Saturday, September 15, 2012

The Waves Keep Rolling In

 Migration usually happens in waves and is not just one steady phenomenon.  These waves tend to follow the weather patterns as high and low pressure systems slide past from west to east.  Interestingly, on the East Coast fall migration tends to pick up after a low pressure system passes because lows spin counter-clockwise, which create north winds in their wake that act as a tailwind.  This probably also happens on the West Coast, but on the Farallones, bird waves usually arrive just as a weak low passes to our north.  This tends to create the ideal weather pattern for us - light southeast winds and high overcast skies.  The theory is that the birds heading south along the coast are flying above the cloud deck and thus don't realize that they are getting blown slightly off course and out over the open ocean.  At dawn, when they fly down through the clouds for food and shelter, they then realize that they are over the open ocean.  If the the height of the cloud ceiling is low enough so that the mainland is not visible (where the birds would rather be), but high enough so that the birds within 10-15 miles can find the islands, a nice fallout can occur.

Since our last post on 3 September, the bird migration has picked up significantly.  The 4th started out with fog, light winds, and only a few miles of visibility.  The poor visibility kept many birds from finding the island, but we did find one arrival that was quite unexpected, a Red Crossbill.  Although there are 57 previous records, the last time that one was seen on the island was in 1998.  The reason for this long gap in occurrence is because they are an irruptive migrant, meaning that they are driven south in large numbers in some years due to a scarcity of food on their breeding grounds.  As you can see from the photo, the bird is not red, but yellow - only the males are red in this species. 

This bird lasted long enough for us all to run outside and photograph it.  As soon as we opened up the mistnets to attempt a capture, it flew off to the north, and we did not see it again that day.

The weather on the following day, the 5th, was nearly perfect for a fallout, with light winds out of the southwest, cloudy skies, and only 10 miles of visibility.  I walked up to the lighthouse to start my shark watch full of expectation, not for sharks though, but birds for the top of lighthouse hill is where most wayward migrants first touch down. As I headed up, I could see a few warblers sallying off the lighthouse for insects, a sure sign that a fallout was happening.  When I got to the top, I found six species of warbler: Yellow, Townsend's, Hermit, Black-throated Gray, Chestnut-sided, and Wilson's.

Shortly after my arrival on the lighthouse, I noticed that there was another female crossbill on the lighthouse.  Since we hadn't banded the one the day before, I was uncertain whether this was the same bird.

However, it sure seemed that the other bird had departed the island.  Without proof, though, we would have to consider it the same bird since there were no intervening days between the observations.  Several days later, I was looking at my photos and noticed that the primaries (outer most feathers on the wing) showed considerably different amounts of wear.  The crossbill on the first day had extremely frayed tips to its primaries,whereas the crossbill on the second day had intact primaries with just a few nicks.

In addition to the above birds, we also found a few sparrows in the genus Spizella, such as Chipping, Clay-colored, and Brewer's.  During the fall, these birds are notoriously difficult to separate from one another, and for some reason on the Farallones, we always seem to get birds that are on the fringe of what is considered "normal" for a species.  On this day we found a Chipping Sparrow, normally very easy to identify, that we initially mistook for a Clay-colored.  The Clay-colored features that it has are the mostly gray nape, relatively strong lines bordering the throat, and general buffy coloration to the mantle and throat.  The features in favor of Chipping, though, were more definitive, such as the blackish line bisecting the eyering and running through the lores, a poorly defined median crown stripe, long wing (2 mm longer than Clay-colored), and gray rump (not visible in this photo).  Could this be a hybrid Chipping x Clay-colored?  Probably not, but only its parents will ever know for sure.

We also found a tricky Brewer's Sparrow.  This species is usually fairly easy to distinguish from Chipping and Clay-colored Sparrows because Brewer's have a full white eyering.  This one, though, had a dingy grayish-brown eyering.  We identified this as a Brewer's through a process of elimination.  It could not be Chipping because it did not have black line through its lores and the rump was brown.  Separating it from Clay-colored was more difficult, but it did not have a distinct median crown stripe, the stripes bordering the throat were fairly weak, and the general coloration was cold, grayish-brown, not the warm, buffy brown usually associated with Clay-colored.  Unfortunately, all the measurements were intermediate between Clay-colored and Brewer's.  What species do you think this is?

The 6th of September was fairly calm, but unfortunately the mainland was clearly visible.  The highlight for the day was a Black-throated Sparrow.  Juveniles of this species are frequently misidentified as Sage Sparrows, but notice the full white supercilium.

September 7th was calm again, but the mainland was still clearly visible.  A few arrivals showed up, including an adult male Cape May Warbler.

The winds were blowing strongly out of the northwest on September 8th and the visibility was good enough for most birds that found themselves over the ocean to continue back to the mainland.  But a Painted Bunting decided the island was as good a place as any and cheered the islanders who saw it.

The 9th through the 11th were mostly clear, with strong to moderate northwest winds resulting in only a few arrivals.  This lull between the waves allows to birds to depart and continue south.  The only notable arrivals this period were a Yellow-headed Blackbird and a Baltimore Oriole.

The next wave began on the 12th, when we were greeted with overcast skies, 6 miles of visibility, and light southeast winds.  Warblers of eleven different species started filtering out of the sky about an hour after dawn, with a Bay-breasted, a Blackburnian, and an American Redstart leading the way and making quick friends. Multi-species flocks, with similar feeding styles, often band together during migration and on their winter grounds to help each other watch out for danger.

Several western migrants were present in the morning too including Wilson's and Hermit.

A few more eastern migrants arrived later in the morning to make for a great little wave.

While we all enjoying this bonanza of warblers, a posse of birders was bobbing around in Mirounga Bay off Saddle Rock hoping that the Northern Gannet would arrive.  Unfortunately for them, they arrived at noon, which was 3.5 hours before the bird arrived.  Thankfully they persevered, and we were able to get them on it as soon as it arrived.

The following day, September 13th, dawned foggy and calm, with few birds around, so we had a Waffle Morning.  The fog lifted after lunch and a few birds began arriving.  As Dan was walking to up to the lighthouse, he suddenly heard the distinctive "jip-jip" call of another Red Crossbill.  This one landed on Lighthouse Hill.  There are several "types" of Red Crossbills that breed in different regions, and each type has a unique sounding call.  We recorded this bird's calls in hopes of figuring out which type it is and where it came from.

The wave continued for one last day on the September  14th.  While eating my Trader Joe's O's, I received a phone call from ex-intern Matt Brady that there was a Yellow-throated Warbler on the lighthouse that he had seen with the webcam.  We all headed up the hill and sure enough, the island's 6th occurrence of a Yellow-throated Warbler was up there. This species breeds in the southeastern United States and is a very rare visitor to California.  In fact, the first record for California was only in 1969, and that record occurred right here on this very island.

When will the next wave happen?  Stay tuned to find out.


ohio girl said...

Very interesting post. I love all the detail and especially the picture of the Magnolia Warbler. Thanks,

John Blakelock said...

I agree. Really enjoyed looking through this. What an amazing collection of birds. Interestingly enough there was recently some Red Crossbills sighted up in Tilden Regional Park around the same dates as the ones on the Farallones. The last reported sighting was from the 16th. I missed them by a couple of days. I also came across your Ebird observations list when searching for Red Crossbills sightings.