Common Eider
Credit: Kirk Rogers

A few days ago someone posted on the Maine Birding listserve a sighting of a large flock of Common Eiders migrating up the Penobscot River in Maine. It brought back memories for me of seeing such flocks migrating up that same river about 25 years in Bangor where I started birding.

This population of Common Eiders is quite an anomaly as the species is famously tied to the sea. Common Eiders are coastal birds that spend winters diving for shellfish in the icy ocean waves and nesting on islands and remote shorelines always within yards of salt water. Even in migration, eiders virtually always will follow the coastline to get where they need to be even if it means a long detour. Until recently no one knew for sure where the eiders that migrated north in spring across inland Maine were going. Satellite tracking has answered the question at least for some of the birds. A number of eiders that were tagged on their breeding grounds along the shores of Quebec’s St. Lawrence Estuary were tracked passing over Maine to and from wintering grounds in southern New England.

Here's a map of one of the satellite-tagged eiders and their annual migratory route:

How a population of Common Eiders would have developed this seemingly anomalous pattern is an interesting question. There are many interesting questions of course. Does an individual bird ever take the shorter inland route one year but the much longer coastal route another? Do all the birds that migrate through Maine breed in the St. Lawrence or do some continue north to the coastal James Bay breeding area?

Feel free to comment below if you have any thoughts on these questions.

On a side note, I found some Common Eiders a few weeks ago while birding near Harpswell, Maine: