My ideas of what exactly makes a species have evolved over time. If I think back to my school days I remember the standard definition of a species was an organism that could breed with one another and produce viable young.
But as time has gone on I've come to realize that the natural world doesn't fit in the neat little boxes that we try and assign for it. We, humanity, have a need and desire to categorize and compartmentalize. Animals, are in a constant (although almost unobservable) state of change, and what we see as species are merely snapshots along this journey, sometimes with a lot of separation, sometimes with a lot of overlap. The dizzying array of gulls is an excellent example. Hybridization is common, and some populations wax and wane depending on the current research and just who is determining what is a species.
This weekend my understanding of just what makes up a species had some more shaking up, as I did some reading in preparation for some summer bird observation.
I've been planning on getting a better handle on ringed plover. Specifically just what populations of the ringed plovers, Charadrius hiaticula (Common Ringed Plover) and Charadrius semipalmatus (Semipalmated Plover) are breeding in the area around Arctic Bay.
Conventional knowledge on ringed plovers on Baffin Island is that Common Ringed Plovers (Charadrius hiaticula) breed on the north half of the island, and Semipalmated Plovers (Charadrius semipalmatus) breed in the south half, with just a narrow area in the middle where both species can be found. However last year I found breeding Semipalmated Plovers near the old Nanisivik townsite, the most northernly known breeding population of Semipalmated Plovers (or so I've been told). In the course of trying to find out more information on the populations I came across a very intriguing paper on polymorphism in the two species of ringed plovers (N.G. Smith, 1968: Polymorphism in Ringed Plovers, Ibis Volume 111, Issue 2 page 177 to 188).
Although the paper is over forty years old (and I had to pay a ridiculous amount to get access to it) it contained some very interesting stuff. Basically, the author, who was researching gulls, came across populations of Semipalmated and Common Ringed Plovers in fjords near Clyde River and began paying careful attention to them.
To summarize what he found, both of the very similar plover species bred in the area, at the same time. Although pair bonding occurred at the same time, C. hiaticula arrived later, and pairs had already formed. The two species would nest in close proximity to each other, and seemed to have the same nesting requirements. Some mixed pairs occurred.
The results of the nesting startled me. In pairings where both male and female were phenotypically C. hiaticula the offspring were all characteristically C. hiaticula. However in mixed pairs the offspring did not show intermediate forms, but were either C. hiaticula or C. semipalmatus, with both types appearing in the same brood. Even more amazing 10 pairs of C. semipalmatus X C. semipalmatus also produced broods that contained both C. hiaticula and C. semipalmatus, suggesting that the gene trigger that produced the C. semipalmatus characteristics would be recessive.
I realize that this is but one study (from forty years ago) but it shows how animals don't fit into neat little boxes when it comes to what we understand to be species. It also shows just how little we know about the birds that breed up here, mostly because there is so little observation being done. And rather than answering questions I had, it raised a whole bunch more. For example, as Semipalmated Plovers migrate south through North America, while Common Ringed Plovers migrate through Greenland and then on to Europe and Africa what happens with the C. hiaticula chicks produced by C. semipalmatus parents. Do they leave their parents and follow other Common Ringed Plovers or do they head south in such small numbers that they aren't noticed? Is there unnoticed multiple mating going on, or are the mixed broods for Semipalmated pairs produced strictly by the pair?
And the biggest question for me still remains. What birds nest here? And how to they interact with each other?