Monday, July 21, 2008

Some Birds Have a Sense of Smell

Turkey vultures are known for finding meals by smell. Other birds, however, have a poor reputation when it comes to their olfactory abilities. That reputation may not be warranted, as recent research indicates that at least some birds can use a sense of smell (abstract).

A genetic study compared chickens, whose full genome has been sequenced, with eight other species from various orders. The study focused on olfactory receptor genes. The number of olfactory receptor genes is thought to correspond with how many distinct scents an animal can smell. The authors found a wide range in the number of olfactory receptor genes present in the eight species (from 107 to 667 genes) and also wide variation in the size of the olfactory bulb relative to the rest of the brain. However, a high proportion of the olfactory receptor genes were potentially functional for all species, suggesting that all birds make at least some use of the sense.

Below is a table of the eight species in the study, ordered by the relative size of the olfactory bulb. (The number of genes is the estimated total number of olfactory receptor genes for the species.)

Species# GenesOB Size
Snow PetrelPagodroma nivea21237
Brown KiwiApteryx australis60034
KakapoStrigops habroptilus66730.2
Black CoucalCentropus grillii
MallardAnas platyrhynchos43019.4
ChickenGallus gallus63814.2
Blue TitCyanistes caeruleus2189.7
GalahEolophus roseicapillus1078
CanarySerinus canaria1666

The snow petrel, a bird that spends most of its time at sea and may need to travel miles to find food, has the largest olfactory bulb. The next two birds, brown kiwi and kakapo, are nocturnal forest birds, so it makes sense that they would have a good sense of smell. Unlike owls, which can use their ears to listen for prey in the dark, kiwis and kakapos eat insects and fruit, respectively. Songbirds, meanwhile, make a relatively poor showing.

There are some caveats here. Since the full genomes for most of the species in this study have not been sequenced, the results are expressed in probabilities. Even the chicken genome, the only one to be fully sequenced, is still undergoing revision. So it seems that the questions of how well birds can smell and how they use that sense still have to be answered.

That said, the conclusions of this genetic study fit with the results of a behavioral study of blue tits. The tits in that study were able to detect a predator's scent around their nesting box and distinguish it from other common scents, such as water and quail. If tits are near the low end of avian olfactory ability and they can do that, imagine how much the birds at the high end of the spectrum can smell.