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It is not uncommon to find dead monarchs lying in the sand along the beach here at Chincoteague National Wildlife Refuge on the southern tip of Assateague Island in Virginia. My data show that this occurs after days with winds from the W, WNW, NNW,or NW with wind speeds exceeding 15 mph.
On north Assateague, the tall primary dune is intact and protects the diverse interdune area behind it. The vegetation on top of and behind the primary dune provides a wind buffer for migrating monarchs. However, when the monarchs reach the recreational beach of south Assateague (where there are 961 parking spaces, horse-back riders, SUV beach lanes, etc) there is no longer a primary dune. The ocean has no barrier and during high tides or storms, the ocean overwashes the beach, goes over the road and inundates the salt marsh and bay. These overwash areas are barren of vegetation.
When the migrating monarchs reach these areas on days with the winds I described above, they cannot fight the wind and get blown out to sea by the thousands. Watermen have told me they have seen monarchs landing on their boats 9-15 miles out.
When the winds shift, either later in the day or the next day, the monarchs make it back to shore. I have stood on shore and counted them as they made landfall and headed toward the closest nectar source, usually seaside goldenrod.
If the winds don't shift, it is likely that thousands of monarchs die from exhaustion trying to make it back to shore. It is after days like this, at low tide, that I find their bodies on the beach. There was one such scenario one year early in my study when I saw hundreds of monarchs in the sand and I assumed they were dead. But on closer inspection I realized they were alive but their wings were waterlogged. Lifting them up and holding them in the wind, I was able to dry and release almost all of them. Once people on the beach saw what I was doing, dozens pitched in to help. When we were done, only a handful of dead monarchs were left on the beach.
Larry Brindza at Kiptopeke State Park at the tip of the Delmarva Peninsula, and Dr. Lincoln Brower did a comparison study on the abdominal fat content of coastal and inland migrating monarchs and found that the coastal monarchs had far less fat.
When Lincoln asked me why I thought that was so, my answer was that they are burning up their fat reserves every time they get blown out over the ocean and try to make it back to land. I have tagged monarchs just after they made landfall, and have documented the tagged monarchs nectaring on seaside goldenrod for a period of 5 days afterward. They stayed to refuel and re-build the fat they lost in the ordeal, even when windspeed and direction were perfect for migration.
That is why I return here every spring and fall to plant seaside goldenrod. I want to ensure that there will always be nectar for those exhausted monarchs just making landfall.