the way that i did it was to go for a older lens .i bought the sigma 135-400 f4.5-5.6 on e/bay ,got it for £180 inc p&p and then had it re-chipped by sigma for £33 ,so for just over 200 quid i have a excellent birding lens ,o.k i still want a bigmos 150-500 but the 700 + price tag is daunting .its not the perfect be all end all but i turn out some reasonable shots with it ,and if you take the average as our little group do that out of 2-300 shots your probably only going to get half a dozen good ones its about right .as someone else said with birding you really need a large apeture but that costs large money so you got to compromise
