Passerine Anatomy: Ten Key Facts

What is a passerine?

Perching birds or songbirds belong to order Passeriformes, which makes up the largest taxonomic group of birds. Passeriforms make up nearly 60% of all birds with over 5,000 species belonging to this group (Fig 1).

Figure 1. Perching birds belong to order Passeriformes. Upper left: American robin (Turdus migratorius) by ibm4381; Upper right: Eastern bluebird (Sialia sialis) by audreyjm529; Lower left: House sparrow (Passer domesticus) by jfinnirwin; Lower right: Common raven (Corvus corax) by Frank Vassen Click to enlarge

Although many songbirds are free-ranging, passerines are also an important part of aviculture. Species such as canaries (Serenus canaria) and Society or Bengalese finches (Lonchura striata ) have been domesticated for centuries (Fig 2).

Figure 2. Passerine species kept in aviaries include the owl finch (Taeniopygia bichenovii) (upper left), red-headed finch (Amadina erythrocephala) (upper right); canary (Serinus canaria) (lower left); Gouldian finches (Chloebia gouldiae) (lower right) Click to enlarge

If you are comfortable with psittacine anatomy and physiology, then you are well on your way to understanding passerines. LafeberVet has listed ten interesting, clinically significant facts about passerine anatomy and physiology.

Physical description

All passeriformes share some common physical traits such as an anisodactyl foot (three toes forward, one toe facing back) and a narrow triangular tongue.

High metabolic rate

Basal metabolic rate is approximately 65% higher in passerines when compared to non-passerine birds. Body temperature also tends to be approximately 2 degrees higher when compared to larger birds (~ 107.6ºF or 42ºC).To meet their metabolic needs, passerines may need to eat up to 30% of their body weight (BW) daily. Compare this with the approximately 10% BW estimated for many psittacines.

Passerine sinuses play by their own rules

Birds possess only one true paranasal sinus: the infraorbital sinus. In most bird species, the paranasal sinuses communicate. Passerines are unique in that the right and left infraorbital sinuses are usually not connected. Therefore in passerine patients suspected to have sinusitis, collect samples from both sinuses when performing diagnostic tests.

Singing is for the boys

Passeriform song is induced by rising testosterone levels. Male songbirds produce a melodic song, while females tend to make a monotonous chirp or single note call. Passerine birds possess complex syringeal anatomy that affords them amazing singing ability. In fact, some birds can sing duets with themselves by using alternate bronchi. Visit the Cornell Lab of Ornithology for an auditory example of a “one bird duet”.

Figure 3. The wood thrush (Hylocichla mustelina) is a passerine species known for beautiful “one bird duets” Image by USFWS. Click to enlarge

Among companion birds, canaries are prized for their harmonious song. Be forewarned that most male canaries do not sing during molt, which can last 6-8 weeks. Older male canaries may also sing less frequently if testosterone levels fall.

Bright colors are for the boys too

As testosterone levels rise, so do the levels of carotenoids that give male passerines bright coloration on their feathers, beaks, and legs. Vivid reds, yellows, and oranges signal to the hen that the male bird is healthy and a potentially good mate.Rising testosterone levels can have a deleterious affect on immunocompetence in many species. The relationship between testosterone and carotenoids, a group of powerful antioxidants, may explain why the immune systems of these colorful songbirds can remain strong.

Cloacal promontory

The distal end of the ductus deferens is called the seminal glomus in many passerine species. During the reproductive season, the seminal glomus enlarges and pushes the cloaca caudally. This prominent, conical swelling is called the “cloacal promontory”. Wildlife biologists in the field use this physical finding to sex songbirds during the breeding season. The region is exposed by blowing feathers on the bird’s vent cranially. The cloacal promontory can also be identified during a veterinary exam, but of course during most of the year the vent region will appear unremarkable.

Renal lobes

The middle and caudal renal lobes or divisions are fused in the vast majority of passerines. Small species may drink up to 250-300 ml/kg BW daily, although some desert species like the zebra finch (Taeniopygia guttata) can go for days or weeks without drinking water.

Gastrointestinal tract

Gastrointestinal tract structure varies with the diet. The gastrointestinal tract shows a great degree of diversity among order Passeriformes. Variations in gastrointestinal (GIT) anatomy and physiology allow birds to fill a wide range of ecological niches and take advantage of a wide variety of foods. A large, muscular, grit-filled ventriculus or gizzard lined with a prominent cuticle or protective koilin layer is present in species that eat hard foods like granivores and insectivores, but not in nectarivores. The largest number of bird species are omnivores, and may be classified as facultative frugivore-granivores or facultative granivore-insectivores depending on the season. Birds that shift seasonally between hard and soft food normally have corresponding shifts in the amount of girt present as well as the structure of the GIT although the gut is most similar to that seen in grain or insect-eaters. Facultative granivore-insectivores have muscular gizzards with hard cuticles during the winter months when they mostly feed on seeds. Koilin is softened and there is little grit within the ventriculus during the summer months when birds eat soft insects.

Additional necropsy findings

The passerine spleen and ceca are small structures that are unlikely to be viewed by imaging, however it is important to know what to expect at necropsy:

1. • The passerine spleen is oblong not round as in psittacines.
   • Ceca, when present, are small and vestigial.

Passerines have seven air sacs

This final point may not be a pressing clinical issue, however no collection of passerine anatomy facts is complete without it:  The cranial thoracic air sacs of most passerine birds are fused to the clavicular air sac, giving these birds a total of seven air sacs instead of the nine seen in psittacines.

Summary

Basic comfort with psittacine anatomy and physiology, will hold the clinician is good stead when studying songbird anatomy. All birds possess a relatively high metabolic rate, however because of their small size the rate tends to quite high in passerine birds. This means that nutritional support must be particularly intensive to allow the debilitated songbird to gain, or even maintain, body weight in a clinical setting. Many other unique anatomic features of the passerine bird, such as the appearance of the spleen, ceca, or kidneys, are generally only pertinent during a gross necropsy.

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