Many people have asked recently, upon noticing a thin layer of ice covering Jamaica Pond, how Jessie is able to survive in the pond in the winter. This is a great question -- thanks for asking!
The main thing to keep in mind is that Jessie probably doesn't actually live full-time in the pond. Surely she spends a good deal of her time there, eating aquatic plants, sneaking up and surprising ducks and geese, searching for a mate, and so on. As we will soon reveal our own theory as to what Jessie is, I don't want to give too much away, but we're pretty sure that Jessie has a layer, den, or lodge somewhere not in the pond -- probably in the woods in the Emerald Necklace or over the town line in Brookline nearby. So if the pond is completely frozen over, Jessie probably is just bored, not in danger.
In addition, if the ice is not too thick, he can easily break through, coming in and and coming out. There are also plenty of spots around the pond where there isn't an ice cover. However, it's probably no coincidence that many of our Jessie sightings in the winter are not actually in or directly around the pond. Due to the warm winter, this year is a bit of an exception, but in general the in-pond reports are positively correlated with temperature.
Hope that helps!