On what bases would you rule out this bird being a European Hobby
OK, there are a few things, I'll list them:
1. The bird in question has a red cere and legs. These would have been yellow in Euro Hobby. I suppose it could be argued that the bare parts of the pictured bird is actually yellow but my opinion is that it is indeed red. Have a look at the difference in colour between the cere and the eye ring. While the female Amur Falcon does have a red eye ring, it can often appear yellow as it is usually not as red as the cere. I believe this to be the case with this bird.
2. The absence of rufous leg coverts and undertail coverts in this bird immediately rules out Euro Hobby. In case of an immature bird, the Euro Hobby would not have shown the rufous vent area but then the bare parts (legs, cere and eye ring) would not have had any colour, it would have been a very pale yellow at best but most likely just grey.
3. The Euro Hobby has more pointy wings than the Amur Falcon. This is a subtle feature and I wuldn't expect everyone to notice it immediately. It also seems to be more apparent when you see the bird fly than when you see it on a photograph. When comparing Euro Hobby to Lanner Falcon this is a very noticeable field feature but I'd say that for EH and AF it is probably not a very reliable feature to look for.
4. The description of the sighting noted that the birds were seen in large flocks. This is typical of Amur Falcon and very atypical of Euro Hobby.
A few additional comments:
1. The time of day could have been a useful indication as well. Bucky didn't mention the time of day but European Hobby is very much a crepuscular bird which means that they are active at dusk and dawn.
2. One question to always ask yourself when deciding between two birds is "Which is the more common one of the two?" I know, this may seem nonsensical because what about those sightings where you do see Euro Hobby? But the point I'd like to make is that Amur Falcon is VERY common while Euro Hobby is rather uncommon. You will probably see 500 Amur Falcons for every Euro Hobby that you see. So when you see a bird like this one, the first thoughts that should go through your head is "Probably a female Amur Falcon" and then you work from there. If it is a Euro Hobby you will eventually arrive at that conclusion but if you immediately start thinking Euro Hobby and ultimately arrive at Amur Falcon you just make life harder for yourself.