Birds' Feet; amazing feats

I've been wanting to do this post for a while, but there's a bit involved in it so I've deferred it a couple of times. However this winter isn't currently encouraging me to get out and about as much as I might like, so this seems like a good time to do it. I hope I can encourage you to stay with me for a story that I find fascinating; that might just be me of course but I hope that at least some of you will join me. There's some unfamiliar terminology but while I won't ignore it (recognising that some readers might be interested in the terms) I'll stick to familiar language as far as possible. 

You might like to tackle this one in a couple of sessions! 

It's worth noting that all birds (unlike most mammals and limbed reptiles) walk on their toes, rather than the soles of their feet. And no, I realise that it doesn't look like that, does it? But, birds are not at all like mammals or reptiles, including in their feet. In fact I should probably put feet into inverted commas each time I use it for birds, but that would be tedious and probably even more confusing.

The 'shin' at the end of the red arrow is no such thing; it is the tarsometatarsus,
formed entirely of bones from the lower foot, so effectively it is part of the foot. 
The joint above it, just below the feathers, is really an ankle. (Above that the yellow 
arrow indicates the tibiotarsus, where the tibia - or shin bone - is fused with yet more 
foot bones, from the upper foot.) This 'extended foot' gives extra leverage for taking 
off for instance, but is the reason for saying that birds walk on their toes. 
And if this is no interest to you, please don't stop reading yet; it's just background.
And the lovely model is a Great Egret Ardea alba, in Canberra.

Birds of course inherited their feet, like everything else initially, from their dinosaur ancestors. In particular these ancestors were the theropods, one of three major dinosaur groups, which included some favourites like the huge Tyrannosaurus rex and the much smaller (and much maligned) velociraptors of movie infamy. One relevant common characteristic was that they all walked on their hindlegs while the smaller front limbs were adapted for other purposes (including being repurposed as wings in due course, but that's another story). They also all had four toes on their walking feet, one backward and the other three facing forward. Most of them still do in fact, but we tend to refer to them as 'birds' these days. 

Forest Raven Corvus tasmanicus, Tasmania. Some 70% of living birds still retain this
foot structure, mostly because it is an efficient way of grasping perches. Needless to say
it didn't evolve for perching on powerlines, but it works perfectly well there too!
For the record this is known as anisodactyly, but I promise that this will be the last
time I use the term. (Probably.)

Superb Lyrebird Menura novaehollandiae, Beowa NP, south coast NSW.
Again for the record, toe number 1 is the hind one; the outer front one on each foot is #4,
then #3 and #2 going to the centre of the body.

Here's a sketch in case you want to refer back to it. I've borrowed these sketches from an excellent 2016 article by Lisa Nupen in African Birdlife. 

 

All the passerines - the most modern Order containing the majority of living birds, often referred to as 'songbirds' or 'perching birds', though neither is very satisfactory - have this 'standard' foot arrangement. Here are some other examples (in each case you might get a better idea by clicking on the photo to enlarge it). 

Noisy Friarbird Philemon corniculatus, Canberra.

Spotted Pardalote Pardalotus punctatus, Canberra, with nesting material.

 

Varied Sittella Daphoenositta chrysoptera, Canberra. This is a flexible foot structure
and can be adapted to clinging to rough surfaces as well as perching on branches.

Various non-passerines (ie all other bird Orders) which perch also have the 'standard' foot structure too, including pigeons.   

Chestnut-quilled Rock-Pigeon Petrophassa rufipennis, Kakadu NP, Northern Territory.
They have retained the classic 'perching foot' even though they apparently never perch in
trees like other pigeons but live among the sandstone gorges and ridges; perhaps it's simply 
that the feet are of no disadvantage here so no need for changes.

However in the 150 million years of bird evolution (give or take) there have inevitably been adaptations to specific requirements; let's stay with the 'standard' foot structure for now, before looking at some entirely different options. Several other disparate major bird groups, such as hawks and eagles plus the unrelated falcons, penguins, ducks, shearwaters and fowl relatives (pheasants, quail, peafowl, partridges etc) also have kept this structure, but adapted it for a range of purposes.

