Our backyard bird feeder has had some interesting visitors in Spring of 2024. Among the common visitors were a pair of Tufted Titmice, Goldfinches, a pair of Mourning Doves, Cardinals, several Song Sparrows, several Common Whitethroat Sparrows, a flock of Brown-headed Cowbirds, a White Breasted Nuthatch, and perhaps just passing through to more northern climes: a Baltimore Oriole, and several Rose Breasted Grosbeaks. Three species of woodpecker were common visitors. The large Red Bellied Woodpecker and two smaller woodpeckers, the Hairy and Downy woodpeckers. These two latter woodpecker species were unusual in that they look almost identical, only differing in size. This is unusual in nature…different species most often look different. Similar looking species are most often are very closely related.
THE WOODPECKERS
One of these, the more common Downy Woodpecker, (Picoides pubsecense) is about the size of a large sparrow, while its look-alike, the Hairy Woodpecker (Leuconotopicus villosus) is closer in size to a Cardinal or Robin. These two distinct species puzzled me, as well as other observers, because they are similar in appearance, are found in almost identical ranges over North America, and occupy very similar habitats. In the natural order of things a species is an interbreeding group of similar appearing organisms which interbreed to produce viable young. They are genetically almost identical and thus have similar morphology. Different species should “look different” and those differences are generally the result of their adaptation to some specific ecological “niche” in which they are best adapted. One must wonder why has Mother Nature generated two near identical (excepting for size), yet distinct species inhabiting similar habitats?
The Downy Woodpecker is about 6 inches long, with a 10 in wingspan, and weighs between 3/4 of an ounce to as much as one ounce. It has a white breast and a typical woodpecker black and white patterned back. Males are distinguished by a red spot on the back of their head. (See data from: Shenandoah National Park Svs).
The Hairy Woodpecker is a medium sized woodpecker of about nine (9) inches long with a wingspan of @ 15 in and weighs in at about 2.5 ounces, or about two and a half times that of its presumable close relative. The distinct size differences around a backyard feeder make it easy to determine which is which. In the field the fact that the bill of the Hairy Woodpecker is about as long as its head is wide, while that of the downy is shorter than its head-width help to make a species determination. There are also several relatively minor and difficult to discern differences in color patterns.
These two woodpeckers, are often seen at the same bird-feeder, they both inhabit the local woodlands, and a literature review of their national range maps indicates that they have almost identical ranges, Both occupy almost all of North America, except where trees and woodlands are absent. The larger Hairy woodpecker’s range includes a bit farther north in Canada and Alaska, and it ranges a bit further south into mountains of central America, as far south as Panama. While the Downy is confined to the forested areas of most of North America. I did consider mass vs surface area factor, such as the fact that the larger bird would be better adapted to a colder climate than the smaller one. But a closer examination of the range maps tended to obviate that reasoning. The Hairy woodpecker range does extend relatively further north—but also further south.
The Hairy Woodpecker’s diet consist of more than 75% larvae of wood boring beetles, bark beetles, ants, moth pupae, insect cocoons, bees, wasps, caterpillars, spiders millipedes and lesser numbers of crickets and grasshoppers
The natural prey diet of Downy woodpeckers is similar with more than 75% is insects and grubs found under or in tree bark, on plant stems and in tree galls. The diet of the the two species are almost identical. So why are there two size ranges?
WHAT IS A SPECIES?
A “species” is defined in biology is an interbreeding group of morphologically and genetically similar individuals which can produce viable, and fertile offspring. Most often (and in obviously in paleontology) scientists often have only the morphology of specimens as a method to establish a “new or different species” from another. In paleontology, scientists have only skeletal or shell remains(or other preservable hard parts or evidences) and thus are confined to base their species determinations on strictly morphological similarity.
