Bird Anatomy
Birdwatching Fundamentals
Course 1 · Chapter 3 · Bird Anatomy: Naming the Parts
Birdwatching Fundamentals 2 covered the kit for seeing birds. This chapter covers what you are looking at. Field guides and experienced birders describe a bird using a small vocabulary of body parts, so learning those words lets you describe a sighting precisely and, in Birdwatching Fundamentals 4, identify it. The chapter covers the bill, feathers, wings, tail, legs and feet, and a few facts about the body underneath.
The Parts You Will See Named
Wikipedia's Bird anatomy article labels a figure of a yellow-wattled lapwing with these parts: beak, head, iris, pupil, mantle, coverts, scapulars, tertials, rump, primaries, vent, thigh, tarsus, feet, belly, flanks, breast, throat, wattle and eyestripe. The article labels them but does not describe each one in its text, so the one-line notes below are my own general knowledge unless marked otherwise (check).
Head
Beak, eye (iris and pupil), throat and the eyestripe, a line running through the eye.
Upperparts
Mantle (the back area), scapulars, coverts, tertials, primaries and rump (check each against a field guide diagram).
Underparts
Throat, breast, belly, flanks, vent and thigh. The tarsus is the part of the leg you see above the toes.
The Bill
The article says the beak "is used for eating and for preening, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young." It has upper and lower mandibles covered by a keratinised layer called the rhamphotheca. Birds lack teeth, and many chicks have an "egg tooth" for hatching. For the watcher, the practical point is that bill shape is a clue to diet and behaviour (my own general knowledge; check).
Feathers
Wikipedia's Bird feather article names two basic types: vaned feathers (also called contour or pennaceous feathers) that cover the exterior of the body, and down feathers beneath them. It also describes:
| Kind | What the article says |
|---|---|
| Flight feathers | Remiges (wing) and rectrices (tail), "the most important feathers for flight" |
| Down | Has no barbicels, so its barbules stay loose and trap air, which insulates |
| Filoplumes | Rarer, hairlike feathers beside pennaceous feathers, often hidden; "one or two" per pennaceous feather at least on the head, neck and trunk; absent in ratites |
| Bristles | Stiff, tapering feathers with "a large rachis but few barbs"; rictal bristles grow around the eyes and bill |
A feather's structure, per the article, is a central shaft (the rachis) with barbs branching from it, barbules branching from the barbs, and tiny hooks called barbicels linking neighbouring barbules. At the base the rachis forms a hollow quill, the calamus, set in a follicle in the skin. Feathers wear out and are replaced periodically by moulting, and new feathers grow from the same follicles. The article gave no moulting schedule in the part I read.
Wings and Tail
The Bird anatomy article says the bones in the wing are "extremely light" and that the digits are "fused together". Its wing section is only a link to a separate article, which I did not read, so details such as secondaries are not covered here (check). Tail feathers attach to the pygostyle, and muscles control the movement of the tail and the spreading of the rectrices.
Legs and Feet
The article says leg bones are the heaviest, "contributing to a low center of gravity", and lists five foot arrangements:
| Type | Toe arrangement | Found in |
|---|---|---|
| Anisodactyl | Three toes forward, one back | Most common arrangement |
| Zygodactyl | Two forward, two back | Parrots and woodpeckers |
| Heterodactyl | Digits three and four forward, one and two back | Trogons only |
| Syndactyl | Second and third toes fused | Kingfishers and related Coraciiformes |
| Pamprodactyl | All four toes may point forward | Swifts |
Keratin scales cover mainly the toes and tarsi, and the scaly covering of the foot is called the podotheca.
The Body Underneath
- "A bird's skeleton accounts for only about 5% of its total body weight", and the skull "usually weighs about 1%" of it.
- Birds are the only living vertebrates with "a keeled breastbone", which anchors the flight muscles.
- Pectoral muscles "make up about 15–25% of a flighted bird's body weight."
Not Yet Verified, and Sources
- Sources. Bird anatomy and Bird feather: the first 100,000 characters of about 190,000 and 198,000 respectively were read, so later sections were not.
- The date from Birdwatching Fundamentals 1 and 2. Settled as 1889 using library catalogue records; one encyclopaedia page gives 1890.
- Not in the articles as read. What each labelled body part is, the bill-shape clue, the knee remark, and the 10% to 15% estimate (all marked check above). The semiplume feather type was not in the text I read.
- Not covered. Secondaries and wing shape, and the soft-part colours used in identification.
Exercises
Name the foot type for each description: (a) three toes forward, one back; (b) two forward, two back; (c) all four toes can point forward; (d) second and third toes fused. Give one group of birds the chapter links to each type.
A bird weighs 100 g. Using the percentages in this chapter, work out the skeleton weight, the skull weight, and the range of pectoral muscle weight. Then estimate the plumage weight using the "two or three times the skeleton" rule.
Without looking back, list as many labelled parts of the bird as you can from the "Parts You Will See Named" list, then write a two-sentence description of a bird in your garden or a local park using at least five of them.
Quick Reference
- Bill: upper and lower mandibles, covered by the rhamphotheca; no teeth
- Feathers: vaned (contour) over down; flight feathers are remiges and rectrices
- Feather structure: rachis, barbs, barbules, barbicels, calamus
- Feet: anisodactyl, zygodactyl, heterodactyl, syndactyl, pamprodactyl
- Body: skeleton about 5%, skull about 1%, pectoral muscles 15–25% of body weight
- Check: the 10% to 15% plumage estimate and the plain-language notes on body parts