Waterfowl Anatomy & Physiology: A Dozen Key Facts

What is a waterfowl?

Waterfowl belong to Order Anseriformes. The nearest taxonomic relatives of the waterfowl are Ciconiiformes such as the heron and bittern. Virtually all anseriforms belong to family Anatidae, which consists of ducks, geese, and swans. Anhimidae is a small family of South American goose-sized birds called screamers. Screamers posses long, thick legs and unwebbed feet. The magpie goose (family Anseranatidae) is another unique waterfowl species native to Australia and New Guinea.

If you are comfortable with psittacine anatomy and physiology, then many features of waterfowls will be familiar. LafeberVet has listed twelve interesting and clinically significant facts about waterfowl anatomy & physiology.

Manual restraint-related facts

1. Bill
   It is helpful to understand the anseriform bill before restraining the animal. The presence of serrated lamellae (Fig 1) as well as a double row of overlapping bristles at the lateral margin of the tongue, allows the bill to serve as a straining organ. Compared to psittacines, a bite from a waterfowl is not a major concern but the lamellae do allow the bird to pinch hard.
   

   Figure 1. Serrated lamellae (arrow) as well as a double row of overlapping bristles at the lateral margin of the tongue, allows the waterfowl bill to serve as a straining organ. The lamellae can also deliver a substantial pinch. Also note the “frontal knob”, a fleshy protuberance covered by epidermis. Image by aussiegall. Click image to enlarge.

   Cutaneous mechanoreceptors, or Herbst’s corpuscles, are present on the bill. A shield-like region of thickened epidermis at the tip of the upper beak called the nail contains hundreds of well-developed sensory nerve endings making this area particularly sensitive to touch. The nail is designed to grasp small, slippery objects (Fig 2).

   

   Figure 2. The nail is a shield-like region of thickened epidermis at the tip of the upper beak that is particularly sensitive to touch. The nail is used to grasp small, slippery objects. Image by BotheredByBees.

2. Wings
   Restraint of waterfowl is relatively straightforward, however swans and geese can use their strong wings defensively. Some birds are capable of inflicting injury through strong wing flapping, particularly the spur-winged goose (Plectropterus gambensis).Swans may also attack people who approach their nests too closely (Fig 3). An adult swan can seriously injure children, and there are records of swans knocking boaters off of jet skis. Most recently there is a report of an adult male drowning after being knocked out of his kayak by a swan.

Figure 3. Swans are territorial and can be quite aggressive when threatened. Image by ellenm Click image to enlarge

Physical examination-related facts

3. Plumage
   Waterfowl possess a dense coat of insulating down and a thick layer of feathers. Clinicians that are used to easily parting psittacine feathers will be surprised by dense waterfowl plumage. In many regions, particularly over the coelom, it is difficult—if not impossible—to visualize the skin. Healthy waterfowl carefully preen and oil their plumage using secretions from the feather-tufted preen glands for waterproofing. Water should normally roll off feathers.
4. Foot/leg
   The bottoms of the feet should have small patterns of scale with no balding areas or scabs. The toenails are short but normally very sharp and can cause painful scratches.Dabbling ducks (genus Anas) like mallards, pintails, and teals surface feed or tip up in shallow water. Their legs are relatively centered on the body allowing birds to take off of the water’s surface at an acute angle. Most diving ducks (genus Aythya) like canvasback and lesser scaup feed by diving into deep water. Legs are located more caudally on the body.
5. Sexual dimorphism
   Ducks are usually sexually dimorphic. Males have iridescent plumage and distinct patterns except for the Pekin duck, American black duck (Anas rubripes), and Muscovy duck (Cairina moschata). Like most avian species, juvenile birds tend to have female plumage. Juvenile males achieve male plumage during puberty. Most ducks reach sexual maturity at one year of age. Geese become sexually mature at two years and swans at five years of age.
6. Copulatory organ
   Most male waterfowl (Box 1) possess an intromittent or erectile phallus, which is a unique corkscrew structure covered with keratinized papillae (Fig 4). The phallus sits on the floor of the proctodeum or the distal-most component of the cloaca. Gentle eversion of the cloaca, performed by applying gentle pressure on each side of the cloaca, can allow visualization of the phallus, even in newly hatched young. Waterfowl possess ten to eleven primary feathers. Most free-ranging species molt all flight feathers at once, becoming flightless for 3 to 6 weeks. Most members of subfamily Anatinae (e.g. shelducks, dabbling ducks, perching ducks, and diving ducks) molt twice yearly. These species go through a breeding or nuptial molt and a non-breeding molt, also known as the winter or eclipse molt.When their breeding efforts are complete, the males of most duck species in the Northern Hemisphere molt from their brightly colored nuptial plumage to a dull, cryptic plumage. Their brilliance is dimmed—they go into “eclipse”. The eclipse plumage is generally retained for a brief time—in many species for as little as 1 to 3 months. Although some species retain this plumage until the following spring.Unlike most waterfowl, screamers experience a gradual molt and they do not pass through an annual flightless period.

