Evolution and a Sighting on the Quarry

Fairly early in the morning, with the angle of sunlight low and coming from behind it, I saw an unusual bird swimming on the quarry pond. It was chunky, low in the water, with a long, slim upturned bill. I thought I was looking at a loon.

To recreate that first impression, here's a photograph of it converted to black and white. Note particularly the silhouette, proportions, and high waterline indicating heaviness.

Juvenile Red-throated Loon

Compare it to this photograph of a loon that visited the quarry earlier in the year.

Female Common Merganser

My camera and I circled around the quarry rim to a position where over-the-shoulder sun gave a brighter portrait of the bird's plumage and, importantly, it's bill. We were looking at a merganser and not a loon.

Another view of the immature loon

Plumage aside, the telltale distinction between mergansers and loons is the bill. Both birds dive for fish. Loons accomplish this with a pointed, conically shaped bill. Merganser bills are serrated and hooked at the tip, for better gripping of their prey. 

In looking into these characteristics I was puzzled that mergansers are described in the 'duck section' of the field guide while loons are grouped with grebes in a different part of the book. At first glance mergansers and loons should be taxonomic siblings. And mergansers don't resemble those other members of the duck family Anatidae with wide flat bills. What places them in that genealogy? 

Wait! One defining characteristic of the Anatidae  is a hard 'nail' at the end of the bill and ridges or serrations along its sides. Dabbling ducks use variations of these features to harvest aquatic vegetation. Over time mergansers have evolved their own configuration to specialize in capturing fish. 

I turned to local naturalist Chris Leahy for help in understanding this. His response is worth including in its entirety and reading with care. "Welcome to the fascinating world of convergent evolution, the phenomenon in which species that are unrelated phylogenetically, evolve similar physical characteristics. A frequently cited example is auks and penguins; these families are not related but because they evolved under similar conditions, they developed a wide range of physical similarities, including appearance, behavioral habits such s diving; adaptations to cold climates and frigid waters, colonial breeding habits,. food preferences, etc., etc. 

"Your loon/merganser example is similar. That mergansers (as well as other diving ducks, e.g. scoters, eiders, etc.) evolved among the waterfowl at some distance from the loons and grebes is well established. The similarity you (rightly) perceive results from both groups evolving general body shapes, habitats, habits (diving for fish), etc. under similar ecological/climatic conditions. However on a closer inspection you would discover many anatomical distinctions, such as bill form and texture, form and location on the body of feet and legs, plumage textures, etc. that belie superficial similarities. When you think about the great diversity of aquatic habitats containing a wealth of food sources, it is not surprising that similar bird types evolved from different lineages."

During its stay here this merganser has taken comfort in the companionship of the preeminent local Anatidae representative, the Mallard.

Initially it stayed as far as possible from human visitors, especially admirers with cameras. By the third day it was following the mallards' preening pattern on an island close to passersby. It seemed to be an example of convergent interests and temperaments reaching back past divergent evolutionary paths. I've only seen loons preen while afloat.