Generalists and Specialists
Every species is unique, even if it has close relatives. And every species lives only in certain places in the world. Some species are generalists that can survive in many kinds of habitats eating a wide variety of food. Other species are specialists, living in a more restricted range with special dietary or habitat requirements.

Rufous and Ruby-throated Hummingbirds are both specialists and generalists! They migrate and nest over a broad geographical range like generalists. They mostly eat nectar and tiny insects, like specialists, but can branch out to take sap from sapsucker drill holes and can even eat willow catkins when a sudden cold snap makes other spring foods scarce.

Any ornithologist seeing a hummingbird for the first time could instantly guess that this bird eats liquid food from flowers, flies long distances flapping every moment, and has some sort of adaptation for staying alive on freezing nights or cold, rainy days. How? By understanding how bird bodies and behavior are adapted to habitat and food requirements. If you look at a hummingbird's body from head to tail tip you can see just how this tiny bird is designed precisely for the kind of life it leads.

Wikimedia: Jeffrey W.
Martin Dollenkamp