On paper, the 701D seems like a relatively poor value unless you absolutely need the HDMI camera triggering and timecode input (no timecode generator).
The lower-cost Tascam's are a better value overall, I think. Besides the 70D and DR-40, the 60D is on sale all the time now.
Here's the thing though - if you're doing ambient / bird song, you're going to want something with preamps that have very low noise at very high gain, and that means a higher class of equipment. That should be your #1 priority here. All of those Tascams will have unacceptable noise for you at the highest gain levels. If you weren't doing ambient recording, I'd say the 60D or 70D would be the way to go, but I don't think you'll be satisfied with the preamps and I doubt the 701D preamps are going to be that much quieter than the 70D, even though its specs are a little bit better.
The cheapest way for you to go is to look for a used Sound Devices MixPre / Shure FP-24 preamp on eBay for $300-$400, which you connect to your M10 with a mini stereo cable. I have that setup, and it works great. More gain than you will ever need, and very, very low noise even when you crank the level. You'll have to use the Tape Out on the MixPre/FP24 with the M10 which will deliver 56dB max gain, but that gain level will be actually quiet enough to use for your purposes, as opposed to the Tascams which will deliver that much gain but have all kinds of noise in there along with it.
You can't do an under-camera mount this way, but you could use a small shoulder bag or backpack like this. I do a lot of marching band recording in the fall, and I sometimes use the FP-24 > M10 in a backpack while using a monopod to hold my camera and mics. You could also look for a MixPre-D which has the DSLR plate, but it's much more expensive and and you'd still have to mount the M10 somewhere.
If you need this to be a one-box solution, I'd keep saving my money and get a used SD 701 or 744, which are going for a little over $1k right now. There's also the Zoom F8 (one for sale in the YS right now) but I haven't seen any reviews of how quiet its preamps are when cranked all the way, since most people here typically aren't doing that. It would allow you the under-camera setup though.