As in winged or 4 legged vermin?

Rats, not really unless it's harvested and clamped badly, or has a lot of leftover maize or Corn cobs that would attract your crows and rats,a messily harvested cornfield. Rats won't go to the effort of digging up tuber when they could burrow into a nice pile of heaped and stored spuds, etc.

Winter crop cover is usually brassicas, like cabbages, kale early turnips, etc so its only interesting to game birds and those that live on greens and weed seeds as many of those winter crops have a weed content too like fat hen. Grain wouldn't be a Winter game crop cover.

No idea at all.

It’s not just about what birds will eat what crops, some will eat/browse on a,b,c, others will target x,x,z at different times of year.

Some birds only target crops in severe weather conditions when hungry and when other options are gone, lots of birds are attracted not for the crop per say but for what’s on the crop, as in seeds/incest’s/grubs/larva, etc,

That would only be a problem if the field was near a house or shed - otherwise the rats will feed plenty of owls etc.

Yes - any food source available in the winter will attract rats and hungry vermin - especially if oats are included in the mix.

https://www.agriland.ie/farming-news/triticale-results-in-less-vermin-in-wild-bird-cover/

Interesting read -

"Rodent Pest Control in Rearing and Managing Game Birds"

https://iasis.ie/Documents/Rodent%20Pest%20Control%20in%20Rearing%20and%20Managing%20Game%20Birds.pdf

Exactly what if it should be, it's a not-for-consumption crop, and to answer another point you made in another point, It shouldn't be planted year in and year out either.If the farmer is doing crop rotation in a field.IE spuds one year, cabbage the next etc. The winter crop or cover should change too. The best cover crop is if you can convince the farmer to leave a strip of unharvested crop in the field as cover which is then ploughed under come Spring.

Rats are just a thing you have to put up with on any farm or crop. But as said, they become food too for owls, foxes, etc and once their food supply is gone they will depart too. Which will be to the farmyard of course if there are open and sloppily stored and supplied animal foodstuffs, household rubbish etc.

If there is Govt involvement then with money,understandable.

The likes of Skylarks and Pipits do - I grow some bird cover on my place in North Mayo and they are in it every year. I see Lapwing and Native Partridges are breeding in similar down the country on NARG projects in places like Carlow, Tipp and North Dublin.

That varies with location - in my place in North Mayo I typically sow in late April, it would be a few weeks earlier (weather depending) further South and East. I work with Spring crops cos they are more attractive to these nesting birds. Some other folks sow winter crops in Oct/Nov. I believe Partridges start pairing up in mid March and clutches laid in April most years.