Sounds like both a blessing and a curse! Firstly, it is so very good of you to allow them to stay - so many homeowners would have been hosing them off immediately. I'm not an expert so I don't know specifically why they may have chosen your property. I suppose the proximity of the window frame to the overhanging eaves is providing them with a ledge, a back wall and a roof - perfect for building. Perhaps there is also a source of good mud nearby, such as a creek, since they need to carry it by the beak-full back to the colony. Apparently they will start laying eggs (3-4) immediately after construction or even before the nest is complete. Babies will hatch within 15-16 days, and fledge 20-25 days after hatching.
The good news is you will have a front row seat to this amazing nature show. The bad news is that they are going to make a big mess. You will need to be careful when removing the nests in the fall since they can contain insects and bacteria. You will also likely want to find a way to discourage the colony from returning again next spring. This will likely require attaching some type of shield above the windows and under the eaves. Yikes!
Would love to see photos over the course of the next several weeks. Best of luck!
-kris
