We know that birds have remarkable vision and hearing. However, the received wisdom from ornithology texts is that birds have a poor sense of smell. This claim is based in part on the rather unimpressive size of the olfactory lobes of a bird’s brain. These lobes extend anteriorly in the skull, underlying the bill where the external nostrils are located.
There are a few groups of birds that do have remarkable senses of smell. The tubenoses (albatrosses, petrels and storm petrels) are the zenith of bird olfaction. Tubenoses forage on the open ocean for small fish and squid near the surface. These prey items give off oils that float on top of the heavier water.
The problem with depending on these oceanic animals for food is that they are notoriously patchy in their distribution. No problem for an albatross or petrel; they wander broadly over the open ocean, smelling their way to a patch of fish or squid.
This method of foraging is made possible by the long, thin wings of these birds. An albatross’s wingspan can exceed 10 feet. By orienting into the ever-present winds on the open ocean, a tubenose gains lift and can then tack in any direction. As they lose altitude, they simply turn into the wind to gain more lift.
The result is an efficient means of flying. A tubenose can cover hundreds of miles in a day scarcely flapping a wing.
If you go on an offshore bird trip, you will see that the leader will occasionally pour fish oils and offal overboard to create an oil slick to try to draw in tubenoses. I vividly remember a pelagic trip out of Westport, Washington when chum was put into the water about 40 miles offshore. Within 15 minutes, 90 black-footed albatrosses were flying and sitting close by. I felt so sorry for one very seasick birder who could not even raise his head to look at the albatrosses.
The turkey vulture also has a remarkable sense of smell. In this case, the vulture is sensitive to ethyl mercaptan, the odorous compound in carrion. Some petroleum companies use this sensitivity to their advantage by putting ethyl mercaptan in the gasoline carried through pipelines. Any pipeline leak is easily found by looking for turkey vultures hovering over the leak.
Recent work has indicated that other birds may have keener senses of smell than previously recognized.
For his doctoral research, Alex Van Huynh showed how black-capped chickadees and Carolina chickadees keep from hybridizing. Apparently, the chickadees have a tough time telling the two species apart like we birders! The chickadees produce a chemical signal (a pheromone) that differs between species. Who knew chickadees might choose a mate by smell?
Ivan Hiltpold and W. G. Shriver at the University of Delaware investigated the importance of smell for birds in finding insect prey in corn fields.
Plants and herbivorous insects are at war. Plants can use structural defenses, like thick leaves or thorns, to try to deter insects. They also can use chemical defense by producing noxious chemicals that kill or deter insects.
The defensive chemicals (methyl jasmonate and methyl salicylate are the most common) are expensive to make and are volatile, so they do not stay in a plant’s leaves for long. Plants, therefore, make them only after insects start attacking. Release the chemical defense!
Here’s the really cool part. The authors found that insectivorous birds were attracted to a mixture of these defensive chemicals. So, the defensive chemicals produced by the plants have two beneficial effects: they deter or kill herbivorous insects and they attract birds to eat the insects.
Ecologists call this three-level interaction a top-down food chain. As you go down the food chain, the effects alternate. For instance, if you increase bird predation, the insects go down and the plants grow better. From the perspective of plants, the enemy of my enemy is my friend.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu
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