Sunday, January 4, 2015

Why The Woodpecker Lost a Toe

I often get asked why the three-toed woodpeckers have only three toes. Here is my best attempt at explanation for why the woodpecker lost a toe (first taking a big step back). The zygodactyl foot of woodpeckers has two toes forward and two toes back (first and fourth toes). This arrangement is excellent for gripping branches and is found in other arboreal bird families such as parrots, cuckoos, and owls. This arrangement also helps woodpeckers to brace themselves on vertical tree limbs. This photo of a preening male Golden-fronted Woodpecker (Melanerpes aurifrons) from Yucatan, Mexico shows the zygodactyl arrangement (apologies for the motion blur).

This next photo of a female Red-rumped Woodpecker (Veniliornis kirkii) from Colombia shows how the zygodactyl arrangement is used for gripping. It is very rare to see a woodpecker positioned upside-down like this and that is because they have other climbing tools at their disposal.

This male Cuban Green Woodpecker (Xiphidiopicus percussus) shows a much more typical woodpecker posture. In addition to the four-toed grip of the feet, the stiffened,pointed central tail feathers brace against the trunk providing further support from below. You can also see one of the woodpeckers’ other amazing adaptations, the long tongue, that enables them to probe for wood-boring larvae.

The toes are not completely fixed in the position relative to the leg and have a little maneuverability that also helps as shown by these Great Slaty Woodpeckers (Mulleripicus pulverulentus) from Palawan in the Philippines.

The zygodactyl toe arrangement has such a great advantage for the woodpecker tree-climbing lifestyle that you might be surprised to learn that not all woodpeckers have four toes. Some woodpeckers have lost a toe along their evolutionary road, which begs the eternal question of just why did the woodpecker lose a toe?  You could perhaps divide up the woodpecker family in the following way:

A)  Small species with four toes and a rounded or square tail (not stiffened). This includes the piculets and the larger but mostly ground-dwelling wrynecks.
B)  Medium-sized species with four toes and a pointed, stiffened tail
C)  Medium-sized species with three toes and a pointed, stiffened tail
D)  Large species with four toes and a pointed, stiffened tail

The differences in the woodpecker family are fascinating. The differences boils down to how much they weigh (how they support that weight while conducting their head-banging lifestyle) and how they forage. Let us look at a few examples from each group to illustrate:

Group A) This female Rufous Piculet (Sasia abnormis) from Malaysia demonstrates the typical stance of the smallest woodpeckers, the piculets. The Piculets are so small that they don’t need to support their weight with their tails and often grip horizontal branches in the manner shown here.

Group A) This male Mottled Piculet (Picumnus nebulosus) from southern Brazil shows how piculets can grip vertical branches and trunks without using their tail as a brace.

Group D) Large woodpeckers need all the help they can get to support their weight and still be able to pound a tree trunk. They do this with a combination of the zygodactyl foot and their stiffened tail as shown by this male Scaly-bellied Woodpecker (Picus squamatus) from India.

Group D) Nonetheless, even among large woodpeckers there are some subtle differences. This photo composite shows two of the largest genera of woodpeckers, Dryocopus and Campephilus. The largest woodpeckers in the world, the Imperial Woodpecker and Ivory-billed Woodpecker (both now sadly believed to be extinct) belong to the genus Campephilus. Being so large, they have developed an extra trick to support their weight. These large woodpeckers use their four toes and their stiffened tail feathers but they also spread their trasometatarsus wide with the joint resting against the trunk as an extra support. In this photo composite, you can see this is the male Crimson-crested Woodpecker (Campephilus melanoleucos) on top and the male Powerful Woodpecker (Campephilus pollens) on the right (notice how the legs are angled wide of the bird’s body.  Although they are almost as large, Dryocopus woodpeckers don’t do this, having a more typical “straight” stance as shown by the female Lineated Woodpecker (Dryocopus lineatus) on the bottom of this photo composite.

These differences seem straightforward enough but it is amongst the medium-sized woodpeckers that things get really interesting. Let us first compare a coupe of similar sized woodpeckers.

Group B) This male Yellow-bellied Sapsucker (Sphyrapicus varius) is a typical medium sized woodpecker in terms of their stance—four-toed grip plus tail support.

Group C) These Pale-headed Woodpeckers (Gecinulus grantia) (male on right with pink crown spot) are slightly larger than the sapsucker above but apparently still small enough to have just three toes per foot as shown in these photos. This may have to do with the bamboo forests they call home or their style of foraging. The key thing is that at this size they are capable of supporting their weight with just three toes plus tail. The three-toed adaptation occurs in several genera and could have evolved more than once.

Group C) There is another group of medium-sized three-toed woodpeckers that have a particularly interesting foraging style, viz. the Black-backed, American Three-toed and Eurasian Three-toed Woodpeckers. I had often wondered what would cause their common ancestor to lose a toe. I only first started down the road of figuring our why when photographing them foraging. Normally, when one photographs a woodpecker feeding there are some shots where the head and neck are blurred that you throw away. Take a look at the blurred photo of a female Black-backed Woodpecker (Picoides arcticus) below and see if you can spot the difference that got me thinking… and reading!

What you may notice in that photo that it is not just the head that is blurred but also the wing tips! That observation is key. When these three woodpecker species forage they “scale” the bark of the tree away, often leaving a visible trace of their handiwork. To do this, they are striking the tree and flicking the bark away, effectively two different motions in one smooth stroke. The ingenious three-toed design functions a bit like a fulcrum with the strong backward-pointing toe effectively pushing the bird off the tree and thrusting them into the stroke with the whole body, followed by the flicking action of the bill on contact. This gives them a surprising amount of force with their deceptively soft tapping and facilitates bark removal by striking and flicking in the same stroke. Whereas other woodpeckers are mostly striking with their head and upper body, these three put their whole body into the stroke and the three-toed design “frees” them up to do just that. As a result the lower part of the body is also in motion during the stroke and the wing tips appear blurred.

This photo of a male Black-backed Woodpecker shows you how strong the rear toe is. As you can also see, they still use their tail as a prop and are similar in other anatomical aspects to their congenitors in Picoides. They also have reinforced skulls and super long tongues like other woodpeckers.

This photo of a male American Three-toed Woodpecker (Picoides dorsalis), shows the end of the sideways flicking motion and the heavily scaled tree trunk as a result. You can see some of the points of impact where the bill hit the trunk before the flick too.

The rear toe can move around to the side when the bird is moving or resting as this male American Three-toed Woodpecker shows. He is using his tongue after flicking a small area of bark clear—listening and probing for food.

So why did they lose a toe? All the better to whack trees with! (in some cases)

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