Do Irish Moorhens migrate at all?

Melodeon

We have what appears to be a pretty prolific breeding population of Moorhens here.
They're making extensive use of an old disused outdoor slurry tank, which currently contains a decade's worth of rainwater and accumulated pond weed and vegetation around the edges.
The thing is, at some point in the reasonably near future, we're going to have to do away with the old tank to make room for another building.

I REALLY don't want to mess them around or disturb them, so I'm wondering if there's any possibility of them taking it in their own heads to move somewhere else at some point?
There are still at least 3 adults on site today, so I'm thinking they're planning on over-wintering here.

Here's a pic from last July of a momma bird with some chicks and some other adolescents/adults in the background:

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Irish Moorhens are sedentary. They may find somewhere else purely out of necessity following loss of their 'habitat'.

Melodeon

Feck, that's what I was afraid of.

We have no immediate plans do do anything with this tank for now, and maybe even for quite a long time, but when might be the 'least-worst' time of year to empty it, thus depriving them of their watery habitat?
Sometime in late-winter/early-spring perhaps, when food supplies are building up again but before they get on with mating/nesting/hatching?

New Home

Would you be able to provide them with an alternative habitat nearby? I confess I don't know the first thing about them, I don't know if they're protected or anything like that, but perhaps if they find something similar to what they have now nearby they may take to it and it won't be as stressful for them to move.