Knowing the nature of birds such as these you will turn up with you 600MM prime and he will immediately swoop out the tree and land on a fence post not 10 feet in front of you, you will be left wishing you had brought your 70-200 from the car 

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html