Nature recordings will require very low noise electronics. The SONY mentioned above is a good starting point. I am partial to Sound Devices. Their SD 702 would do the trick for you. Here is a link:
Sound Devices 702 - High-Resolution 2-Channel Compact Flash Field Record
These devices are rarely if ever discounted so find a dealer you like. You will need mics. I would recommend DPA 4060's. Link:
DPA Microphones 4060 Omnidirectional Miniature 4060-FM - B&H
These are also price controlled so find a dealer you like. You may as well get the SMK kit as you will need the phantom power adapters. The other gear in the kit may be helpful, too. Link:
DPA Microphones SMK4060 Stereo Microphone Kit SMK4060 - B&H
For just under $3K you have a rugged setup, low noise package. Folks will chime in and say that these mics are not low noise. A search of this board in this forum reveals the following nature recordings with these mics:
Nagra LB - DPA 4060 - Sound Sample of a Forest
There are other recordings with the 4060's. Check them out and make up your own mind.
Disclaimer: I have Sound Devices recorders and am very pleased with them. I have DPA mics and am very pleased with them.
As always, YMMV