Birds like lyrebirds and the megapodes (mound builders) move great quantities of ground litter with their feet - either looking for food (lyrebirds) or creating nesting mounds (megapodes). Unsurprisingly their feet (and legs) are particularly large and strong relative to their body size.

Superb Lyrebird again, also at Beowa NP. 

Orange-footed Scrubfowl Megapodius reinwardt, Darwin.
This is not a large bird, about chook-size ('chicken' for my
non-Australian readers), but its incubating mounds are enormous (see below). 

Orange-footed Scrubfowl mound, Darwin.

Birds of prey - especially the families that include falcons and eagles/hawks - utilise their '3-1' feet for catching, crushing and carrying often large prey. 

Wedge-tailed Eagle Aquila audax, Sturt NP, far north-western NSW.
The big eagles have hugely powerful clawed feet.

Australian Hobby Falco longipennis with Diamond Dove lunch, Karumba, north Queensland. 

Black Bittern Botaurus flavicollis, Kakadu NP, Northern Territory. The bitterns are a
group of herons which live among reeds and typically have long slender toes for 
climbing reed stems.

Comb-crested Jacana Irediparra gallinacea, again in Kakadu NP. The jacanas form a 
family of eight species of waders, found across the world's tropics, which have enormously
extended toes which spread the bird's weight and enable it to walk on lily pads. 

Birds that swim have, unsurprisingly, developed adaptations to their feet to assist with this energy-consuming lifestyle. Some groups, such as ducks, gulls and penguins, have retained the standard '3 toes forward, 1 back' structure, but added skin webs between the front three toes for added efficiency; the technical term is palmate. 

Freckled Duck Stictonetta naevosa, Canberra.

Fairy Penguin Eudyptula minor, Canberra Zoo (sorry, I try to only use 'wild' photos).

Silver Gull Chroicocephalus novaehollandiae, south coast NSW.

This webbing need not be complete to be beneficial. Magpie Geese Anseranas semipalmata, form an entire family of primitive water birds, not true geese or ducks. Their webbing is partial, probably better adapted to walking in mud than swimming.

This partial webbing is reflected in their species name; semipalmata  means 'half-webbed'.

These partial webs can also be seen in other birds which do not swim but walk in water on soft substrates, which would be much harder if they sank in with each step. Herons and some sandpipers are examples of this. 

White-faced Heron Egretta novaehollandiae, Canberra.

 

Semipalmated Sandpiper Calidris pusilla, Galápagos.

Other unrelated waterbird groups, including grebes and coots, have instead developed lobes of skin around each toe, but the toes are free of each other. 

Eurasian Coot Fulica atra, Canberra. This photo is one that would benefit from
being clicked to expand.

Australasian Grebe Tachybaptus novaehollandiae, Territory Wildlife Park, Darwin.
Aquariums can be very useful! The toes are separate again, but they are only spread out
when the grebe is pushing through the water. Even the rear toe has some lobing. 

Several other waterbird groups (including cormorants, darters, pelicans and gannets) have gone 'all the way' here, by bringing the fourth toe around to the side, and extending the webbing to it too. This form is named, descriptively, as totipalmate

Blue-footed Booby Sula nebouxii, a member of the gannet family, Galápagos.
They dive into the water from on high, and pursue fish under the water.

Great Cormorant Phalacrocorax carbo, River Murray.
(Another one where a 'click and expand' would make it clearer.)

Australian Pelican Pelecanus conspicillatus.

Another variation on the basic structure is known as syndactyly, where the two inner front toes (#2 and #3) are fused by skin for most of their length. It is suggested that by forcing these toes to act together it makes for a stronger grip, but I don't think anyone really knows. It can be seen in wood hoopoes, hornbills and kingfishers.

Again my thanks to Lisa Nupen (see above).

Green Wood Hoopoe Phoeniculus purpureus, Lake Nkuru NP, Kenya.
Sydactyly is seen most strongly in wood hoopoes and hornbills; this is the 
best I can offer of these, but click on the photo and you'll see the fused toes quite well.

