Saturday, November 3, 2012

The World of Nuthatches

Nuthatches (Sittidae) are a northern hemisphere family that are most commonly found in temperate forests. Of the 28 species of Nuthatch in the world, only two have ranges that extend south of the equator (Blue Nuthatch and Velvet-fronted Nuthatch, both of which occur in Southeast Asia and also on Sumatra and Java).  Only four species occur in the Nearctic, the remainder occurring in Eurasia (one, the Algerian Nuthatch, occurs in northernmost Africa but only within the Palearctic Ream and not within the Afrotropical Realm). As you can imagine, that means that a large percentage of the world’s nuthatches occur in Asia and this family is thought to be “Old World” in origin. Of the 28 species, 14 occur in the Oriental Ream and 10 in the Palearctic Realm of which 6 in Asia, hence there are no less 20 species of nuthatch in Asia. The Himalayan region, with a total of eight species, has the greatest number of species of nuthatches of any area, although the ranges of all eight do not overlap.

Although not a particularly large family, and not very diverse in their size, plumage colouration and morphology; wherever they occur, the most distinctive feature of nuthatches is their behaviour, especially the way they climb both upward and downward on branches. This immediately distinguishes them from all other “climbing” birds such as woodpeckers and treecreeper, that only move upward on tree trunks and branches (or at least position their bodies upward), although woodpecker and treecreepers, like nuthatches, will forage along the underside of branches. The difference is in part anatomical: since the majority of the “climbers” including most woodpeckers and treecreepers, have pointed stiffened tail feathers that act as a brace when climbing. Nuthatches on the other hand rely on their exceptionally strong toes to grip to the bark, whether going up or down. There is also a behavioural difference in that nuthatches have evolved a unique “stride” to accompany their anatomical adaptations, i.e. when walking downwards they are able to place one foot directly below the other and hence walk in a straight line downward. This technique is presumably important for balance and for maximizing the effectiveness of their toe grip. These anatomical and behavioural characteristics are illustrated by four photos below: a Sulphur-billed Nuthatch of the Philippines (note large toes and ability to “hang”), followed by a Blue Nuthatch of Southeast Asia (note downward motion), a Pygmy Nuthatch of North and Central America (note downward-pointing posture with the legs aligned in a straight line one below the other and hence ability to hang on while walking down the tree trunk) and a female White-breasted Nuthatch of North America (note how the large hallux toes clings to bark).

One of the most widespread of the nuthatches is the Eurasian Nuthatch. The first photo is from South Korea, but this species occurs across temperate Asia and Europe and even a tiny amount into the northernmost Africa. The first photo shows their remarkable ability to seemingly hang off the side of tree trunks using their strong toes. The second photo shows the western European subspecies for comparison (note the difference in underpart colouration).  The second photo also shows a behaviour of food storage – ramming a food item into bark crevices either to assist in cracking them open or for storage – a habitat that presumably earned this family their name.

Of course the short, straight, chisel-like bills of nuthatches are designed for probing into bark but several species show a willingness to forage on other substrates (as opposed to just probing into tree bark). This Eurasian Nuthatch is picking tasty morsels off a rocky outcrop. Other species such as the two “Rock Nuthatch” species (Eastern and Western) are more specialized in this technique.

Some of the Old World nuthatches are very similar in appearance and require a little care in identification. This is a Chestnut-vented Nuthatch (photo taken in China) that is similar in appearance to Eurasian Nuthatch (the distinctive undertail coverts not visible in this photo).

Another similar species, whose range partially overlaps with similar species like the Chestnut-bellied Nuthatch is the White-tailed Nuthatch shown here. This species has a predominantly Himalayan distribution and I took this photograph in northern India.The white base to the tail feathers that gives this species their name are seldom visible in normal field conditions.

The identification of nuthatches is simplified by the fact that in most parts of the world there are no more than two or three species that coexist. This is presumably due to similarities in foraging niche. Nonetheless, there are certainly differences in niche that reduce competition where overlap occurs. Where I live in North America, for example, the ranges of the Red-breasted and White-breasted Nuthatches overlap partially and you do sometimes see them together at a feeder in winter. Nonetheless, the former is primarily a bird of boreal/coniferous forest and the latter primarily of deciduous forest. Here is a photo of a Red-breasted Nuthatch on a pine tree and a White-breasted Nuthatch on a riverside deciduous tree, both taken in southeastern Manitoba, Canada. The White-breasted Nuthatch in this photo is a male and, if you like, try comparing with the photo of a female White-breasted nuthatch above to see if you can spot the differences.

In Central Asia, the White-cheeked Nuthatch is remarkably similar to the White-breasted Nuthatch of North America, although the more eastern subspecies, like this one shown from western China, have more extensive rufous below. If you can imagine this bird without the rufous underparts, you’ll understand what I mean.

This Yunnan Nuthatch, endemic to southwest China, is a small species, but this series of three photos illustrate their ability to probe into large seed cones to retrieve large seeds. 

North American observers will recognise this behaviour – in this case, the diminutive Pygmy Nuthatch illustrates the same technique…

One of the most distinctive species is the Velvet-fronted Nuthatch shown here. This is a widespread Southeast Asian species (this photo taken in India). The Sulphur-billed Nuthatch shown above was once considered conspecific with this species, though it differs in having a yellow as opposed to a red bill.  This photo further illustrates the utility of the probe-feeding strategy (in this case for prying open a tree wound) so typical of this fascinating family.

So ends this glimpse into the life of the Sittidae…

1 comment:

Nature Blog Network Birdwatching Blogs - BlogCatalog Blog Directory Fatbirder's Top 500 Birding Websites