Lifestyles

Of a Feather: Bigger and smaller, but so similar

By BILL CHAISSON
By Bill Chaisson

Telling similar looking species apart can be difficult, but it is perhaps most difficult when the main difference between two species is that one is simply larger than the other, while they are otherwise similarly marked, or nearly so.

The words “greater” and “lesser” are often deployed in order to distinguish between these pairs, but occasionally the bestowers of bird names have been cleverer.

My favorite example of this came to mind the other day when a particularly large hairy woodpecker alighted on our suet feeder. As we all know, very small hairs (and feathers) are called “down,” so calling a miniature version of the hairy woodpecker, the downy woodpecker is better than average wordplay.

These are two very common woodpeckers with almost complete overlap in their distributions; both are likely to visit the same bird feeder. Their plumages are nearly identical; the downy has three dark bars or spots on the edges of its white outer tail feathers, while the hairy’s are entirely white. The downy has a conspicuous tuft of white feathers in front of the eye, while the hairy’s is less so.

But the most reliable fieldmark is the size of the bill. The hairy’s is not just larger, it is relatively larger, nearly as long as its head. Roger Tory Peterson referred to the downy’s bill as a “little bark sticker”; it is a mere stub of a beak.

Cultivars of plants are often much larger than the wild species from which they were derived. Everything is bigger, the flowers, the leaves, the whole plant is taller and the stems thicker. This occurs because of polyploidy. Normal plants and animals are diploid; there are two copies of every chromosome. In plants the chromosomes are identical in form in male and female plants; there are no X and Y sex chromosomes. Therefore, when various processes result in triploid and tetraploid plants, there are simply three or four copies of every gene to create that much more of all the proteins needed to make the plant. Hence the plant is simply bigger.

In animals, the presence of sex chromosomes complicates matters. Some triploid animals exist, but for some reason they are mostly lizards. They have all-female populations that reproduce by parthenogenesis, an asexual process. Researchers have crossed the triploid females with diploid males of a related whiptail species to produce tetraploid offspring that are fertile, but all of this is unknown in birds, which are descended from dinosaurs, not lizards.

In birds, the large/small species pair phenomenon has more than one explanation. In the hairy/downy pair it is not because they are closely related. Within their genus, they are quite far apart. Rather, it has been shown that the downies acquired several traits that caused them to mimic the appearance of hairies. At present it is not known why. Recent studies at feeders have shown that hairy woodpeckers are very aggressive, and it therefore might be advantageous for downies to be mistaken for hairies. But the matter is not settled.

What of the greater and lesser scaup, two common diving ducks that can be told apart only by head shape? The greater scaup is bigger, 18-20 inches versus 16-18 inches, and its bill and neck are proportionately larger and thicker. But these features are hard to gauge from a hundred yards away, which is a common distance over which to see ducks.

The lesser scaup has a straighter neck and a more squared-off head, with the highest point above the nape, toward the back of the head. The greater scaup’s head is more rounded. Unfortunately, this is only apparent when the birds are resting. When they are actively feeding and diving, the head shapes are nearly the same.

The color of the heads was once thought to be different, greenish in the greater and purplish in the lesser, but in fact both species’ heads can appear to be either color.

Unlike the woodpeckers, the scaups are each other’s closest relatives. But while the greater scaup has a Holarctic distribution, found in the Old and New World, the lesser scaup is found only in North America (Nearctic). And while the greater scaup regularly hybridizes with the tufted duck in Europe and the lesser scaup hybridizes with the ring-necked duck in North America, cross breeding between the scaups is unknown. In North America their ranges overlap broadly, and they are often sighted together, but they remain separate species.

In contrast, the ranges of the greater and lesser prairie chicken no longer overlap at all but did historically. They are not actually that different in size; the greater is only an inch longer, but it does have significantly longer wings and is heavier. When their ranges did overlap, they remained separate by habitat preferences and mating rituals, which are elaborate. The greater prairie chicken is found in areas with more rainfall on tall-grass prairie, while the lesser ranged further west into more arid grasslands. The two species are closely related, and some taxonomists believe the lesser to be a subspecies of the greater.

The greater and lesser sandhill cranes are always classified as subspecies, but the differences among populations are clinal, which means they change gradually across the species’ distribution. There are six subspecies altogether, some which might not be real, but merely transitions between greater differences.

The sandhill crane is distributed widely from north to south across North America, with the northern populations tending to be larger and the southern populations tending to be smaller. This is an example of Bergmann’s rule, which states that animals of larger size will tend to be found in colder environments because their surface-to-volume ratio is lower, allowing them to retain heat more efficiently.

In sum, species pairs must be evaluated one at a time. Looking alike is not a guarantee of relatedness. But there is always some adaptive reason for it. Otherwise, it would not persist.

Bill Chaisson has been a birdwatcher from the age of 10. He is a former managing editor of the Eagle Times and now works and lives in the town of Wilmot.

Avatar photo

As your daily newspaper, we are committed to providing you with important local news coverage for Sullivan County and the surrounding areas.