I'm sorry Chris, but IMHO this is quite poor advice. Allowing the camera to choose both aperture and shutter speed for subjects that flit around one moment, briefly pause, then take off in flight is asking for trouble. Sure, you will get the odd shot where the camera has replicated optimum settings, but far too often you are going to be stuck with inappropriate settings for the subject.
....Yes, I recall starting off using P on my daughter's 100D for a group portrait session outdoors and allowing the camera's software to select the aperture-speed balance and it takes away a lot of control. It's rather like leaving my car in fully automatic (great for town and traffic queues) for fun bends rather than popping her into the gears and being in the right gear at the right instant by being in fuller control.
With this lens and the subject matter you need to find a way of keeping your shutter speed up to avoid any motion blur. If your subject is stationary, then providing you have sufficient support to your rig then you can lower it to allow for a smaller aperture and more depth of field, and there are numerous ways of achieving this, but using 'P' is right at the bottom of the list. A lot of successful photographers use semi-auto modes these days, but with birds, you'll find very few good photographers using 'P'.
....Keeping my shutter speed up by having control of it is why I chose Tv prog. But I usually use Av prog on my 70-200mm with some +/- ev. It seems that even at f5.6 the 400mm delivers better than expected DoF (see my garden's fence post pic below).
Robin, I'd strongly suggest that for starters you get used to using Av mode. The canon 400mm f5.6 is well regarded as a lens that is sharp wide open, so unless you're shooting in nice bright light - very rare at this time of year! - then start off shooting wide open. Your 70d will perform to a decent standard at ISO 1600 no problem providing you get the exposure right and do not have to crop too much. I've also read your comments elsewhere that a little bit of noise does not bother you. Keep an eye on your shutter speed, use good support and away you go. Then, as you get used to your gear you can look at when to lower the ISO for a better quality shot, and start using exposure compensation to fine tune your camera to get the results you're after.
....Yes, I'll try going back to Av and set my ISO to Auto with a ISO 1600 max ('auto max' is a useful option on the 70D). To begin with I will maximise my chances by continuing to use the support of my monopod - I'm sure it's what delivered a stable shot with my 400 lens wide open and only 1/200 shutter speed - I was pleasantly surprised and that was at the minimum 12ft distance as well, and in poor light. It was an experiment to show myself what would happen. Boring fence post subject but here it is:
^ Canon 70D, 400mm f5.6L at minimum 12ft distance, ISO 800, f5.6, 1/200, on monopod.
Take your gear down to the local pond and start practicing on the ducks / geese/ gulls etc. You should be able to look at framing these as you see fit, therefor reducing the need to crop too much, then once you're happy with your results, move maybe to somewhere that has a feeding station and allows you to get close to smaller, more difficult to shoot, subjects.
....Local ponds around here are much wilder - Cormorant, Otter, Kingfisher, wild Mallard, Water Voles, Moorhen, Dragonflies (in flight season) - and hence trickier but I understand the principle. Loads of Seagulls, so. My garden is well stocked with a feeding station - The regular Dunnock, Robin, and Blackbirds are the most cooperative. Lots if Tits but they're very fast.
More than anything, post your results here, and be prepared to listen to the crit given. There are a couple of photographers here that have done just that and are improving at a rapid rate of knots
....I hopefully have already shown that I am prepared to listen and learn - An earlier thread in which Jeff 'the black fox' has been particularly helpful and whom I blame for spending my money on the 400mm f5.6L from TP's Classifieds!:
http://www.talkphotography.co.uk/threads/a-grey-day-in-west-bay.522243
I'm raring to go (my daughter is fed up with me wanting to photograph everything) and just need it to stop raining and have some decent light.
One thing though - I am very happy indeed with my ability to capture a moment and to compose a picture which puts the subject in context and with atmosphere, even if heavily cropped at times.
I will be posting more as and when and appreciate everyone's input