This one obligingly sat on fence post in garden whilst it drove us slowly insane! Lol
Strangely it is my second most popular vid!!
2013 photos & vids here
eff37 on Flickr
I live in a chalk quarry surrounded by scrub & trees up top ... I have this 'enchanting' call morning, noon & night this time of year ... occasionally get to hear double-cheep reply by female!
No House Martins unfortunately!
I think this thread has gone off on a tangent? Chaffinches, incl males, give a "pink/fink" or "pink, pink/fink fink" type call. They don't nest in martin nests of sparrow terraces though. Great tits have many calls, incl two syllable. As they've been mentioned nesting in a box, it's also possibly them.
I am afraid my reaction to my dad several years ago, when he told me about sparrow v martin conflict in the 1950's in Batheaston was exactly the same. "Ok, thanks. I'll keep an eye out".
The problem is that it's very nature means it's largely done by stealth. Daylight starts 4am ish. Sparrows trashing martin nests take no more than a couple of mins. Likewise, a feeding adult being attacked could be over in seconds. Unless you're in the garden at the time, you will be oblivious.
The other problem is that when you see it, it is too late.
Here is the official RSPB info. It was updated/corrected fairly recently. I am guessing this was done after the BTO did several years of research, which I gave input into via surveys.
https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/house-martin/conflict-with-sparrows/
When it is too late, and sparrows are actively attacking martins and their nests, if you spot it in time, and still have any house martins left, you will need to get your ladder out, or pay for someone else to. Here is my latest thread with my experiences last year. It is very similar to the year before....and more successful, but equally challenging as the year before that. i.e. if people leave sparrows to get fully established in house martin colonies, they are wanting a job for life to then try and keep an semblance of martin colony.
https://community.rspb.org.uk/chat/f/the-tea-rooms/197081/another-year-same-results
I'm spending time posting all of this as you raised concerns about your house martins, so care for their survival. Obviously up to you if you want to leave it to pan out. There may be reason why sparrow numbers won't take off. In which case, my typing is unnecessary. However, I still maintain that it is not in martin conservation interest to be encouraging house sparrows.