Also, the Nikon 80-400, while the current version is too slow for birds there should be a new version on the market come this autumn or winter. It might be worth saving up until then. It's a little faster (larger maximum aperature) than the two lenses mentioned above and superior optically.
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You can get the same sensor quality of the D90 with Nikon's newest entry level cameras, the D5000 and D3000. I would not consider the older D60 unless it is considerably cheaper than either of these. Nice camera but uses an older generation 1 sensor technology.
good luck,
Rick