In reply to Paul I:
Paul I said:
From what you are suggesting the ones which need the feed lose out to the well fed birds.
It is more a case of those that are generalist, or adaptable, or can benefit from direct human action that are increasing in numbers (on the whole). Those species that will never come to garden feeders....e.g. waders! need other help, most especially habitat creation and/or protection, human disturbance/hunting reduced and predator restriction/control at nest sites.
As prev., humans feeding birds in gardens 24x7x365 is supplementary feeding. It's not really a conservation thing or necessary to keep species from going extinct. In a small number of cases, targeted feeding is of conservation value, e.g. feeding in areas of cirl buntings & turtle doves.
Swifts, house martins, curlews, whinchats, spotted flycatchers, cuckoos etc etc don't need or want humans to feed them. A level playing field is one of the things that may help reduce their decline, alongside habitat protection, providing nesting sites etc.
It is perfectly natural for predators to raid nests. It is not perfectly natural for booming great spotted woodpecker populations for example, to rampage through house martin colonies. That would have been unusual 50 years ago.
Species benefiting from humans feeding 24x7x365 in gardens are wood pigeons, great spotted woodpeckers, magpies, nuthatches, blue tits, feral pigeons, jackdaws, goldfinches, house sparrows, robins, blackbirds etc. Very hard to make a case for feeding them to conserve their populations. Almost all of that list have done very well in recent decades. Blackcaps you referred to, also doing very well, in part due to humans feeding over Wintering populations (as opposed to UK breeding blackcaps which aren't benefiting from human feeding in UK).
There are exceptions. Starlings and chaffinches have declined but also benefit from feeding in gardens. The latter is declining in part due to disease.....anything to do with disease spread at feeders? Only science that no one is paying for will prove it one way or another. Greenfinches benefited from feeding in 1970's. Remember the red netting peanut feeders? They then paid for the bird feeding by being heavily wiped out by disease and only recently are showing limited signs of population recovery.
People over estimate the importance of humans feeding birds in terms of 'helping'. As I said, most species get no benefit at all, and if anything, become worse off as a result.
Re sparrows v martins, sparrows throwing nestlings or eggs out of martin nests are one way of spotting the issue. Sparrows also attack martins as they arrive or leave nest sites, and routinely wait above the nests for them to arrive. Once I see a sparrow enter an active martin nest, I know it's pretty much game over for that martin nest. Only once or twice have martin nests survived sparrow intrusion here that I've witnessed.