The sweet hum of songbirds

By Dr Leo JosephSeptember 10th, 2021

The origin of the world’s songbirds is a story that began close to home. Their special ability to detect the sweet taste of sugary food is a similar tale.

A sweet song

When European ornithologists first encountered Australian birds in the 18th and 19th centuries, they gave them names that likened the “new” Australian birds to familiar northern hemisphere species. We still use many of these names today, such as wren, robin, treecreeper, quailthrush, cuckoo-shrike, magpie and chat. These names led to the scientific idea that Australian songbirds were derived, in an evolutionary sense, from their northern hemisphere namesakes.

This assumption took a long time to shake. But molecular research in the 1970s and early 1980s began to reveal that the evolutionary sequence was the other way around. Over the past two decades, research in the world of birds and ornithology has confirmed that songbirds, more technically known as oscine passerine birds, had their evolutionary origins right here in Australia. Australia’s songbirds represent the survivors of the ancestral species that left Australia and gave rise to the songbirds of the northern hemisphere. You can learn more in Tim Low’s book Where Song Began.

A black and white bird with yellow wings feeding on a white grevillea flower.

A New Holland Honeyeater enjoying the sweet flavour of a grevillea. Photo uploaded to iNaturalist by Clark Edmeades CC-BY-NC-ND 4.0 (Int)

Why do hummingbirds have a sweet tooth?

All birds of the world, songbirds included, completely lack the genetic and biochemical mechanisms that most mammals, including us, use to detect the sweet taste of sugars. These mechanisms appear to have been completely lost during the evolution of the branch of dinosaurs that evolved into birds.

This observation led Dr Maude Baldwin, now of the Max Planck Institute for Ornithology in Germany, to pose a question about hummingbirds, which are not songbirds. How do the hummingbird species of the Americas detect the sugars and sweet taste of the flowers that are so essential to their survival?

Umami: a taste sensation

Dr Baldwin and her team discovered that hummingbirds, during their evolution, co-opted another gene widely used in the animal kingdom to detect umami, or savoury tastes. We use it too; think of the meaty, earthy tones of miso soup or bone broth.

The team found that in hummingbirds a number of mutations have occurred in the umami-detecting gene. The result was they found another way to detect sweetness. Once that was in place, the ecological stage was set for the ancestors of today’s hummingbirds to radiate evolutionarily into the many species and bizarre forms that we know today.

What about the other birds of the world that feed on flowers and presumably must be able to detect sweetness? Could the same be true of songbirds? When I contacted Dr Baldwin to to ask how Australia’s songbirds and even many of our parrots might be detecting sweetness, she was way ahead of me. She had already made a trip to Australia to gather samples to answer exactly this question.

An Australian origin of song and taste

A rainbow lorikeet in a tree.

The ability to detect sweet flavours is important to survival for many parrot species. How do they achieve it? Photo uploaded to iNaturalist by Victor W Fazio III CC-BY-NC-ND 4.0 (Int)

In research published earlier this year, Dr Baldwin’s team looked at the evolutionary family tree of the world’s thousands of species of songbirds. They found that songbirds’ ability to detect sweetness first arose early in the evolution of Australian songbird species. This means that Australia gave the world both songbirds and their very special characteristic: the ability to detect sweet tastes.

While similarly co-opting an umami-detecting gene, songbirds do not use exactly the same mechanism as hummingbirds to detect sweet tastes. This means the mechanisms have arisen independently in songbirds and hummingbirds.

It is not a simple challenge for evolution to change the genetic mechanism for tasting umami into one for detecting sweetness. But once it has evolved, this ability can be lost and regained relatively easily. This explains the loss and gain of the ability to detect sweetness among different species in the songbird family tree.

One of the next steps in this research is to ask how Australia’s parrots detect sweetness. Parrots are close relatives of songbirds and many species feed on flowers or the sweet lerp and honeydew produced by insects. At the Australian National Wildlife Collection, we have contributed some samples to Dr Baldwin’s team. We look forward to more exciting developments.

Leo Joseph is the Director of the Australian National Wildlife Collection at CSIRO in Canberra.

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