Paleontologists thus often have to decide if similar “appearing” species are indeed distinct “species”. Scientists who operate in the field of science nomenclature and who make these distinctions are often divided onto two classes themselves, often being grouped as either “lumpers” or “splitters”. The former are more likely to combine similar specimens into one species, and the latter tend to use even minor variations as a reason to create a “new” species. In more modern times genetic analysis of DNA of living species has often provided data to confirm or deny these species relationships.
Species may evolve from one species into another as a result of geographic separation or other factors which act to prevent interbreeding of the pool of genetically similar organisms (species) and thus over time genetic isolation and changes occur resulting in morphological variations. Thus on the physical margins of the gene pool, or when geological or other phenomena separate species so they can not continue to interbreed and exchange gene material—a new species may come into existence.
WHY THEIR MORPHOLOGY IS SO SIMILAR
I was puzzled by the fact that these two very similar appearing woodpecker species, with almost identical diets and which occupy overlapping ranges and almost identical habitats—as far as is known. Based on observations of the species appearance, one should conclude they were very likely closely related species. A first assumption I made this morphological similarity was that this phenomenon was a manifestation of the process of “speciation” or the formation of a new species.
I reasoned, that perhaps, the size differences (six inches long for the Downy and closer to nine inches long for the Hairy) were related to differences in exploitative behavior. Since both species have a similar diet, foraging among tree trunks and branches for grubs, caterpillars, and wood boring insects for their caloric intake. This led to my original hypothesis: the Downy was likely adapted to exploiting the more numerous smaller branches and limbs, while the Hairy exploited larger branches tree trunks and perhaps favored more mature forests.
ADVANTAGE/DISADVANTAGE OF SIZE?
One might reason that the smaller, lighter Downy had more “recently” evolved, possibly as it adapted to exploit the more prevalent in modern times second or third growth forests. Thus, this closely related smaller species was better adapted to exploit grubs and boring insects in the younger, smaller diameter branches of trees and perhaps also more likely to find appropriate sized nesting sites in a less mature forests, or human managed forests with fewer old, dead or dying trees of late maturity. This was an example of “speciation in process”. The process of a new species evolving from an older species in response to alterations in environment-such as biological, environmental and physical changes.
On the other hand, the Hairy was better adapted to forage for bugs and wood boring insects on older, more mature and often more common larger diameter trunks, boles and branches of trees in more mature perhaps climax forests—which in present times are more restricted in range. Thus, I reasoned, perhaps we are seeing the development of a new species better adapted to the “altered, less mature more likely managed forests” and suburban wooded environments of more modern times.
Woodpeckers often feed clinging precariously to a near vertical tree trunk or branch. Their feet, armed with sharp claws are well adapted for this purpose. Most species have four toes with two forward and two rearward toes which enable a secure grasping grip on rough tree bark. This secure footing coupled with its rigid tail feathers, which are used to press into the tree or limb at an angle, and in this way act as strong brace against their feet. The tail brace insures a secure perch for feeding, pecking holes to excavate grubs, wood borers and larvae and for excavating nest holes in trees.
In this position, with their sharp pointed beak, they can actively extract boring insects from tough bark, excavate nest holes, and —in the face of a threat—sidle horizontally left of right around the circumference of the trunk or limb, thus placing the mass of the branch or tree trunk they are exploiting between them and a threat from a predator, such as owls, hawks or even arboreal snakes. It is not uncommon to see woodpeckers sidling horizontally around the circumference of a branch to keep a threat such as a human observer or predator at bay.
I reasoned, that the size variation between the Hairy (larger) and Downy (smaller) versions of these two closely related species (I assumed) were possibly an adaptation of one of these two species to changes in forest maturity, assuming less mature forests would have greater opportunity for smaller woodpecker species while more mature climax forests might favor the larger Hairy species. Thus the reduction in area of old growth, climax forests might have favored the development of smaller woodpecker species better able to exploit smaller diameter limbs, branches and tree trunks.
