A tool for one job
Why Are Bird Beaks All Different Shapes?
You can guess what a bird eats from its beak alone, because the beak is the whole eating apparatus.
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Step 1 of 5
A bird has nothing else to work with.
No hands, no teeth, no lips. Everything a bird picks up, breaks open, carries, builds with or feeds its young comes through the beak.
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Step 2 of 5
Seeds need a nutcracker.
A seed is a hard shell round a good meal. Seed-eaters have short deep beaks that work like pliers, with the muscle to crack a shell in one squeeze.
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Step 3 of 5
Nectar needs a straw.
A flower keeps its sugar at the bottom of a narrow tube. Nectar-feeders have long thin beaks and a tongue that reaches further still, matched to particular flowers.
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Step 4 of 5
Meat needs a hook.
A hunting bird's beak curves down to a point, so it can hold and tear rather than crack or probe. It is a cutting tool, and it is useless on a seed.
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Step 5 of 5
It is the classic evidence for evolution.
On one group of islands, finches from one ancestor have beaks from heavy nutcrackers to fine probes, each matching the food on its own island. Bird beaks measurably change within a few dry years.
A beak is bone under a layer of the same material as your fingernails, and it keeps growing. A bird that stops wearing it down gets into trouble.
The short version
A beak is a bird's only hand, mouth and tool, so its shape is built for one food — pliers for seeds, a straw for nectar, a hook for meat — and it can measurably change within a few dry years.
Try it yourself
Try picking a raisin out of a narrow bottle with tweezers, then with pliers. Same food, wrong tool.