When I come across animal or bird or bug injured and in pain, no resources available nearby to shelter, do I leave it to nature or kill it to end its misery. What is Buddhism view on mercy?
Definitely no killing. That's the first among the Five Precept to be observed. Do what you can to help. If it's a large animal, call the local animal shelter helpline. If it's a small animal like a bird or a bug, at least move it to a safer place like under shade of a tree to avoid the hot sun and to prevent people from stepping on it, cover it with some large leaves, etc.. Any kind of help no matter how small might still bring great peace to other beings.
Apart from breaking the first precept and creating bad Karma for yourself, there's no guarantee that by killing you are going to reduce it's suffering. What if it was born in a realm lower than the animal realm?
So if you can't think of any way to help, practice Upekkha: Beings are owners of their deeds. Whose [if not theirs] is the choice by which they will suffer or become happy?
You can also contemplate on yourself not being free from such suffering in the future and use it as a motivation to progress in the path.