Animals eat different things. Every kid knows about herbivores vs. carnivores. Strangely, the other type of diet variation—diet breadth—is much less generally known. Generalists have broad diets, being able to eat a wide variety of things, while specialists eat only a few types of items. Anteaters and hummingbirds are specialists; the seagull who flew off with your lunch is a generalist.
Being a generalist gives a species a lot of advantages, especially in unpredictable environments. The more you can eat, the less likely you are to run out of food. An anteater without ants will starve, but a seagull without fish can eat crabs, or carrion, or Cheetos.
Generalists tend to be the species that are good at invading new habitats, because they are flexible in what they eat. They also tend to be the species that do well in urban environments—essentially a “new” environment, from the long perspective of evolution. Raccoons are generalists. Pigeons are exceptional generalists, not only able to subsist on trash, but able to raise their chicks on trash too—by feeding the chicks a protein-rich “pigeon milk” that the parents produce. (Most other birds have to feed their chicks insects, which may be difficult to find in the concrete jungle.)
Given all of these benefits, why do specialists exist at all? Why don’t the generalists outcompete them and fill the world entirely with flexible, omnivorous crows and gulls?
Because there are some serious downsides to that diet flexibility. The better you are at eating everything, the worse you are at eating any one particular thing. In scientific terms, the “handling time” of your food items goes up: it takes you longer to 1) decide if something is food, and 2) figure out how to eat that thing.
An oystercatcher, specialist in opening and eating shellfish, doesn’t have to spend a lot of time wondering if shellfish are food, and can open them and eat them quickly.
A gull isn’t so efficient.
You can see the advantage of specialists in their specialized morphology. An anteater’s long sticky tongue and hefty digging claws makes him excellent at breaking into anthills and consuming their residents. A hummingbird’s long bill, curled and forked tongue, and ability to hover suits him perfectly for retrieving nectar from flowers.
In any shellfish-eating race, the oystercatcher would beat the gull every time. Which is okay: the gull can eat something else.
Specialists succeed by being excellent at one or a few things. Generalists succeed by being okay at a lot of things.
This can involve learning: as you encounter new foods and struggle with how to eat them, you discover new strategies.
For a bird without a specialized shellfish-opening bill, shellfish locked in their hard shells are a particularly difficult puzzle. You can carry them around all you like, but if you can’t get inside, you aren’t getting food.
Being a dietary generalist may be one driving force behind intelligence. The need to constantly solve food-related puzzles, to be creative and flexible, requires brain power. Some of the most striking feats of intelligence in animals involve using tools to get food: dolphins use sponges, crows use modified leaves, chimpanzees use sticks. Social learning, a prerequisite to what we call culture in humans, shows up again and again in animals solving food problems and then copying each other’s solutions.
Sometimes, as in the cases of tool use, these solutions looks very impressive. Other times, the solution just looks like an animal doing something it was really never built to do.
Take these gulls, who have oh-so-creatively decided to imitate swallows in order to eat the tiny insects swarming on the mud flat. Never mind that their bills are the wrong shape, and they aren’t built to fly low to the ground like swallows. They just open their mouths, spread their wings, and run through the swarm.
Then again, an animal doing something it was never really built to do describes a fair amount of technology-assisted human activity. So maybe I shouldn’t judge these muddy, bug-spattered gulls.