When we think about birds, we often picture their colourful plumage: the iridescence of a peacock's tail or the electric blue flash of a kingfisher. Or we might consider how they use voices, from the song of the nightingale to the coo of a dove or the shriek of a jay.
Author
- Joey Baxter
PhD Candidate in Biosciences, University of Sheffield
So it's easy to imagine that vision and hearing must be the senses these birds use to explore their environment and interact with each other. However, smell is also vital to birds for navigating, foraging and even communicating. Yet this sense is often underestimated or ignored entirely.
Some of the blame for this long-standing underestimation can be assigned to influential 19th century naturalists like John James Audobon, whose early experiments on turkey vultures led him to conclude that they could not smell and must use sight to locate their carcass suppers.
He presented vultures with paintings of dead sheep, which they pecked away at. But when he shrouded putrid carrion with plant material the vultures ignored it. However, later work revealed flaws in Audobon's research - these birds prefer fresher meat and locate it using scent, even when it is visually obscured.
The turkey vulture's keen sense of smell was put to use by oil company engineers in 1930s California. Workers were having trouble with leaks along a 42-mile-long natural gas pipeline but noticed that vultures would often congregate around these leaks. Natural gas alone is odourless, but a chemical called ethyl mercaptan is added so humans can detect its distinctive eggy smell at close range. Ethyl mercaptan is also released by decomposing meat, so vultures associate it with food. The engineers used this to their advantage, intentionally pumping through large doses of ethyl mercaptan and observing the vultures to pinpoint and repair leaks.
More recently, research has explored the many ways that birds use their sense of smell in the wild. At the University of Sheffield, I am investigating whether long-tailed tits, a small UK garden bird, might use their sense of smell to recognise family members. Like we humans often help close family with childcare, long-tailed tits will feed chicks belonging to siblings, parents and children during the breeding season. How these little birds identify who is and isn't a close relative is not entirely clear yet, but their scents may hold the key.
Starlings and blue tits , meanwhile, use scent to seek out aromatic plants such as yarrow, hogweed and elder, which they weave into their nests. The strong-smelling compounds in these plants defend their chicks against parasites, in the same way that we might use citronella to ward off mosquitoes.
Hoopoe chicks manufacture their own chemical defences. These are colourful birds with a long, curved bill and a distinctive orange crown of feathers. Young hoopoes produce a thick, dark, foul-smelling substance called preen oil from a gland just above their tail that contains bacteria. These beneficial bacteria break the preen oil down into pungent chemicals that keep germs at bay.
New Zealand's national icon, the flightless kiwi , is mostly nocturnal and feeds on worms and insects found underground, so cannot rely on vision when foraging. Instead, kiwis have nostrils at the very tip of their long bills, which they probe the earth with to sniff out their subterranean prey.
Crested auklets are small, black seabirds that smell like tangerines. This odour is produced by special feathers and is involved in social communication. Both male and female auklets will rub their bills into the nape of another bird's neck to get a good whiff, using this smell to assess their quality as a potential mate. So, it pays to produce a good strong dose of this natural cologne.
Petrels and shearwaters fly across hundreds of kilometres of open ocean in search of sustenance, using their sense of smell to detect dimethyl sulfide (DMS), a chemical produced by phytoplankton (microscopic plant-like organisms). The odour of this chemical, often compared to that of boiled cabbage, signals where in the sea is likely to be rich with food. Additionally, the varying intensity of this chemical allows them to create an olfactory map in their heads for navigating back to their nests on land.
Sadly, this impressive olfactory ability can land these birds in trouble. Ocean plastic causes blooms of phytoplankton, which pump DMS into the air in unusual quantities. Seabirds can be confused by these chemicals that are usually associated with food and will often consume the plastic, which can be fatal. Because of their reliance on scent for foraging, DMS-sensitive birds, which also include albatrosses, are nearly six times more likely than other species to ingest plastic.
Like birds, humans have been historically underappreciated when it comes to smell. An idea that - again - largely stems from the pontifications of 19th century scientists.
Humans, however, are sensitive to an enormous range of odours . One experiment showed that, when blindfolded, human participants could track the scent of chocolate across a field. We use our sense of smell all the time in our daily lives - sometimes without fully realising it - in avoiding danger (noticing the smell of smoke), selecting food (passing up off milk or picking a particularly ripe orange) or even choosing a partner. Research suggests people are often drawn to the odour of those with a very different set of immune genes to their own.
So, even for animals that don't have a dominant sense of smell, odours form a key part of the way they interact with the world.
Joey Baxter receives funding from UK Research & Innovation (UKRI), via the Engineering and Physical Sciences Research Council (EPSRC).