The larger Hairy woodpecker would be less likely to exploit smaller branches for food, which would not offer adequate protection from predators, while the smaller Downy could effectively do so. In addition, a similar reason may be proposed for finding appropriate nest hole sites for the two species. The smaller Downy might have an advantage in having more potential areas for food exploitation, and for nest sites in less mature forests.
My hypothesis was that what we are observing in these two species of almost identical appearance, except for size, was the process of evolutionary adaptation of an earlier, larger species (Hairy)—which had adapted to the old growth primeval mature, climax forests of North America, was evolving into a more recent variant species (the Downy) better adapted to the more recent forests of North America…where over the last several hundred years mature old growth climax forests have been cut down giving way to second and third growth forests and urbanized environments. Perhaps the Downy was an example of “speciation” or the origin of a new species more closely adapted to life in the present day forests.
NEW DNA EVIDENCE UPENDS A HAPPY HYPOTHESIS
Then an uninvited fact came to my knowledge and obviated my hypothesis. In 2015 biologists studying the genetics of these two woodpeckers concluded that the Downy and Hairy Woodpeckers —were NOT closely related at all! In fact, they were so different genetically that they would most likely be, in time, assigned to completely different genera. This threw a monkey wrench into the workings of my, “speciation hypothesis. So what is happening? Perhaps these new data on woodpecker genetics reveal an even more interesting story.
A NEW HYPOTHESIS: MIMICRY A FORM OF CONVERGENT EVOLUTION
There are many examples in nature of convergent evolution. A process in which different species, genera, even classes and orders of animals evolve similar morphology, structures, or color patterns in response to predation, to a changing environment, to competition, or adaptation to similar habitats. One classical example is this form of evolution is that of fossil ichthyosaurs and modern marine dolphins. Ichthyosaurs which lived in the Mesozoic Era (200 Mya) and the modern day Dolphin are an example of convergent evolution. Both species were adapted to similar marine habitats and method of predation. Reconstructions of fossil ichthyosaurs indicate that they would have looked very similar to living dolphins. Both evolved long toothed jaws, similar shaped flippers and shaped tails. Their morphology was similar because they inhabited similar environments and had a similar niche in their environment. There are several forms of convergent evolution.
MIMICRY: OR “COPYCAT” LIFE INSURANCE.
Mimicry is a special form of convergent evolution, in which a species evolves a morphology and appearance similar to another usually unrelated species. In this form of convergent evolution copying the appearance of another species provides a level of survival advantage for the mimic.
LOOKING TOUGH OR DANGEROUS MAY SAVE YOUR LIFE
In one form of mimicry, a species with no chemical defense mechanism evolves to converge on the appearance of another species which has developed a poison or unpalatable chemical defense system. One well-documented example of this form of convergent evolution is that of the harmless King Snake which has evolved a color pattern which resembles the poisonous Coral Snake. This form of mimicry is called Batesian Mimicry. (Though recent investigations reveal a more complex relationship which includes several look alike species which resemble each other..not all of which are poisonous)
Or in another similar case, the Viceroy butterfly evolved to closely resemble the Monarch butterfly. The assumed advantage for the Viceroy is that by looking like the mildly poisonous or foul tasting Monarch it gains a survival advantage. The Viceroy improves its survivability by being less likely to be attacked if it looked like the (unpalatable ) Monarch. But recent experimental accounts reveal that both species are either poisonous or unpalatable. A more recent explanation is that the two species appear to be mimicking each other. In this form of mimicry both species gain a survival advantage. Predators are more likely (and quickly) to learn to avoid the general orange and black color patterns displayed by Viceroys and Monarchs (both unpalatable) than they would if these two species each had to depend only on their own species deaths or unpalatability by predation to develop a warning to predators. Thus this mutual form of mimicry is an advantage to both species.
In the case of the Hairy and Downy Woodpeckers we may have an exceptional example of convergent evolution. In the same environment or ecological niche two different species have evolved to have almost exactly identical colors, male /female identity markings, black and white ladder striped dorsal patterns and other anatomical features. Why? We must assume that these patterns, colors, etc. must have evolved as a result of the fact that they somehow provide some form of adaptive advantage to the species.
