In the breeding season, birds use song as a means of recognising and identifying the species and sex of the individual singing. They can then act accordingly, for example; if a breeding-age female bird hears a male’s (in most species it is just the males who sing) song, she can approach him with the intent of mating, or if a male hears another male’s song in his territory, he can choose to fight, sing back or flee. These are the fundamental purposes of bird song (Catchpole, 1995).
The Phylloscopus Leaf Warblers are a genus of small Old-World passerines. Most are migratory, especially those at higher latitudes. They mainly inhabit forests on both the breeding and wintering grounds and are insectivorous (Baker, 1997). Because many species in the genus are so similar in visual appearance, vocal communication is very important in interactions between individuals of the same species, so while there is a lot of variation in each species’ song, there are elements which stay constant and can be used to specifically identify an individual (Tietze et al., 2015).
Because birdsong is a means of social interaction, the fact that it can be flexible and alter in meaning with changes in the social environment is vital.
Section 1: How Social Environment Affects the Song of an Individual in the Short-term
It would be beneficial to a singing bird if it could change its song immediately because of changes in its social environment, this would allow it to convey information without risking injury and a waste of energy on physical actions (Falls, 1969).
In the Willow Warbler P.trochilus, male songs produced in agonistic situations with other males are markedly different to the ‘spontaneous’ songs produced in normal life. Agonistic songs are shorter in duration (x̅ = 2.32s compared to x̅ = 3.14s) and number of syllables (x̅ = 13.46 compared to x̅ = 19.34) than spontaneous songs. However, the volume and length of the songs increase over the period of the interaction. Immediately before physically attacking another male, the regularity of songs is lower than in normal life, but this then increases to above the normal regularity after the attack. There is also a song element, known as the A-syllable, which is rare in spontaneous songs but very frequent in agonistic songs (immediately before an attack, frequency = 0.80). It is uttered 3-5 times in the first half of the song and therefore completely changes the structure and conveys a very specific meaning to other Willow Warblers. This meaning is that the singer is going to attack (Jarvi et al., 1980).
Wood Warblers P.sibilatrix also change their songs in interactions with other males. In an experiment, Szymkowiak and Kuczyński (Szymkowiak and Kuczynski, 2017) found that birds increased the rate at which they sung by an average of 0.84 songs per minute during a simulated interaction with another ‘male’. This behaviour also correlated with the intensity of their aggressive response. Additionally, Szymkowiak and Kuczyński showed that males reacted more aggressively to playbacks with a higher song rate (this meant a faster approach to, more attacks on and more flyovers of a dummy)(Szymkowiak and Kuczynski, 2017).
Clearly, when a territorial male is approached by another male, leaf warblers often change their song to convey meaning. The changes that occur depend on species and the nature of the interaction (within a species). I have not found any reference to song changing in an interaction with a potential mate, but this could be because different vocalisations or behaviours other than true song have evolved for this purpose. Indeed, Simms (Simms, 1985) states that Willow Warblers have a ‘thin shrew-like note’ and a ‘plaintive cheep’, both uttered in display. They also have a well-developed and characteristic courtship display which involves a chase and wing-waving, although courtship behaviour is clearly quite variable as Simms (Simms, 1985) also says that there is sometimes a ‘little display flight with a burst of song’.
Section 2: How Social Environment Affects the Song of an Individual in the Long-term
Birds may also need to alter their song over the course of a breeding season or period in a breeding season because of prevailing conditions in their social environment. The primary cause of this is to increase their chances of mating.
Willow Warblers change their song in response to the density of the population they are residing in. Due to environmental factors and mortality of individuals in this migratory and philopatric species (Hedlund, 2017), density will change from year to year and even within a breeding season. In a study by Goretskaia (Goretskaia, 2004), birds spent a lower percentage of their time singing (median 58% compared to median 68%) during observation periods in the lower density population. It is worth noting that in the high density population, 51% of singing time comprised of duels with other males, whereas this percentage was only 7% in the low density population. The structure of the songs were also different between the study areas, in the low density area, the median number of ‘similar elements per initial phrase’ was 3-4 in the low density area and 4-7 in the high density area. The difference remained significant when females were excluded from the analysis and Goretskaia concluded that the number of duels directly affects the length of the initial phrases of the song. In high density areas, the variability, which is most prevalent in duels, and the number of double-songs (two songs in one) also increased.
