On Thursday evening, July 26, I headed down to Montrose Beach--Chicago's most famous birding beach--after work to look for a couple of uncommon shorebirds--American Avocet and Willet--that had been reported there earlier in the day. The avocet was easy to find, but the Willet was nowhere to be found. Eventually I found it among a patch of weeds, dying. It died overnight; the next day I brought it to the museum (where we checked it for parasites and prepared it as a study skin) and started wondering: how did it die?
On Saturday, I learned that a Montrose volunteer had found and picked up a dead Lesser Yellowlegs on the beach. Another dead shorebird at Montrose. I picked it up from him; like the Willet it showed no external signs of trauma. Sunday evening I had a chance to get back to Montrose Beach. I found a dead Sanderling. A third dead shorebird. Something was definitely fishy.
I was lucky to run into the retired waterfowl biologist Bob Montgomery yesterday, who suggested that botulism may be the culprit. He had seen a video of the Montrose yellowlegs feeding on maggots, which in turn were feeding on a fish carcass that had washed up on the beach. If the fish died of botulism, the toxin-producing bacteria could be transferred through the maggots to the birds that ate them (like in the photos below, which I took at Montrose on Thursday), according to the National Wildlife Health Center. Fishy indeed.
On Thursday there were about twenty migrant shorebirds at Montrose; only three were juveniles. Yet two of the three shorebirds that have been found dead were juveniles; perhaps they have weaker immune systems than adults? Given how many of the shorebirds were feeding around the dead fish last week, it seems likely also that more birds were killed by botulism at Montrose, but we didn't find the carcasses.
The good news is that this threat to the local shorebirds has passed, at least for now. The fish are now meatless; without meat there is no more botulism-contaminated food source for the migrating shorebirds. But it is something to be alert for in the future.