Further to my previous post, it's also worth bearing in mind that rejection/acceptance isn't as black and white as it may seem. Magazine editors will often choose a poem because it fits in well with others that have already been selected for a certain issue. On the other hand, of course, a poem might miss out because it doesn't work alongside previously chosen pieces.
This question of fit is also relevant in terms of the transition from magazine to collection. There are occasions when a poem that appeared in a top journal just doesn't pay its way in the context of a full-length manuscript. In an opposing sense, meanwhile, certain pieces are destined never to be accepted for stand-alone publication but turn out to be crucial to the flow of a book. They bounce off and enrich the poems around them.
Of course, all the above is easy in theory and difficult to judge on a case-by-case basis. I think many poets can recall having agonised over a poem's possible worth!
This, I'm afraid, is utterly depressing and terrifying in equal measure. I'm used, like any other birdwatcher, to reading about worrying declines in all ...