That's neat. They're not all clearly identifiable to me of course (not being English), but I can immediately pick out the ones like goldfinch, kingfisher, magpie, green woodpecker, etc (basically, nine out of 25 are obvious to me). I could probably get more if I put some effort into it.
So in case anyone's interested or wants to google them to see how the pictures match, here's what I believe they all are (in the spoiler). I think they're quite cool little illustrations - if I had more time and artistic ability it would be quite cool to make some as a quiz.
Going across 1: Hawfinch, European Bee-eater, Bluethroat, Nuthatch, Magpie
2: Jay, Bullfinch, Red-backed Shrike, Green Woodpecker, Greenfinch
3: Great Spotted Woodpecker, Hoopoe, Woodpigeon, House Sparrow, Kingfisher
4: Jackdaw, Great tit (still not happy with this one, but I believe that is what it's supposed to be), Waxwing, Blue Tit, Goldfinch
5: Robin, Chaffinch, Goldcrest, Mallard, Wren
2: Jay, Bullfinch, Red-backed Shrike, Green Woodpecker, Greenfinch
3: Great Spotted Woodpecker, Hoopoe, Woodpigeon, House Sparrow, Kingfisher
4: Jackdaw, Great tit (still not happy with this one, but I believe that is what it's supposed to be), Waxwing, Blue Tit, Goldfinch
5: Robin, Chaffinch, Goldcrest, Mallard, Wren