Care for muscovy ducks contained by winter?

Should I have more of a heat source for them in the winter after just a little dog house for them to get out of the weather within? We live in the Chicago area, so winter can sometimes get pretty brutal.
Answers:    They are a drastically hardy breed of duck. We had 6 of them 'dumped' at our pond last winter by some scummy person... We have no shelter and no way to provide them shelter. We did, however have an aerator on the pond, so it never completely froze. They other had water and a place to swim. We fed them every morning, and lots of people threw bread out for them also. They were very comfortable and survived a bad winter here in central Ohio. We did find them a shed to go into this year - but they won't go in! We enjoy lots of hay in the shed, which helps to keep them thaw out - if they should go in! We're glad in a minute that they were dumped - they are great animals and keep the bug population down in the summer.
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