why are small birds like humming birds are not found in polar regions?Explain.

Thermoregulation is energetically expensive for many organisms.This is particularly true for small animals like shrews and humming birds.Heat loss or heat gain is a function of surface area.since small animals have a larger surface area relative to their volume,they tend to lose body heat when it is cold outside.then they have to expend much energy to generate body heat through metabolism.this is the main reason why very small animals are rarely found in polar regions