Avian Physiology Part II: Feeding and Digestion Sonia M. Hernandez

What GI characteristics are uniquely avian? No teeth Little saliva, few taste buds Mammals: chew first, chemistry second Birds: chemistry first, grinding second

Into the mouth and… Down the mucous-gland lined esophagus – Very distensible – Can produce “milk”

Crop Expanded esophagus Stores, moistens and softens food Regulates flow Sometimes specialized

“Stomach” Two-chambered Proventriculus  glandular – Gastric juices, peptic enzymes – Very large in fish-eaters, carnivores Ventriculus  muscular Shrike can digest mouse in3 hrs!

Intestine Length varies depending on diet – Short  fruit, meat, insects – Long  seeds, plants, fish Ceca – Bacteria aid in digestion

Intestinal function Extremely efficient Rapid transit time Combine active and passive transport of nutrients into enterocytes

Assimilation Raptors 66-88% of energy Herbivores 60-70% of young plants Can change seasonally – American robins improve lipid assimilation when eating berries in fall

Eating for… Birds eat to fulfill calorie requirements Exceptions: – Willow ptarmigan  nitrogen and phosphorous – White-crowned sparrow  amino acids

Sucrose Songbirds cannot eat – No sucrase – Results in diarrhea Hummingbirds – Assimilate 95-99% of nectar energy from sucrose nectar – Also absorb glucose

Energy Balance Ideally a balance between between intake and expenditure Before migration, need to eat more to store as fat reserves

Foraging time Amount of time feeding decreases with increasing nectar from each flower

Fat reserves Typically kept at a minimum – Songbirds, 10% for winter needs – Bulbuls, 5%, to make it through the night

Fat reserves and Hoarding Large birds can survive longer without eating 10g warbler at 1-9C will die in 1 day without food Male emperor penguin fast for days and lose 45% body mass

Fat reserves and Hoarding Acorn wood pecker, shrikes, Crested tits

Water economy High metabolisms require a lot of H2O – Particularly in warm climates – Especially bc of evaporative losses Water acquired from food

Evaporative water loss at non-stressful ambient temps decreasing sharply with increasing size of small birds. metabolic water production (cross-hatching) offsets this loss High metabolism=more metabolic H2O than other verts

Drinking water The drier the environment, the more need to visit surface water regularly

Excretion Kidneys – Excrete uric acid – Twice as much nitrogen as urea – Can be excreted in semisolid form – Comparison: Birds use 1 ml of water to excrete 370 ml of nitrogen Mammals need 20 ml – Further concentrate uric acid in cloaca through reabsorption of water

Too much water… Hummingbirds consume huge quantities of water – Intestines can allow water to pass through without processing by kidneys – Have highest evaporative loss

Can’t concentrate… Loops of Henle are very short