I've got a small Holly shrub called Ilex Little Rascal, which doesn't grow very big. It has sharp leaves but I don't know if it would be big enough for you.

There's Berberis thunbergii atropurpurea 'nana' which only gets about 12- 18 inches, but it must have the 'nana' bit in the name tag or you'll end up with the 'senior' version that gets about 5 feet. It's very prickly, but deciduous, though the prickles don't come off, they stay on the plant, so its prickly all year. Sadly though, I do not think prickly plants will deter wood pigeons.

Ruscus aculeatus, Butcher's Broom might fit the bill. Give it a Google for more information. An old allotment friend used to spread thorny rose cutting between the rows to deter the pigeons but not a good idea if you have children.

I was going to suggest one of the Berberis species like Bamboo, otherwise there is a prostrate form of Pyracantha or you can just put in 'prostrate evergreen shrub' in your search engine and you will probably get hundreds!
Not sure anything is put pigeons off though.

Yea, thinking about it, there's Berberis bagatalle, only gets a foot high, or Berberis 'admiration' which gets a bit taller, around 2 to 2.5 feet, or Berberis 'Golden Nugget' with bright yellow foliage, up to a foot high - but all only colour up properly if they receive half a day's sunshine, in a shady spot, they all look green. All deciduous too though.

Thanks everyone, plenty to be looking into there. I could try the thorny rose cuttings as I have quite a few roses in another part of the garden, but not near the bird feeder. I rigged up a pigeon deterant with netting and canes, in the hope they stop trying...trouble is I feed lots of beautiful birds and I don't want to discourage them, or even stop feeding...problems, problems...

It boggles my mind that any bird smaller than a pheasant could possibly compact the soil! Are you sure that something else isn't responsible?

No Tug, definitely Wood Pigeons, I've watched them landing like a jumbo jet then waddling round flattening the plants and compacting the soil!

Well, I can see them flattening the plants, especially if the plants are young seedlings, but they would only compact the top few millimeters of soil. Hmm..true, that would interfere with sprouting of tiny seeds, but shouldn't affect established plants. I wouldn't take the soil down to that fine a tilth anyway--it's bad for soil organisms. I would worry more about them eating seeds and young seedlings. Here in the desert, people who garden in areas with high bird populations build chicken wire cages around their beds. Birds are rarely much bothered by thorns anyway, since their feathers and scales protect them.

Well, all I can say is you should see my garden every morning. Plants flattened not just seedlings. The are also so clumsy when they come into land that they've even broken stems on taller plants. I know why they come, it's because I have a bird feeding station and all my various food stuff is in special feeders some obviously gets knocked to the ground. Other birds, the ground feeders, take advantage of this, which I haven't a problem with, they don't cause any damage. The same cannot be said for the Wood Pigeons the soil is flattened and plants even killed. I just wish I could get rid of them. I don't think chick wire all over my garden is the answer.

Hmm..I just looked Wood Pigeons up, like I should have from the beginning! :} That's a big Pigeon!! The only thing that I can think of to do is to move the feeding station farther away, or move the garden. I'll keep looking for bright ideas, though.

I love wood pigeons, love that cooing sound in the summer, but there's no denying they'd make a good sized roast...

Half a wood pigeon with orange glaze, a small baked potato, green beans with slivered almonds and pearl onions...mmm.... :)

Bamboo...try 'liking' at 4am...!

Tug; Now you're talking. I just wish I could catch one. My OH has suggested he borrows his friends air-rifle, but with our luck, the dead bird will end up falling in our neighbours garden...not a good idea. We could do with a hawk...

Its not illegal to kill wood pigeons with an air rifle - but only if the pellet from it does not leave the boundary of your own property. If you try shooting one, miss and the pellet goes over the fence to next door, that is an offence, so if you have a small garden, probably best not to risk it.

As for 4 am, what wakes me up around that time is fox activity, what a racket...

One of two things, here:
The nicer one is the Southern Mockingbird. It has a beautiful song to wake up to, and nice to fall back asleep to, too.
Not so nice are the schizophrenic homeless, arguing with their invisible friends. I don't blame them, but in this day and age, why are people this ill left to wander the streets all night, and then die of heat stroke the next day! It is positively Dickensian!

We also have a fox visits our garden Bamboo, but she doesn't wake us up.

Bird song is lovely Tug, but not the loud Wood Pigeon, who always seems to sit on the chimney and then it echos throughout the home.

Thinks it's a rock star, does it!? That's like the Gila woodpeckers here, sitting on the air conditioner, and rapping on the metal. At least they don't do it at night!

Thank goodness for that!