Please help me identify this caterpillar.

[birdling]

It is yellowy green, no hairs and found on a wall not near any likely plants.
We would like to see if we can hatch it, but if we can't find suitable food, then we'll have to let it go.

Does it have a spike at the end? If so, probably a hawk moth caterpillar (several types). How long is it?
www.google.com/search?q=hawk+moth+caterpillar&tbm=isch&chips=q:hawk+moth+caterpillar,g_1:green:pMGQRWHvQaA%3D&rlz=1C1CAFA_enGB716GB717&hl=en-US&ved=2ahUKEwj20_v-j9HoAhXQ0YUKHcP_BD8Q4lYoAXoECAEQFg&biw=1007&bih=608

No, there's no spike.

It's about 2.5cm long