These sometimes (but not always) take several years to germinate. An extreme case took SIX years although some species showed after as many months.
Any suggestions on dormancy breaking other than just leaving the pots outside? Why are different species behaving so differently?

I can't answer your question but you sure must have a lot of patience to wait so long!

Have you tried lightly scarifying the seed coat? One site I looked at recommended rubbing with sandpaper, then sowing in containers of vermiculite and setting them in a refrigerator for 3 to 4 months before putting them in a warm spot to sprout.

As I recall, the cotoneaster in my mother's garden in BC was very popular with the birds when the berries were on it. She did used to see small seedlings under the larger bushes there.

Perhaps the seed is a type that needs to pass through the gut of a critter before it will germinate?

Rather than wait for birds to eat your seeds then trying to collect them . . . (just kidding) maybe fermenting them (as seed savers do for tomatoes) will help speed up the process? Mash up a few ripe berries, add a little water and maybe a touch of sugar, put it some place warm-ish, let it get stinky, then try planting the seeds.

I have been raising Cotoneaster species for about 20 years so any recalcitrant seeds just get left in the box outside until they wake up!
There is a species of tree native to Madagascar whose plentiful fruits stopped producing young plants for 300years. It was eventually realised that the Dodo's digestive system was necessary. Importing some Turkeys enabled new trees to generate. This is absolutely true.

Gordon, when you collect the seed, do you remove them from the berries?

Thinking of how it might happen in nature, when the berries fall to the ground.

Elaine,
Sometimes I bury the whole fruit and sometimes the cleaned seeds;there seemed little difference. I think some scarification could well be the best especially if the seeds are not absolutely fresh. Wish I had been more scientific in the past but then I was busy working then.

Good to see another of Winston's quotes.

Gordon.

Yes, Winston had a large repertoire of great quote-ables, didn't he.

My friend who collects tomato seeds says that the acid in the fruit scarifies the seed as it decays and ferments, if that is any help. She's a biologist and was looking at the seed under a microscope for some other reason when she discovered this.

Elaine,
Now this acid idea must be correct.
1. Gibberelic acid (GA-3) is sometimes used to break dormancy.

2. This explains the Dodo anecdote.(Mauritius actually...) Stomach contents are extremely acidic).

3. Our local sewage works has millions of tomato plants every Autumn.

4.Scientists are usually right when talking Science!