They are lovely looking birds but they do nest raid as mentioned above. There are loads of them here (far more than when I was young) and since I've been shooting these and magpies I've noticed an increase in songbirds in the area.
Sounds like worst kind of confirmation bias to me.
I am unfamiliar with any strong scientific evidence that corvids cause any appreciable decline in song bird populations. But I am happy to learn if you know of any.
People see song bird numbers decline and corvid numbers rise and assume the two are linked. Not the case. There does not have to be causation even if there might be correlation.
Modern agricultural methods are the most likely explanation for song bird decline. Corvid intelligence the most likely reason they are able to better adapt to their changing environment.
Jays in particular are pretty shy birds and mostly survive on woodland nuts, they are highly unlikely to be in direct competition with the songbirds at your feeders as they generally prefer larger nuts and being away from man.
Corvids are themselves predated by even larger and rarer species, by reducing their numbers you inadvertently remove a potential source of food for these.
In fact corvids are some of the most intelligent animals on the planet, they have huge brains in relation to their size and have been seen to be able to solve complex problems using tools (even in the wild). Would you be so willing to kill a great ape or a dolphin (comparable intelligences) to 'save' song birds? I'd hope not.
Finally if you really want to help song birds you'd be much better off planting hedgerows, but please don't take my word for any of this, just spend some time researching it properly with an open mind.
