Two bird species sing more complex songs in simpler communities
Causes of the latitudinal gradient in birdsong complexity assessed from geographical variation within two Himalayan warbler species. Singh, P. & Price, T. D. 2015. IBIS 157: 511-527. DOI 10.1111/ibi.12271
Some bird species sing what appear to us to be very simple songs of a single note, which is repeated a few times. Others, such as the varied and melodious song of the European Robin, are much more complex.
That this means something for the birds as well as us comes from the finding that the further north from the tropics one is, the more birds tend to sing complex, diverse, songs. We dub this the complexity rule. Although the pattern is not universal, it rivals the famous Bergmann’s rule for its generality (Bergmann’s rule states that body size tends to increase as one goes to higher latitudes). But why is this the case?
We found 10 different hypotheses to explain why species at higher latitudes should sing songs of greater complexity. For example, females of several species have been shown to prefer complex songs over simple ones in laboratory tests. Given this, the fact that many species in the north are migratory and start to reproduce quickly after they reach their breeding grounds may put a premium on rapid pairing, thereby increasing competition among males for mates over a short timescale. A second explanation is that the presence of a food flush at higher latitudes may reduce costs of singing complex songs. The problem is that these and the other 8 explanations are correlated with each other, hence hard to tease apart.
We studied geographical variation in the songs of two Himalayan birds, the Grey-hooded Warbler and Blyth’s Crowned Warbler (Fig 1). These closely related species are among the most abundant of all birds in the Himalayas, with the Grey-hooded Warbler occupying an elevational range of ~600m-2,000m and the Blyth’s occurring above it from ~2400m-3100m. Although the Himalayas are often perceived as running from east to west, they actually take a northwesterly turn, and the western Himalayas are much more temperate-like than the eastern Himalayas. We found the complexity rule holds: both species sing more complex songs in the northwestern Himalayas than in the eastern Himalayas. In this study, however, many previous explanations cannot apply. For example, our measurements show there is less food in the northwest.
Listen to the songs of our study species:
Blyth’s Crowned Warbler Phylloscopus reguloides © Pratap Singh
Grey-hooded Warbler Phylloscopus xanthoschistos © Pratap Singh
One hypothesis stands out. This is that the presence of less than half the number of species in the northwest than the east means that songs are less masked by background noise. Some years ago, Haven Wiley argued that in the presence of a cacophony of sound simple songs containing repeated notes should be favoured because, if only part of the song is heard, the singer can still be identified. Applying this reasoning, restrictions on how complex a song can be should be stronger in the east than in the northwest. We measured sound amplitude at multiple sites in both locations and found the ‘sound space’ in the northwest was particularly reduced in amplitude in the 2.5-5 Khz frequency range at elevations where the Grey-hooded Warbler occurs. Remarkably (to us) the Grey-hooded Warbler in the northwest not only sings with more notes in its song, but over a broader frequency range, expanding its bandwidth down into the vacated 2.5-5 Khz zone (Fig 3), exactly the region that is quieter in background noise in comparison with the east. This provides some supporting evidence that songs are more constrained by noise in the east.
We also asked how differences in song complexity along the Himalayas arose. Many bird species start their song with a distinct alerting note, designed to draw attention. The Grey-hooded Warbler is the only species we are aware of in which each male has several different alerting notes (Fig 3). One alerting note precedes one song type, and another precedes a different song type in the male’s repertoire. We discovered that some of the alerting notes of eastern birds are actually present within northwestern individuals’ songs. Apparently, one way complexity has been built up is by the alerting notes moving into, and replacing, one of the repeated notes in a song, with another alerting note added on at the beginning. The Blyth’s Crowned Warbler has generated its complexity quite differently. In this species, the northwestern form appears to have copied songs from other warbler species, with the result that the few songs in its repertoire are more different from each other than they are in the east.
We have no idea if the different ways the two species’ songs have become complex reflect different functions of these alternative forms of complexity (e.g., one may be more valuable in male competition and another in female choice), or if the pressure was simply “be complex” and different species achieved this using methods available to them. For example, the Blyth’s Crowned Warbler is a typical bird in that the alerting note is the same in front of all of its songs (and the same in all Himalayan populations), implying it could not build up complexity in the way we infer has been partly responsible for Grey-hooded Warbler complexity. But whatever the driving force, the movement from simplicity to complexity creates diversity. Songs of the two species in the northwest might be considered more different from each other than they are in the east, a pattern that has been shown in other studies of bird songs from different latitudes.
Our results thus link the complexity rule to yet another latitudinal pattern, the most famous one of all. This rule is that fewer species are present at higher latitudes than lower latitudes: the so-called latitudinal gradient in species diversity, first discussed by Alfred Russell Wallace and whose causes are still much debated. At higher latitudes the relatively few species there apparently allow a male there to more fully express himself, and this may be part of the reason that the “the sweetness of his notes may yeeld a delectable resonancie”, as Thomas Coryat said about the European Robin in 1611.
Top image: Blyth’s Crowned Warbler Phylloscopus reguloides © Vijay Cavale
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