The pupils of Springfield School, asks :-
We feed all sorts of food scraps to all sorts of birds that visit our school. How can a bird eat all these different types of food which are foreign to them?
James Briskie, an ornothologist at the University of Canterbury's Zoology Department, responded.
The types of food that birds eat can vary quite dramatically from one species to the next. Some birds have a very specialised diet. For example shags and pelicans feed exclusively on fish. Other birds are more generalised. A New Zealand robin feeds mostly on insects but it will eat berries from time to time. Bellbirds and tuis like nectar and honeydew, but will also eat insects and berries.
Gulls will eat almost anything. Such a strategy is called omnivory (omni is Latin for "all" and vor means "to devour"). Omnivorous animals, such as humans and gulls, readily exploit new food sources. You dont see shags begging for your left-over fish and chips but a gull certainly will.
The breadth of a bird's diet is typically reflected in the structure of its digestive system, starting with the mouth. Seed eaters (eg a goldfinch) have a short conical beak for cracking seeds. Insect eaters (eg a grey warbler) have a thin, forceps-like, beak for picking insects off leaves. Meat-eaters (eg a harrier hawk) have a sharp, hook-like, beak for ripping into flesh.
Birds that feed on fruit, meat and insects (all rich in nutrients) have short intestines, whereas those that feed on seeds need longer digestive times and therefore longer intestines. Some birds, like the hoatzin in the Amazon, even ferment leaves in their gut much like a cow.
Gull intestines are about average length for their body size, a reflection perhaps of their need to deal with a wide variety of food items of average digestability.