Why the bill is so large and brightly colored is still debated and may be complex.
The bill has forward-facing serrations resembling teeth, which historically led naturalists to believe that toucans captured fish and were primarily carnivorous, but today we know that they eat mostly fruit.
As there is no sexual dimorphism in coloration it is unlikely to be a sexual signal; It does aid in their feeding behavior (as they sit in one spot and reach for all fruit in range, thereby reducing energy expenditure).
It has also been theorised that the bill may intimidate smaller birds, so that the toucan may plunder nests undisturbed.
Also, the beak allows the bird to reach deep into treeholes to access food unavailable to other birds, and also to depredate suspended nests built by smaller birds.
|Includes CC-BY-SA content from Wikipedia's Toucan article (authors)|