Swarms

Gulls at McBeth Point, Lake Winnipeg

For most of the past month I seem to have been on the road in various parts of central Canada. The other morning, as we drove just north of Lake Huron near St. Joseph Island, we were struck by the abundance of Sandhill Cranes. Every kilometre we would see these magnificent birds fly past, in groups of two to five. They all seemed to be headed northward, probably leaving safe roosts along the lake to forage in the farmland and woods.

Since I was in the passenger seat and had nothing better to do, I began to look up Sandhill Cranes on my phone. I discovered that they are widespread creatures, occurring across much of North America and part of Siberia. And they really are abundant; although we tend to see them in small numbers in most of Canada, they can occur in huge flocks in certain parts of the migration season. So I guess it isn’t really surprising that I was seeing so many in Ontario, after observing family groups in central Manitoba last month.

Sandhill Cranes in the Grand Rapids Uplands of Manitoba, August, 2012

It is wonderful for the mind to be free to wander, so the Sandhill Crane got me interested in its larger relative, the elegant white and black Whooping Crane (the former is Grus canadensis, the latter, Grus americana). Why is the  Whooping Crane one of the poster children of endangered species, while Sandhill Cranes are common?

I had assumed that this was because Whooping Cranes had been so ruthlessly hunted through the nineteenth century, while presumably their Sandhill cousins had been better at avoiding human hunters. But this turned out to be an inaccurate perception. Certainly the cranes, like other large birds, were hunted indiscriminately by our meat-hungry ancestors. But apparently Whooping Cranes were always rare, and there may have been only 10,000 or so of them even before Europeans arrived here. They were, perhaps, a species that was already on the brink of natural extinction. An endangered species-in-waiting, if you like.

Considering such things intuitively, it is easy for us to understand that we could, through steady hunting pressure and destruction of natural habitats, drive a species of 10,000 down to a “rump” population that, at its lowest, consisted of just 15 adult Whooping Cranes in 1938. And it is also (relatively) easy to understand that the Sandhill, with its far larger populations split into family groups and flocks across North America, has not suffered the same fate. But how could it be that we managed to drive an even more abundant species to complete extinction, and in a relatively short time period?

Of course, I am now thinking about the Passenger Pigeon (Ectopistes migratorius).  Prior to European colonization, it is estimated that there were 3 to 5 billion of these birds, and that they made up 25 to 40 percent of all land birds in the US! How could something once so outrageously abundant possibly be gone?

The answer comes from behaviour. The Passenger Pigeon existed only in immense flocks that moved from place to place across this continent, stripping bare the fields and woodlands as they went. This strategy apparently served the pigeons well in pre-settlement times, but it made them a perfect target for market meat hunters. A flock hundreds of kilometres long and including upwards of a billion birds makes for a “can’t miss” target, and to a single viewer it appears to be a nearly infinite resource (does this sort of attitude seem familiar when considering some of the resources we are exploiting nowadays?).

But Passenger Pigeons could only survive in those big flocks. At some point in their decline there may well have been only one large flock in existence, and once that was hunted the remaining groups of stragglers were doomed. A swarming species cannot survive if there are no longer enough creatures to swarm.

Canada geese in the Grand Rapids Uplands of Manitoba, August, 2012

What is the moral that paleontologists can draw from this? Those of us who study ancient creatures like to put forward ideas about how those creatures lived and why they became extinct. Swarming might have been critical to some organisms, but perhaps we just don’t recognize it.

Some fossils occur in very large numbers, but how can we actually demonstrate that the animals lived in swarms? I think that I can suggest jellyfish swarms when I see hugely abundant fossils at a single horizon in an ancient tidal channel (as I did last month), and  deposits such as dinosaur bone beds may provide evidence of mass mortality of large groups. But clusters of fossils can occur for a great variety of reasons, and processes such as time-averaging are not the friend of those trying to tease out the signals of ancient ecology.

It is infinitely easier to watch the living cranes . . . or the humans . . .

Strange ritualized swarming behaviour observed in Homo sapiens, Palace of Versailles, France

© Graham Young, 2012