For the use you describe you really need to specify the technical requirements of your sonogram analysis.
From this you can build a spec for the recording equipment you need to generate precisely what you require to be successful. This will certainly remove a lot of the risk of purchasing unsuitable equipment and possibly save you money by not buying more than you strictly need. I am very worried that you are looking at more and more expensive recorders without any obvious justification.
For successful lab work you should be able to provide a technical evaluation of your equipment selection process. If nothing else it will give any papers you author geater credibility.
Have you read papers/thesis published at other Universities on similar topics, these will give a much clearer view of what is required for your work?
The Maranz is certainly a respectable recorder and you may never have to buy anything better, however, when you are starting out the limiting factor will be your lack of practical experience, usually followed by the microphone or microphone/pre-amp combination.
Assuming that the microphone you are using has an output that is within the range of the recorder pre-amps and is compatible physically, it is unlikely that you will be able to tell the recordings from, say, a Sony M10 or the Maranz pmd-661 Mk2 apart.
The limiting factors will be the microphone and any lack of linearity or noise in the recoding process, however, for birdsong recording the base cutoff suffered by somerecorders is at frequencies you may not be interested in. You requirements specification will help you here.
The internal mics on the original 661 allegedly picked up hum from the recorder making it fairly important to use external mics. It is possible that this may have been fixed on the Mk2.
As I said at the beginning, if you really want XLR capability, something like a Zoom H4n may be all you need for less than half the price of the Maranz, but you need to assemble your facts before you take any decision at all.