Blue-eared Kingfisher Alcedo meninting, Sabah, Malaysian Borneo.
Again, enlarging both these photos will enable you to better see the toes.

Red-backed Kingfisher Todiramphus pyrrhopygius, central Australia.

There is another widespread perching foot form which has evolved separately in various non-passerine bird groups. This one (zygodactyly, again just for the record) features two toes opposite the other two, such that toe #4 has joined #1 at the back, with of course #2 and #3 still facing forward. This also gives an excellent grip, it seems. There's no suggestion that it's better than anisodactyly (sorry to mention it again!), just a different answer to the same question. Nonetheless it presumably derived from the 3-1 ancestral foot, so must have been of some advantage - there's still much that we don't understand. 

Here's Lisa Nupen's sketch of it.

It has arisen in groups as diverse and familiar as parrots/cockatoos, owls, woodpeckers, cuckoos, turacos and toucans. In parrots it is a valuable tool for manipulating food items, but that seems less relevant to other groups which have the arrangement. Here are some examples of all those groups displaying their toe set-ups. 

Golden-shouldered Parrot Psephotellus chrysopterygius, Artemis Station, Cape York Peninsula,
Queensland. This is a highly Endangered species.

Female Red-winged Parrot Aprosmictus erythropterus, Boodjamulla NP, north-west Queensland.
She is demonstrating the efficacy of the grip.

Yellow-tailed Black-Cockatoo Zanda funerea, manipulating a banksia cone with his foot, 
rotating it to allow the bill to access the seeds. (This was happening alongside our balcony;
the black bill tells us that it's a male.)

Crested Coua Coua cristata, a cuckoo, Ankarana NP, Madagascar. 

 

Pacific Koel Eudynamys orientalis, another cuckoo, south coast NSW.

Yellow-throated Toucan Ramphastos ambiguus, north-eastern Andes, Ecuador.

White-bellied Go-Away Bird Crinifer leucogaster, a turaco, Buffalo Springs NR, central Kenya.

Little Woodpecker Veniliornis passerinus, Pantanal, south-western Brazil.

Pale-billed Woodpecker Campephilus guatemalensis, Carara NP, Costa Rica.
I have included this because it doesn't seem to be the normal woodpecker foot structure (see
previous photo). A few species of zygodactyl birds can swing their back toes forward, 
but not woodpeckers as far as I know, and in any case all four toes are pointing forward here. 
I assume that here it's to do with the broad vertical tree trunk, but I've not seen this mentioned. 
Any input welcomed! 

Pearl-spotted Owlet Glaucidium perlatum, Tarangire NP, Tanzania.
This little owl is a 'hawk owl', one of the two families of living owls,
and by far the most species-rich with 230 species.

Eastern Barn Owl Tyto javanica, central Australia. The barn owls
comprise the other owl family, but with only 20 species across the world.
I think it's interesting that both families display the same foot structure, though
they apparently separated some 40-50 million years ago. Either this was the structure
of their ancient common ancestor, or they both independently evolved it later.
The first seems to me to be more likely.

And just before we leave the 'two forward, two back' foot model, there is a rather unexpected variation on the zygodactyl, which appears in just one Order of birds, the trogons, which arose some 50 million years ago. They are colourful long-tailed birds of tropical forests across the world, except for Australia/New Guinea. In the trogons the inner front toe (#2) has swung to the back, rather than the outer #4 of the others we've just been discussing.

Again my thanks to Lisa Nupen (see above) for this illustration of trogon-specific heterodactyly. 
And in case you're wondering how we know which toe was originally at the front, 
the four toes of all birds with the original structure conveniently have unique 
 and fixed numbers of bones, from two to five.

We wouldn't think this would make much difference from zygodactyl feet, but it seems to, and not as we might expect. They can perch OK, but walking along a branch is almost beyond them, and to turn they must half-flap to get around. I'm not going to attempt an explanation of this, but I thought you should know... 

