Hi Pauline,
I don't have any experience with Nikon, or DSLRs for that matter. But I have done a bit of research on them. Can you tell us what camera you're moving up FROM?
I'm sure the D40 and D60 are both excellent, but you should look into the cost not just of the camera but also the lens(es) you may want or need for birding. Long lenses for DSLRs are often more expensive (even MUCH more) than the camera body. And the learning curve to use a DSLR well can be considerable depending on what you're using now and how much experience you have.
Many regular contributors here post truly excellent bird photos captured with their DSLRs and will likely have some good advice for you. But some folks also struggle to master them. It requires both money and skill to get the most from them.
Hope this helps,
Kenn
Kenn & Temple
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