Bird Digestive System

The caeca (also spelled ceca) are a pair of blindsacs or lateral dilatations of the gut, marking the beginning of the rectum.  They are used as an aid in the digestive system of some birds to help the enzymatic breakdown of materials like cellulose.  When the caeca are large the rectum is shut off from the ileum or small intestine by a valvular sphincter, which allows the faecal matter to ascend from the rectum into the caeca, but prevents it from passing back into the ileum. The caeca vary extremely in size in the different groups of Birds - they attain their greatest size in those that are herbivorous, are small or hardly functional in most that live on animal food, and are altogether absent in fruit eaters and grain eaters. There are, however, so many exceptions to this broad generalisation, that an enumeration is advisable, especially since a certain taxonomic value cannot be denied to these organs.

It is highly probable that originally all Birds possessed caeca, and that, according to the diet, these were either further developed or reduced in size or even lost ultimately. Hence the mere presence of caeca in a bird is of less taxonomic value than their state of development; they are either functional, or without function; their absence is only the last step of their degeneration.

1. The caeca are large and of great functional importance in:
  • Struthio,
  • Rhea,
  • Apteryx,
  • Crypturi,
  • Gallinae,
  • Pteroclidae,
  • Grallae,
  • and Anseres.
The abovementioned birds are chiefly herbivorous.  Also the caeca are such in many worm-eating Limicolae, for instance in:
  • Avocet,
  • Lapwing,
  • Ringed Plover,
  • Oedicnemus,
  • Thinocorys,
  • Attagis,
  • and the Corncrake.
The caeca are also large and important in owls, and in the following birds who are strictly insectivorous:

2. The caeca are distinctly functional, but comparatively short, in:

  • Casuaris,
  • Dromaeus,
  • Grus,
  • Turnix,
  • many Anatidae (vegetable-eaters with a great predilection for animal food),
  • Limicolae
  • Rallidae,
  • Numenius,
  • Totanus,
  • Gallinago,
  • Chionis,
  • Porphyrio,
  • Porzana;
  • The piscivorous Spheniscidae,
  • Pelicanus,
  • Podicipes,
  • Uria,
  • Colymbus;
  • Merops,
  • and Phoenicopterus.

3. The caeca are quite degenerated and functionless, being either:

(a) reduced to small wartlike or vermiform appendages, as in some of the:
  • Spheniscidae,
  • Herodii,
  • Pelargi,
  • Steganopodes,
  • Laridae,
  • Strepsilas,
  • Limosa,
  • Scolopax,
  • Parra,
  • Rhinochetus,
  • many Columbae,
  • Accipitres,
  • and Passeres;
(b) they are entirely absent, as in many of the:
  • Columbae,
  • Psittaci,
  • Musophaga,
  • Corythaix,
  • Pici,
  • Alcedinidae,
  • Bucerotidae,
  • Upupidae,
  • Colius,
  • Cypselidae,
  • and Trochilidae.

4. Sometimes one caecum remains in a rudimentary condition and the other one has disappeared. This is the rule in almost all Herodii and in Procellaria, but occasionally met with in Steganopodes, Podicipes, Strepsilas, and in Atrichia.

The greatest development of the caeca occurs in Struthio, Rhea, Tinamus, and Meleagris, their aggregate volume equalling or even surpassing that of the rest of the intestinal canal, the caeca in these cases, especially in Ratitae, showing numerous transverse constrictions and sacculations, which increase the absorbing surface.

A certain correlation exists between the caeca and the length and width of the rectum.

The examples above seem to show that caeca are not required for the digestion of meat, fruit, and grain. Fish-eating Ducks have considerably shorter caeca than their strictly vegetarian relations.  The same remark applies to those Waders which live upon molluscs and other soft-bodied invertebrates.

The presence or absence of the caeca being thus explained by the food, a clue will occasionally be afforded to the systematic position of birds in which they appear against reasonable expectation.  It is clear that a change of diet may be accomplished in a much shorter time than it takes to modify the various digestive organs. For instance, the exclusive meat-diet of the Birds-of-Prey has reduced their caeca to mere rudiments, and it is more than improbable that the insectivorous habits of many of the smaller Falconidae will ever redevelop these organs, especially since these birds throw out the indigestible parts in pellets. Owls now cannot be distinguished from Diurnal Birds-of-Prey by their diet - they possess large caeca, and cannot therefore be derived from the Accipitres, which have lost them, nor is it probable that Owls and Accipitres came from one common stock and are collateral branched, because in this case both would be of equal age, and we should have to assume that the meat-diet had in one branch suppressed and in the other branch preserved or even increased the caeca. We can only conclude that the Owls are descendants of a stock of birds which, like the Nightjars, lived on chitinous insects (Beetles, Moths), and that they, like Podargus, as shown by its predilection for mice, comparatively recently took to the flesh of vertebrates.

As may be expected, the members of any large and much diversified group of birds, like Waders, Pigeons, Spheniscidae, and, others, have caeca in various stages of development.


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