Over much of this winter, I heard from Seanetters telling me they were finding what seemed like fewer dead birds on their beaches. This was mainly from the “hotspot” beaches that typically produce a few birds per walk throughout the winter, when bird mortality is generally highest. Many of these Seanetters went many weeks without finding any birds. Now, as spring advances, our northern cohort of Seanetters are seeing the opposite in many places–sometimes over a dozen birds dead on a stretch of beach. We are also hearing from nature centers and other groups who are receiving reports from the public about what seems like a spike in mortality. So is it?
Before I begin, I offer the caveat that we have not actually analyzed these numbers since they are only now coming in, but I can give some impressions and some hypotheses. First let’s look at what’s been turning up. On some beaches, it seems that a particular species dominates among the carcasses. Ray Bosse, walking along Buzzard’s Bay in Massachusets, found eleven Canada geese on his beach on March 22. Ray’s beach does not typically turn up many birds. Compare these snapshots from our database for Ray’s beach during the winter of 2014 vs. the winter of 2015:
2014 was a fairly typical year for Ray–a few birds here and there. 2015 looks a bit different. Ray found no birds from November though the end of February, and then an uptick began, culminating in that big day on the 22nd. Compare Ray’s numbers with those of Warren Mumford, walking in Chatham, MA, a part of the Cape facing out toward Nantucket. Warren’s beach is a fairly reliable producer of dead birds, though not in huge numbers all at once. Then, on March 25th, he too saw a sudden influx, finding 17 dead birds on his beach.
These kinds of spikes draw our eyes, but are they reflecting current, ongoing mortalities, or something else? To figure that out, we need to look at just what kinds of birds were found on each beach, and in what condition they were in. When we do this, we find some differences between beaches. On Ray’s big day in Buzzard’s Bay (a fine title for a morbid children’s book about bird carcasses), he found mostly one species–Canada geese (CAGO). Warren, however, found a grab bag of different species, basically representing the usual species that turn up on his beach: eiders, White-winged scoters, and gulls. When we see so many different species, it does not rule out a disease process, or other common cause of death, but it makes it far less likely. Few causes would impact everything from American Black Ducks to Herring Gulls to Common Eiders. The other point to consider is the condition of the carcasses. Warren’s birds, in addition to being all sorts of species, are in varied states of decomposition and degradation. Some are very weathered, and almost mummified. Others are intact, and look fairly fresh. This tells us that these birds did not die all at the same time.
A weathered Common Eider carcass. Likely many months dead.
A much more recently dead juvenile gull.
We appear to be seeing accumulated mortality over time, but revealed to us all at once.
In Ray’s case, the geese are all at just about the same level of degradation, suggesting they died within a much narrower timeframe. Multiple specimens of one species, and all dying around the same time raises our eyebrows a bit higher. For this reason, we are planning to collect a few of these geese and perform necropsies on them. The condition of the carcasses will preclude any advanced diagnostics, but we can hopefully look for signs of trauma, and also assess the nutritional state of the birds. It is very possible that the cause of death in these birds was starvation. The harsh winter, one in which many of our sheltered bays (and not so sheltered ones) actually froze over entirely, and massive snowfalls this year, severely reduced the available grass forage for the geese. It’s our current, working hypothesis. Even if the birds died over the course of a few weeks or even a month, the cold and the ice would have preserved them fairly well. They may even have been on the beach the entire time, just concealed by ice and snow and only now becoming visible. This effect might be expected to be even more pronounced in the bays where dead birds were entirely prevented from washing up by the extensive ice sheets from late January through just a week or so ago. On ocean facing beaches, where open water persisted all winter, are these spikes due to birds that were dead on the beach but hidden by snow? Or are these birds drifting in from other previously ice-locked areas?
What even the larger bays looked like this winter. How many dead birds could be locked in that ice?
If the ice-imprisoned carcass hypothesis is correct, then these high numbers of dead birds on many beaches may represent weeks or months worth of mortality all released at once for the finding as the ice and snow rapidly melt. I look forward to seeing what we find in the CAGO from Ray’s beach to see if we seem to be on the right track in our line of thinking. Watch this space for more news.