Over the past 205 years, the lake has frozen over 162 times and remained open 43 times. Though the 43 open years aren’t even distributed –  65% of these have occurred within the last 50 years, 90% in the last 100 years. And this has had a significant impact on our resident birds. For many birds that depend on wetlands for food and shelter (herons, egrets, bitterns, rails, ducks, terns, sandpipers, etc), the impending freeze up of their habitat in the fall signals that it’s time to migrate to more favorable habitats either on the coast or farther south. This is particularly true for shore birds that rely on the shallow parts of rivers, ponds, and lakes which freeze up first and more reliably. These birds tend to be more synced up with the calendar (day length change) than the weather for coordinating migration times.

Mallards flying in to roost for the night in the water treatment facility (Perkins Pier, Burlington)

Should I leave or should I stay?

But for those birds that utilize deeper open water for food and/or shelter, like mergansers and mallards, they’ll postpone their migration until the last minute until their habitat is mostly frozen over. With the lake remaining open longer and longer – or not even freezing over at all – it’s possible for these birds to stick around through the whole winter. These open patches of water are critical habitat in the winter, and it’s why so many congregate at the ferry crossing down in Charlotte (eBird checklist) where boats keep the water from freezing even in colder years or down in Burlington where the lake is widest and least likely to freeze.

Cedar checking out the mergansers with his binoculars (Perkins Pier, Burlington)

Finding food in cities

It’s definitely a tricky balancing act for these waterfowl, weighing whether to not migrate and conserve energy but risking their habitat freezing quickly, or to spend energy migrating to safer habitats but return later for breeding. Mergansers, as with many of the other ducks you’ll find in Vermont in warmer winters, are hunting for food in the water and require plenty of open water to hunt for fish. A mostly frozen lake is a useless lake to these piscivores. Well in advance of the lake freezing over mergansers start making their way south (or to the coast) where they’ll find plenty of open water for hunting.

A pair of mergansers foraging in the shallows (Perkins Pier, Burlington)

In our latest season of The Single Acorn Podcast, we’ve been exploring the challenges and opportunities associated with urban environments. And mallards have certainly be up to the task. Unlike mergansers and many of our other native ducks, mallards are synanthropic, meaning that they are better off living in or around humans and can cope, even as their natural habitat freezes over. When most aquatic vegetation dies back in the winter, their winter diet simply shifts to gleaning from agricultural fields (including silage), eating handouts (like bread), enjoying the delights of other urban leavings, and foraging in the last remaining open water for scant aquatic vegetation. So the lake freezing over might not be a big deal for food, so long as they have a place to sleep.

From 2014, a few hundred mallards warming themselves in the settling ponds of the Burlington Main Wastewater Treatment Plant (Perkins Pier, Burlington)

And indeed, as they might not be as tied to water for foraging, they’re still reliant on open water for roosting. My first winter in Burlington I was running along the bike path at night and heard the squawkings of mallards in the settling ponds (above) at the wasterwater treatment plant. That year, there was far more ice coverage on the lake and I counted over 1,000 ducks in a single “pond” enjoying the warm bath. The pond provides protection in numbers and a warm environment for sleeping. In the morning the ducks scatter to forage. But in warmer winters, like this one, open water isn’t as limiting of a commodity and so there are far fewer in the settling ponds (compare the above image to the one below).

A few mallards warming themselves in the settling ponds of the Burlington Main Wastewater Treatment Plant (Perkins Pier, Burlington)

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