by Matthew Cobb
Over at The Atlantic, Rebecca Rosen has a nice piece about how nearly 900 homing pigeons got lost after being released from a site in Jersey Hill in upstate New York between 1968 and 1987. The disappearance of these pigeons were noted because they were part of a long-term experiment by Cornell professor William T. Keeton, into pigeon navigation. The explanation, as outlined by Rosen, has recently been put forward by Jonathan Hagstrum from the US Geological Survey, in a recent article in Journal of Experimental Biology. And the answer is ‘infrasound’. Hasgtrum’s abstract says:
Jersey Hill lies within an acoustic ‘shadow’ zone relative to infrasonic signals originating from the Cornell loft’s vicinity. Such signals could arise from ground-to-air coupling of near-continuous microseisms, or from scattering of direct microbaroms off terrain features, both of which are initially generated by wave–wave interactions in the deep ocean.
Hagstrum modelled what might have been happening to the sound produced by these various sources and concluded:
little or no infrasound from the loft area arrived at Jersey Hill on days when Cornell pigeons were disoriented there, and that homeward infrasonic signals could have arrived at all three sites from directions consistent with pigeon departure bearings, especially on days when these bearings were unusual. The general stability of release-site biases might be due to influences of terrain on transmission of the homeward signals under prevailing weather patterns, whereas short-term changes in biases might be caused by rapid shifts in atmospheric conditions.
The pigeon-lover Darwin could never have imagined it!
Interestingly, the catastrophic disappearance of a flock of birds played an important role in providing empirical evidence that natural selection could shape species. In 1898, American naturalist Hermon C Bumpus published an article in which he pointed out that, at the time, although many thinkers used natural selection as the framework to interpret the world,
‘we forget we are really using a hypothesis that still remains unproved and that specific examples of destruction of animals of known physical disability are very infrequent’
In February 1898 there was a terribly violent snowstorm and at the end of the storm, a group of 136 sparrows that had been hit by the storm were brought into Bumpus’ lab at Brown University. 72 of the birds survived, 64 perished. What separated the living from the dead? Bumpus measured each bird according to 10 criteria (size, weight, beak length etc). What he found was that the birds that died were generally those that departed most from the mean – the largest, the smallest, the lightest, the heaviest, etc. However, he also noticed that some birds with ‘extreme’ characters did survive, as long as they did not have several extreme variants.
Bumpus concluded with a view that natural selection tended to favour a ‘type’, rather than extreme variants:
‘Natural selection is most destructive of those birds which have departed from the ideal type, and its activity raises the general standard of excellence by favoring those birds which approach the structural ideal’
(This is the only sentence in the article that is in italics)
This raised a problem, which Bumpus did not address, and I invite readers to comment on below the fold. If natural selection favours a ‘type’ and is therefore clearly conservative, as it was in the case of these sparrows, how can selection explain ‘progressive’ evolution, and the appearance of new types? Remember, this was a real and perplexing argument a little over a century ago, decades after Darwin’s approach had become widely accepted.
If you want to read Bumpus’ paper and explore his data, Clark University and UWC have put the material on line as a way of getting their students to study exactly what happened on that snowy night in 1898, and what it means for our understanding of natural selection.
If you want to know more about the way that 19th and early 20th century thinkers tried to find evidence for natural selection, I strongly recommend Jean Gayon’s book Darwinism’s Struggle for Survival: Heredity and the Hypothesis of Natural Selection (Cambridge). Declaration of interest: I translated it.
h/t Ben Goren for the pigeons.