Figure 4. Phallus of a mallard duck (Anas platyrhynchos). Bar measure equals 2 cm. Photograph by Patricia Brennan. Click image to enlarge.

7. Precocial chicks
   Waterfowl chicks are precocial, or more specifically nidifugous which means that youngsters can eat, swim, and dive almost from hatch. Precocial chicks have a large yolk sac, which makes up 12% to 25% of body weight, and enough body fat that they are able to survive for several days without eating if need be.

Anesthesia-related facts

8. Tracheal modifications
   One unique feature of the waterfowl respiratory tract is the syringeal bulla, an enlargement of the trachea at the level of the syrinx. An asymetrical, completely or partially ossified syringeal bulla is present of the left side of trachea in male ducks of many species (Fig 5). Bilateral syringeal bullae are present in both male and female swans (Fig 6). The syringeal is used to produce the loud, booming sounds often considered synonymous with waterfowl.
   

   Figure 5. The syringeal bulla in the duck is an asymmetrical enlargement of the trachea. Copyright 2012 Jakowski, Richard M., DVM, PhD. From Jakowski RM. OCW Veterinary Gross Pathology Image Collection. Published in Tufts OpenCourseWare (2005-2012). [Retrieved 12/17/2012]. Reproduced with permission of the author and publisher. Licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License. Click image to enlarge.

   

   Figure 6. Bilateral syringeal bulla from a male trumpeter swan (Cygnus buccinator). Photograph by H.J. Barnes. Click image to enlarge

   Swans possess a particularly long trachea. In fact, the trachea of the trumpeter swan (Cygnus buccinator) extends into the sternum and turns on itself (Fig 7 and Fig 8). Additionally, the ruddy duck (Oxyura jamaicensis) has an inflatable tracheal sac.

   

   Figure 7. Trachea of the trumpeter swan (Cygnus buccinator): 1) Glottis, 2) keel, the right side was removed to show the embedded tracheal loops, 3) caudal loop of the trachea, 4) dorsal loop of trachea just before exiting the keel, 5) syrinx with syringeal bulla, cut at the level of the primary bronchi. Photograph by James Smallwood. Click image to enlarge.

   

   Figure 8. Trachea of the tundra swan (Cygnus columbianus): 1) Glottis, 2) keel, the right side was removed to show the embedded tracheal loops, 3) caudal loop of the trachea, 5) syrinx with bilateral syringeal bulla, cut at the level of the primary bronchi. Note this trachea lacks the dorsal loop present in the trumpeter swan. Photograph by James Smallwood. Click image to enlarge.

9. Diving reflex
   General anesthesia can trigger a diving reflex in waterfowl, and resultant apnea and bradycardia so be prepared to monitor the patient carefully and to perform positive pressure ventilation.
10. Pectoral muscle mass
    Free-ranging waterfowl, particularly geese and swans, are heavy bodied birds that may be prone to hyperthermia due to their dense plumage and large size. Conventional wisdom has also advised against maintaining waterfowl in dorsal recumbency as they may lead to difficulty ventilating the patient. A recent study in raptors found positioning in sternal recumbency resulted in the greatest lung and air-sac volumes and lowest lung density compared with positioning birds in right lateral and dorsal recumbency.

Necropsy/survey radiographs-related facts

11. Gastrointestinal tract
    The crop of ducks and geese is a simple fusiform widening that usually is not palpable on physical examination. Also, to aid in the digestion of fibrous plant material, geese and swans possess well-developed ceca. A gallbladder is also present in ducks and geese.
12. Lymphatic system
    Lymphatic hearts are lymph node-like structures found in the caudal coelom of waterfowl.

References