I just passed one of the town parks, and there was a flock of Canada geese sleeping on the frozen lake. It seemed like that would be a rather cold place to sleep, is there a reason beyond the possibility of predators why they would do that?
The lake is completely frozen, and the surface is hard with no slush. I am not certain if the geese live there year round or not, but the city did pay $6,000 for a border collie to run the geese off.
There are a few reasons why Canada Geese sleep on the ice. michael has covered them, so this is supplementary.
In simplest terms, it's because they live in the water, and are able to tolerate frozen water for periods of time if fresh is not available. Geese on the ice may be resting or preparing to move on to un-frozen bodies of water when need be. Huddling their bodies together on the ice of a shallow pond can cause it to warm up, especially along the edges, and increase their food supply, so sleeping on the ice can merely be a step towards thawing it out.
As michael pointed out, their bodies are built to stay warm. They have insulated underbellies, and enough feathers, generally between 20,000 and 25,000, to keep their upper body temperature around 104°F (40°C). (Goose down has long been considered the warmest filling for things such as jackets and sleeping bags. After hunting nearly decimated the population of Canada geese, they've become protected, and synthetic warming materials are used instead.)
Geese also have an interesting type of circulation through their feet that transfers warm blood back up into their bodies. Since their feet can't freeze, they often sleep with them tucked underneath their bodies. This heat-transference phenomenon is one of the reasons why sometimes you'll see them standing on one foot with the other tucked up under the belly, especially when the ground is frozen. It's also helpful for living on the tundra, where the ground is frozen much of the year.
Canada geese live year round in much of the United States, including the western part, where I believe you live, so the group on that pond may winter in the area, unless of course nasty people come along and make them move! When in flight (or during migration when applicable) they stay within a certain distance, generally a "bird's-eye view" of bodies of fresh water. They'll either stop and settle in, or at least rest and eat.
For further confirmation that geese sleep on ice, I turned to research done by an organization called Wildfowl & Wetlands Trust (WWT), which is a charity dedicated to saving wetlands in several countries in the United Kingdom.
One of their projects is the annual publication of a scientific journal called Wildfowl. In a past journal entitled Geese in cold weather, authored by Jules Philippona, it was reported that some geese do indeed sleep on the ice. The article cites several scientific papers written between 1962 and 1964.
Note that although Canada Geese have become commonplace in the United Kingdom, and are among the 27 breeds observed and studied, the following quote doesn't specifically mention them.
Geese mostly have their roosts on shallow waters. They also often sleep on ice (Brotherston, 1964, Markgren, 1963, Mathiasson, 1963, Rutschke, 1962). In 1962-3, as well as in other years, geese often used their roosts when these were frozen over. Sometimes even new roosts were formed when the water is ice-covered, as occurred on the Ijsselmeer near the new Noordoostpolder.
The Ijsselmeer near this polder is not suited for roosts, for the water is deep and ships often pass. When the water is ice-covered, however, thousands of geese sometimes sleep there. Then they feed in the pastures and fields of the neighbouring Noordoost polder.
In the severe winter of 1962-3 geese were seen to roost in their feeding grounds:22nd February, 1963 : Etten. A flock of 480 Whitefronts and 20 Beans feed on sprouts. Afterwards when the evening twilight has come, they fly 50m. away and settle on the snow. An hour later they are still there. It is quite certain that they will pass the night here, although some farm houses are within 150m.
In the spring, several Canada Geese and/or Trumpeter Swans stand on frozen ponds as a way to stake out their nesting and feeding turf for the following spring, summer and early fall until they raise their new family(s) and teach them how to fly and then join a larger flock to learn V formation and fly south again. Usually these turf-staking on ice geese either raised a family on this frozen pond the summer before and/or were raised on that pond the year before. Kindo' like coming home for the summer and letting other couples know this is our pond. Canada Geese and Trumpeter Swans in the wild (not as much in parks) are very territorial from the early mating season until they join V-formed flocks in the fall
Because it works I guess. Evolution favours the behaviours that leads to survivability. If the birds that slept on the ground survives less than the one on the pound, it will lead to more and more birds sleeping on the pound cause that's what works and will be transmitted to future generation (either by learning or genetics).