The social environment of a juvenile male Common Chiffchaff P.collybita during its first 44 days of living affects the song it sings for the rest of its life. When birds were hand-reared, and hence did not hear the songs of other males during the first 44 days of their life, their songs differed significantly from normal. But, when taken from the wild when the birds were 44-100 days old, and kept in acoustic isolation, there was not a significant change in their songs (Thielcke and Zimmer, 1986).
The breeding distribution Iberian Chiffchaff P.ibericus is normally restricted to Iberia and North Africa (Svensson, 2010) but, being a long-distance migrant on a north-south axis, they are prone to overshooting their breeding grounds and turning up well north of their normal distribution (Pérez-Tris, 2010; Cottridge, 1997). I personally observed an extralimital vagrant Iberian Chiffchaff in Yorkshire singing the typical song of its species, a perfect copy of the song of Common Chiffchaff and intermediates between the two. This phenomenon is known as mixed-singing and is relatively frequent in areas where Iberian and Common Chiffchaffs, which are expanding their range and infilling areas previously occupied by Iberians, overlap in distribution (Dubois, 2008) and when Iberians occur as vagrants (Collinson, 2008). It is thought that when there is a shortage of potential mates of the same species, such as is occurring during vagrancy to areas where Common Chiffchaffs are present and in the areas of Common Chiffchaff distribution infilling, Iberian Chiffchaffs will incorporate elements of Common song, which they will be hearing from the Common Chiffchaffs inhabiting the area, into their own in an attempt to attract a female Common Chiffchaff and pass on their genes in a non-assortative mating system (Dubois, 2008). According to Thielcke (Thielcke, 1983), the phenomenon of new vocal traditions arising in areas of poor cultural transmission, such as in the dual situations of Iberian Chiffchaff, is one which can allow the creation of new dialects within a species which can then persist.
Mixed singing also occurs in Willow Warblers, which incorporate elements of Common Chiffchaff song into their own. It is becoming more frequent in Britain in line with the retreat of Willow Warblers from the south and east and the resultant infilling of the free Phylloscopus niche with Chiffchaffs. Willow Warblers can increase the complexity and variability of their song between their first and second years, probably using the songs of neighbouring territorial individuals as templates (Gil et al., 2001). Because Willow Warblers are philopatric (Hedlund, 2017) and, in the very closely related and similar Chiffchaff (Simms, 1985), the main period of song learning is within the first 44 days of life (Thielcke and Zimmer, 1986), the lack of singing Willow Warblers where juveniles are growing up could result in the juveniles incorporating Chiffchaff into their song because Chiffchaffs are the predominant perceived territorial competitor. This has an advantage in that the mixed song acts as a stronger deterrent to Chiffchaffs, which could be occupying potential Willow Warbler territory, and result in fewer costly physical conflicts (Lawn, 2018).
Seemingly, an evolutionary advantage exists for birds which are able to adapt their songs in the long term to the prevailing social environment, whether this is by sexual opportunism in an unfavourable social environment (Dubois, 2008), such as with Iberian Chiffchaffs, or adaptive territoriality in a species like the Willow Warbler (Lawn, 2018). On the other hand, learning of the song has been shown to be directly affected by the social environment (Thielcke and Zimmer, 1986).
Ultimately, song changes as a result of the prevailing social environment because the singer has to adapt to manipulate the social environment to its own benefit; to convey meaning in individual interactions, to better defend territories and to increase the chances of attracting a female. Song change also occurs as a function of social environment in ways in which the singer cannot manipulate the social environment reciprocally and is necessarily required to change. Additionally, severe abnormalities in the social environment can directly affect song.
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