Date: 9/29/19 12:01 pm From: Alvaro Jaramillo <chucao...> Subject: RE: [MBBIRDS] Monterey County vs. Santa Cruz County fall migration mystery
Migratory passerines do not need updrafts to migrate, good wind direction yes, but the updraft issue is not a factor. Also keep in mind that we have precious few migrant passerines on the coast itself. The volume of migrants here is very low compared to the volume one finds out east (head out at night to listen to nocturnal migration, nearly nil in many coastal sites). Also the Western migration system is quite different, with birds using shorter flights, and having a generally shorter distance from breeding to wintering areas compared to the East. Finally, it is key to separate what is a good vagrant trap as opposed to a migrant trap. The two can sometimes be the same place, and often are in the East, but in the west they can be different. The behavior of mirror image disoriented eastern vagrants is not going to be the same as that of a Western migrant that is a bit too far west of its regular route.
I bet there are very few western migrants crossing Monterey Bay because they are migrating inland, not on the coast. Vagrants on the other hand, they will be over the water or hugging the coast. It is the reason why I have seen multiple Tennessee Warblers in my yard, but no Hermit Warbler.
From: <mbbirds...> <mbbirds...> On Behalf Of Larry Corridon
Sent: Sunday, September 29, 2019 11:14 AM
To: Lee Jaffe <leejaffe54...>
Cc: Lisa Larson <lisafaylarson...>; Randy Wardle <wrwardle...>; MBB <mbbirds...>
Subject: Re: [MBBIRDS] Monterey County vs. Santa Cruz County fall migration mystery
At Cape May, the migrating birds do choose for the most part to to fly across the bay rather than fly around Chesapeake Bay as crossing the Bay is roughly 30 miles, about the same as Monterey Bay. However, to fly around Chesapeake Bay is close to 200 miles while flying around Monterey Bay is perhaps 45 miles (my guesstimate.) A highflying bird in Santa Cruz should, on a clear day, see that it can follow the coastline rather than fly 30 miles over cold water, which most birds would avoid as there are no updrafts to assist their flight.
Another factor at Cape May is that many birds choose to fly across the Bay after stopping to feed and rest before attempting the flight. Then, when a cold front from the Northwest brings favorable winds to help the birds, they choose to fly, which accounts for their extremely high bird counts on certain fall days. (In fact, birders around Cape May pay close attention the weather and “flock” there in great numbers when a Northwesterly cold front arrives.)
(Having just returned from birding at Cape May I can attest to the above, both from a class I attended by Clay and Pat Sutton, authors of “Birds and Birding at Cape May”, and from having the opposite of a Northwestern cold front. We had mild sunny days and although the birding was good, it was not spectacular with no large numbers of birds flying south.)
I really don’t know the fall wind patterns for our area, but they might play a part in when and where the birds choose to follow a migration route south.
Finally, I’ve not heard any reports of great numbers of birds spotted by pelagic boat trips flying over the water that would confirm most birds flying across the Bay during the fall migration.
That being said, Why would they stop on the south end of the Bay? From there, if they continue to follow the coast, it becomes very mountainous. Even turning inland would require flying over the coastal mountain range. Would a bird following the coast see the coming mountain range choose to stop in Monterey and Pacific Grove to rest and feed before tackling flying over mountainous territory?
I'm assuming that the rarities are traveling with, or at least following the same migration pattern as, less-rare birds. In other words, the rarities might be seen as a "marker" highlighting a more general pattern. Are there more birds showing up in Monterey than Santa Cruz overall? If so that might support Randy's and Lisa's theory about Monterey being the shortest route.