Hearing the tweeting, chirping, singing, and general ruckus birds make each morning is a gift for all early risers (but less so for those of us who prefer to sleep in).
The incredible cacophony of sound is called the dawn chorus, but why do birds do it? What are the sounds that birds make anyway? And why do they all sound so different?
What’s the difference between bird song and bird calls?
Bird songs and calls aren’t the same thing. All birds call, but only some sing: songbirds – known as passerines – make up more than half of all bird species.
Songs are generally more complex, consisting of several notes that are usually learned. A songbird may spend weeks listening and months practicing before they’ve perfected a particular song. Song is also usually for the purpose of mating – whether that be attracting a mate or marking territory against competition.
On the other hand, calls are a single note. Sometimes they’re learned. For instance, a just-hatched baby bird is already capable of calling and begging for food. In some species, for example superb fairy wrens (Malurus cyaneus), the begging call is taught by the mother when the baby is still an embryo in the egg. In other species the begging call is innate.
Why do bird species sound so different to each other?
The sounds animals can make depend on the structures they have to make them. Humans have a larynx, whereas birds have a syrinx.
The bigger the syrinx, the deeper a bird can sing – think of the range of a cello versus a violin. A common raven (Corvus corax) (which can weigh up to 2 kilograms) will produce sounds much lower than the golden-crowned kinglet (Regulus satrapa), a songbird that only weighs about 5.5 grams.
Research also suggests that there might also be species specific sensory preferences: bird brains seem to influence bird song.
A 2020 study showed that baby zebra finches (Taeniopygia guttata) raised with adult Bengalese finches (Lonchura striata domestica) will learn to sing the other species’ song. However, over four subsequent generations of teaching the song to offspring, the zebra finches started to drift back towards singing their own species’ song – without having heard it!
Some birds can even form geographically distinct dialects within different populations, like human accents. A 2012 study that looked at silvereye populations in rural and urban areas found that birds living in a noisy city shift their calls and songs in response to background noise. Some increased the minimum frequency of their songs – which might minimise interference from low-frequency noise that makes it difficult for other birds to hear them – while others altered the tempo or used specific syllables or songs that transmit well through particular acoustic environments.
Why do birds make so much noise at sunrise?
The dawn chorus is an incredible phenomenon, and there’s not yet consensus among scientists as to why it occurs.
According to Dr Dominique Potvin, an evolutionary and behavioural ecologist at the University of the Sunshine Coast, one hypothesis is that birds call at dawn because sound travels well due to a combination of a few different factors.
Early in the morning, there aren’t a lot of other organisms making sound. This is because animals like frogs and invertebrates – animals without a backbone, such as insects – are cold blooded, so they don’t have the external energy to make a lot of noise before the sun comes up.
Another factor is that there’s more moisture in the air, and sound travels slightly faster through humid air because it’s less dense; water molecules take up less space than oxygen, nitrogen or carbon dioxide molecules.
The dawn chorus might also be a way for one bird to communicate to others that it is well-fed and comfortable, signalling fitness to mates and competitors. A 2006 study into silvereyes (Zosterops lateralis) showed they’re more likely to sing a long dawn chorus if they’re really well fed. But if they’re hungry, they tend to sing a really short song (or none at all) because they need to go to get food as soon as the sun comes up.
Part of the function of the dawn chorus could be in establishing territory and reminding other birds that you’re still there and haven’t been killed in the middle of the night!
Professor Sonia Kleindorfer, an organismal systems biologist from Flinders University, says that another way to think about this phenomenon is to ask whether there’s a mechanistic explanation for why sunlight might trigger singing behaviour from birds?
For instance, light might trigger hormone production that increases the motivation to sing.
Like humans, birds have an internal biological clock, or circadian rhythm. This is a complex system that includes the retina, the pineal gland and the hypothalamus in the brain, which converts light signals into electrical signals to stimulate physiological processes.
The key messenger in this process is the hormone melatonin, and research in zebra finches has shown that when the pineal gland is removed, circadian rhythms of singing stops, but administering melatonin restores it.
Artificial night lighting has even been shown to affect the timing of dawn singing. Research in European songbirds has shown that artificial night lighting leads to an earlier dawn song, ranging on average from 10 minutes for the song thrush (Turdus philomelos) to 20 minutes for the robin (Erithacus rubecula).
Many of these, and more, potential explanations could apply both at the same time and to different extents for different bird species.