songs early in the breeding season (when paternity is not decided) and stable singers to have increased reproductive success. We found strong support for plasticity in stability for two key song characteristics: minimum frequency and phrase length. Males were plastic because they became more stable over the season, contrary to expectations. Food supplementation did not affect body condition but increased stability in minimum frequency. This treatment effect occurred only in 1 year, implying that food supplementation affected stability only in interaction with (unknown) year-specific ecological factors. We found no support for directional, correlational or fluctuating selection on the stability in minimum frequency (i.e. the song trait whose stability exhibited cross-year repeatability): stable singers did not have higher reproductive success. Our findings imply that stability in minimum frequency is not a fitness quality indicator unless males enjoy fitness benefits via pathways not studied here. Future studies should thus address the mechanisms shaping and maintaining individual repeatability of song stability in the wild.

The role of singing style in song adjustments to fluctuating sound conditions: A comparative study on Mexican birds.

Many bird species adjust their songs to noisy urban conditions by which they reduce masking and counteract the detrimental impact on signal efficiency. Different species vary in their response to level fluctuations of ambient noise, but it remains unclear why they vary. Here, we investigated whether noise-dependent flexibility may relate to singing style and signal function of the flexible acoustic trait. Species with highly variable songs may generally be more flexible and strongly repetitive singers may be more limited to stray from their stringent patterns. We exposed males of four passerine species with contrasting singing styles (repertoire size, immediate or eventual variety singing and syllable diversity) to three experimental sound conditions: 1) continuous urban noise; 2) intermittent white noise and 3) conspecific song playback. We found no spectral or temporal changes in response to experimental noise exposure in any of the four species, but significant temporal adjustment to conspecific playback in one of them. We argue that the consistency in song frequency and timing may have signal value, independent of singing style, and therefore be an explanation for the general lack of noise-dependent flexibility in the four species of the current study.

Experimental evidence for real-time song frequency shift in response to urban noise in a passerine bird.

Research has shown that bird songs are modified in different ways to deal with urban noise and promote signal transmission through noisy environments. Urban noise is composed of low frequencies, thus the observation that songs have a higher minimum frequency in noisy places suggests this is a way of avoiding noise masking. Most studies are correlative and there is as yet little experimental evidence that this is a short-term mechanism owing to individual plasticity. Here we experimentally test if house finches (Carpodacus mexicanus) can modulate the minimum frequency of their songs in response to different noise levels. We exposed singing males to three continuous treatments: low-high-low noise levels. We found a significant increase in minimum frequency from low to high and a decrement from high to low treatments. We also found that this was mostly achieved by modifying the frequency of the same low-frequency syllable types used in the different treatments. When different low-frequency syllables were used, those sung during the noisy condition were longer than the ones sang during the quiet condition. We conclude that house finches modify their songs in several ways in response to urban noise, thus providing evidence of a short-term acoustic adaptation.