Hen anatomy

Bowl at crop height

Hens share many of the organs we have, which perform many of the bodily processes we perform.  

However, there are some exceptions.

Teeth 

Mature hens do not have teeth like we do. They pick and swallow whole pieces of food, which is then ground up in one of their stomachs called the gizzard. 

Larynx  

Hens create sounds from much lower down in their airways. They have a syrinx at the base of their trachea (windpipe) where it splits to join each lung. They don't make noises using their larynx like we do. 

Air sacs 

This is a flight adaptation in birds. They have thin-walled little sacs of air that are attached to their lungs. The walls are so think they look like cling film. They use their air sacs to enable them to achieve continuous movement of air in the lungs so they can extract much more oxygen from the air than we can. This makes them far more efficient at breathing so they can fly for long distances, if they weren't hens of course. 

Hollow, aerated bones 

Their long bones are hollow e.g. femur, tibia, humerus, and are linked to their airsacs. This keeps their bones much lighter than ours, enabling them to generate lift easily and fly. 

Crop 

This is a thin-walled sac at the base of their neck. They use it to food storage temporarily. It is so thin that you can easily feel whether a bird has eaten grains, or has eaten pellets and had a drink, turning it into porridge consistency. Birds will stock up on food during the day and empty it gradually overnight. Therefore, healthy hens will go to bed with a full crop which should be empty, in a normal bird, by the morning. If your hen is not going to bed with a full crop or is waking up with a significant crop fill in the morning, she is likely to require veterinary attention. 

Two stomachs 

The first stomach is the proventriculus which can be likened to our stomach. It releases acids and enzymes to mix with the food to start the digestion process. 

The second stomach is a thickly muscled sac called the ventriculus or gizzard. This is where grit is stored to act as the hens "teeth" in griding the food up with the acid and enzymes for digestion. It is lined with a tough fibrous layer to protect the muscle from being damaged by the grit. 

Gut movement 

Birds are adapted to be light for flight. One of the heaviest parts of them is their gut due to all the food and water inside. Their intestines are much shorter than ours would be if we were the same size as a hen. Hens have adapted and developed a very clever way of moving the content of their intestines both forward, towards the vent, and backwards, towards their head (peristalsis and retroperistalsis). They do this to increase the time their food spends in the intestine, enabling them to extract more nutrition from food over a much shorted length of intestine. 

Caecae 

These are two blind-ending sacs at the end of the small intestine, where the colon is. They are the site for fermentation, where bacteria help with digestion and the formation of essential nutrients like fatty acids and vitamins. Urine is also concentrated in the caecae. 

Single ovary 

Although cockerels have two testes, hens only have one ovary and oviduct. This is found on their left hand side.  

Kidneys (no bladder) 

Hens have two, long lumpy kidneys that stretch all the way along from their last rib to their tail. They produce uric acid, which is a solid and insoluble, so hens don't have a bladder to store liquid urine like we do. Instead, the urates and excess water that leaves their kidneys goes back up the intestinal tract into their caecae to maximise water absorption. It is then excreted with the faeces as a white urate cap. If a bird has been drinking lots and doesn’t need the extra water, it will pass a very watery poo. This isn't actually diarrhoea, it's just because they don’t have a bladder so have to "pee" on their "poo".