I spend most of my time doing entomology, I have been noticing in the past ten years or so how real the decline in insect and numbers has been. As an example I was doing some sweep-net survey on a chalk grassland site in southern England in May 2018, in two hours I didn't record a single hoverfly species where I'd had fifteen species in the same area, same corresponding week and same conditions, despite the sun being out, the temp being 22oC and there being next to no wind. It didn't improve as the summer progressed. While these are not Little Owl food it does speak of how dire the situation is becoming.
There's also been mass decline in dung beetles which Little Owls do feed on (despite there always being livestock on my previously mentioned site which had eight pairs, I virtually never see any dung beetles there in my weekly and sometimes twice weekly visits), mostly I suspect because of increased use of worming boluses etc. of livestock, particularly Avermectins, there was one school of thought some time ago in entomology circles that this could have been part of the reason Red-backed Shrikes became extinct in the UK.
I'm sure someone better informed than me could probably point to a large scale decline of earthworms too, how often do we see gulls following the plough these days? virtually never, as a result of firstly artificial fertilisers being used in preference to farmyard manure, and second because if manure is used it's usually devoid of insect use due to worming of livestock. There simply aren't the soil invertebrates to attract feeding birds anymore.
