Charles Darwin was a great experimenter. In his later life at Down House, he conducted scores of weird and wonderful experiments on pigeons, fowl, plants, seeds, dogs, his own children, you name it; he would experiment on it. But he also found time to conduct some experiments during the Beagle voyage. He even got to perform an experiment on that most iconic of South American birds, the condor.
Darwin describes his condor experiment in The Voyage of the Beagle. He gets off to what would nowadays be thought of as a pretty bad start:
April 27th. … This day I shot a condor. It measured from tip to tip of the wings, eight and a half feet, and from beak to tail, four feet.
He then describes the range and habits of condors before getting on to his experiment on some live, captive condors:
Remembering the experiments of M. Audubon, on the little smelling powers of carrion-hawks, I tried […] the following experiment: the condors were tied, each by a rope, in a long row at the bottom of a wall; and having folded up a piece of meat in white paper, I walked backwards and forwards, carrying it in my hand at the distance of about three yards from them, but no notice whatever was taken. I then threw it on the ground, within one yard of an old male bird; he looked at it for a moment with attention, but then regarded it no more. With a stick I pushed it closer and closer, until at last he touched it with his beak; the paper was then instantly torn off with fury, and at the same moment, every bird in the long row began struggling and flapping its wings. Under the same circumstances, it would have been quite impossible to have deceived a dog.
…a classic blind test—although it seems strange to use the phrase when experimenting on the sense of smell.
Darwin goes on to observe:
The evidence in favour of and against the acute smelling powers of carrion-vultures is singularly balanced. Professor Owen has demonstrated that the olfactory nerves of the turkey-buzzard (Cathartes aura) are highly developed, and on the evening when Mr. Owen's paper was read at the Zoological Society, it was mentioned by a gentleman that he had seen the carrion-hawks in the West Indies on two occasions collect on the roof of a house, when a corpse had become offensive from not having been buried, in this case, the intelligence could hardly have been acquired by sight. On the other hand, besides the experiments of Audubon and that one by myself, Mr. Bachman has tried in the United States many varied plans, showing that neither the turkey-buzzard (the species dissected by Professor Owen) nor the gallinazo find their food by smell. He covered portions of highly-offensive offal with a thin canvas cloth, and strewed pieces of meat on it: these the carrion-vultures ate up, and then remained quietly standing, with their beaks within the eighth of an inch of the putrid mass, without discovering it. A small rent was made in the canvas, and the offal was immediately discovered; the canvas was replaced by a fresh piece, and meat again put on it, and was again devoured by the vultures without their discovering the hidden mass on which they were trampling. These facts are attested by the signatures of six gentlemen, besides that of Mr. Bachman.
The degree to which certain birds use smell to detect food is still a controversial topic. Most birds seem to have a poor sense of smell, but others such as kiwis and certain sea birds do seem to make use of it while foraging/hunting for food. Although turkey vultures seem to have a good sense of smell, experiments have shown that it does not appear sufficiently acute to detect odours from high altitude.
167 years after Darwin performed his condor experiment, the controversy continues.