Northern Violaceous Trogon Trogon caligatus, south-western dry coastal forest, Ecuador.

The final variation among four-toed birds is shared by two very different orders of birds, but only by some members of them. These are the African mousebirds, and the widespread swifts. Here all four toes are now on the front, to give improved 'hanging and gripping'. Swifts typically nest and roost on cliff or cave faces, and mousebirds scramble constantly through foliage, often feeding hanging upside down. I can't offer a photo of swift feet (nor do I ever expect to be able to I'm afraid!), but I can show you a mousebird's feet.

Speckled Mousebird Colius striatus, southern Kenya.

To finish I'm going to share some birds which have done away with a toe (or even two in one case). In general birds which do a lot of walking across the ground tend not to have retained the hind toe, which I imagine can interfere with the rocking movement of walking. As we might expect this process didn't happen overnight but began with reduced hind toes, which also tended to move up the back of the leg to clear the ground, as it were. Examples include gulls, sandpipers and the pheasant family (including junglefowl and their domestic descendants, peafowl, francolins, partridges, quail etc). 

Lava Gull Leucophaeus fuliginosus, Galápagos; the raised small toe #1 can be seen
clearly here. While not hard to see in the central Galápagos, this is authoritatively
described as the world's rarest gull, with less than 600 individuals remaining (by the 
Cornell Laboratory's Birds of the World).

Common Sandpiper Actitis hypoleucos, Darwin. (Again, click to see it properly, on the right leg.)

Cape Spurfowl Pternistis capensis, Cape Town, in the pheasant family. This group of 
francolins is even named for the prominent spur on the back of the tarsometatarsus 
(see first photo in this post). In some family members, including peafowl and junglefowl,
the spurs are used in territorial or courtship combat.

Brown Quail Synoicus ypsilophorus, north coast NSW. This is my best offering of
the shrunken hind toe of a quail - I haven't had much luck with photographing them.
Again, click and enlarge to see it.

As mentioned earlier, other ground-living birds have lost the back toe entirely to become tridactyl (ie three-toed). These include nearly all plovers, plus bustards, stone curlews, emus, rheas and cassowaries. Additionally there are outliers among sandpipers, woodpeckers and jacamars which have shed a toe among generally four-toed groups.

The Sanderling Calidris alba (here in the Galápagos) is alone among sandpipers
in completely lacking a hind toe.

Crowned Lapwing Vanellus coronatus, central Kenya. 
It is typical of most plovers in having only three toes.

 

Bush Stone-curlew Burhinus grallarius, tropical coastal Queensland, showing its lack of a hind toe.
(This was alongside our cabin, between us and the four-wheel drive.)

Buff-crested Bustard Lophotis gindiana, central Kenya.
(Yet again you might like to click and enlarge to check the detail - or lack of it.)

None of my photos of Australian Bustards Ardeotis australis actually show the feet -
they're all hidden in grass. However these footprints in gravel soil in the Great
Sandy Desert of Western Australia make the three-toed point clearly.

 

Southern Cassowary Casuarius casuarius, Atherton Tableland, north Queensland.
You can't actually see where the back toe isn't (!) but the massive running (and kicking)
front toes are worth admiring. Their grouping, the flightless ratites and South American
tinamous, are the most ancient of living bird groups.

Emus Dromaius novaehollandiae, Gundabooka NP, northern NSW. These also run,
very effectively indeed, on their powerful three toes.

And finally, the ostriches, the two species of which have perhaps the most ancient lineage of all living birds. They are the only ones to have gone one step further in toe reduction, sprinting at up to 70kph on just two toes.

Common Ostrich Struthio camelus, Lake Nakuru NP, Kenya.
Chicks plus a parent's huge two-toed foot.

If you're still with me at the end of this foot marathon, thank you. I hope it's been worth the journey; at worst I've enjoyed it, both searching through my photo files for appropriate illustrations, and learning more about the topic than I already knew. However it would be even better if you've enjoyed it too!

NEXT POSTING THURSDAY 2 JULY

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