I always tell emerging poets who ask for my advice on getting published that they should aim to have a track record of poems published in magazines first. I think that still holds true, but it’s also the case that most collections contain poems that wouldn’t fit in magazines.
Is there a difference between the ‘magazine poem’ and the ‘collection poem’ then? It’s not a matter of quality; some collections contain very good poems that have not been published in magazines. Equally, some poems that have been in magazines aren’t good enough to fit in collections – maybe you spot an uncorrectable flaw in them once they’re in print.
Sometimes magazine poems have a topicality about them, especially if they’re in a webzine or other online publication. Sometimes they’re just too carefully tailored to suit a particular magazine or editor. Sometimes you realise you’ve moved on since you wrote the poem, and it’s no longer a good reflection of your voice or style. Maybe it’s the subject matter: if your collection is a themed one, that limits the range of poems that will fit into it.
Sometimes you have ‘pet’ poems that you like, but which somehow keep getting rejected by editors. I would argue that if their quality is high, there’s no reason to exclude them from a collection.
Fashions have changed over the years too. Norman MacCaig only put previously unpublished poems in his collections. When he discovered one that The Scotsman had published, he took it out.
In my own collections, I suppose the average is around 20% having previously appeared in magazines. I think that’s pretty general. And I don’t have any qualms about revising such poems, if I feel they need to be improved. Most years I have around 10 poems in magazines, and a lot more that don’t make it.