MOST WOODPECKER SPECIES LOOK ALIKE, WHY?
Given this we might wonder, why are almost all woodpeckers black and white? Why do most have ladder-like black and white dorsal patterns, almost all are dark dorsally and have white underparts, almost all have some form of red-colored patches on their heads.
Protective coloration is often claimed as the reason. Specifically a form of coloration called disruptive coloration—patterns which help to break up the shape of the animal (woodpecker) in the eye of a predator. In the environment of tree trunks and tree limbs this contrasting “color” pattern, though making the species more visible to us, tends to hide the bird in plain sight of predators by disrupting the predator eye in such as way that the outline of the bird is obscured.
Take for instance a man or woman dressed all in black, at night against a dark background, this person would be nearly invisible. But in a forest during the day or an open field, black would do nothing to conceal the outline of this human. However, suppose this person wore a black costume with one white leg and a wide white sash across its breast. In the dark one would see the white parts clearly, but the one white leg and sash would not signify “human” to an observer. In the light of day, the same disruptive patterning would also serve to confuse the eye of the beholder, making the identifiable “external form” of the costume wearer as “human” less likely to be identified rapidly, and more difficult to identify as human. This effect would remain were the costume wearer to move into a wide diversity of background colors. And also any dark patches in the background would tend to greatly enhance the camouflage effect.
This form of camouflage known as “disruptive coloration” may be the underlying reason that Hairy and Downy woodpeckers who inhabit almost identical niches in the forest…have near identical color pattens. If one ignores their differences in size which can be ascribed to exploiting different parts of the forest tree -trunk-branch and limb continuum, but their almost identical camouflage patterns are likely a result of very effective protective disruptive coloration pattern that probably protects them from predation. This pattern appears to have some selected for advantage in the North American forests where they are found.
PROTECTION IN A GANG: MULLERIAN MIMICRY
But this does not seem to explain why so many woodpeckers species have very similar markings and feather patterns. Perhaps this phenomenon is a fine example of Mullerian mimicry. With many woodpecker species having similar patterns, predators may quickly learn to avoid attacks on species which have black and white patterns, ladder-like black and white back patterns, a red patch on the head, etc. But why?
Perhaps this “similarity phenomenon” among woodpeckers is the result of mimicry. The larger woodpecker species such as the Pileated, Redheaded, and Red Bellied woodpeckers are well protected with long, sharp pointed bills, and powerful neck muscles able to powerfully drive their sharp beaks. Their bill or beak is covered with durable keratin, has a sharp chisel point and is driven by powerful neck muscles which in total effect, can be a effective protective weapon that may dissuade predators from attack.
Could it be that by advertising “I am a woodpecker with a dangerous weapon on my nose” warns predators away. Woodpeckers as a group may be an example of converging evolution resulting in similar feather patterns — i.e. they mimic each other—because these patterns provide a survival advantage. Looking much like a long billed Hairy, Red-bellied or Pileated woodpecker might just save a smaller woodpecker from attack—just enough times to make its similar plumage an inheritable advantage. Then it takes only long periods of time—geologic time—thousands of years and generations to result in similar feather patterns and colors in the woodpecker gang.
Thus it now seems that the Hairy and Downy woodpeckers have slowly evolved from very different unrelated species to look like each other for several reasons. Woodpecker colors and feather patterns must provide a level of protection from predation, and as well, these particular patterns are especially effective in the North American woodlands that both inhabit. The differences in their size may be simply their more specific adaptation to different sizes of tree limbs and branches which they exploit for food and nest sites in their native forests.
These musings are those of a curious mind, and seem to lead to more questions rather than certainty. This is a good and natural process. One is always fascinated by the complexity and questions wonderful Mother Nature awards us with. May we all wise up and protect her better than we do now.
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