North they go

It is that time of year again. Across the northern hemisphere the poleward procession of the snow-line is giving way to a swarm of migratory birds. We are reminded of that fact every time we see a bird, or hear a sound, that seems out of place. Migrants have been passing through Catalina since mid-April and have been doing so in waves. We had one such wave today, bringing with it Townsend's Warblers, Hermit Warblers, Black-throated Gray Warblers, Western Tanagers, and plenty of other species that I'm sure we missed. Over the years we have caught many of those migrants and, before releasing them unharmed, have had them pose for a few photos.

Hermit Warbler

Warbling Vireo

Townsend's Warbler

Wilson's Warbler

Orange-crowned Warbler

Ok, so I know what astute readers out there must be thinking: "Aren't you there studying a breeding population of orange-crowned warblers? Why is that last photo lumped in with the other migrants?" Well, believe it or not - on this same day last year - we caught a few orange-crowned warblers of the Alaskan/Canadian race that were migrating through Catalina (the above bird is smaller and has duller plumage than our local breeders). We don't know where those birds ended up, but they do serve to highlight just how delayed the northern breeders are. Here most orange-crowned warblers have raised a batch of young (i.e. a brood) by early May, but the Alaskan birds have just started to make their way north. In a few short weeks we will catch up with some of those birds in Fairbanks, and will start studying them just as their migratory journey ends and their breeding season begins. Until then, we will continue to enjoy the passing of migrants on Catalina Island.

To breed or not to breed?

Wow, it sure has been a while since I last posted to this blog! Sorry to anyone who was anxiously awaiting news of the warblers on Catalina Island. Assuming such a person exists, I can only imagine they had two options running through their head while pondering my post-less blog for the past two months: (1) the birds aren't breeding (again) and she has nothing to write about, or (2) the birds are breeding and she is just too darn busy. If you chose option number 2 - ding ding ding - you are correct. The birds did breed and we have a mountain of data to show for it.

We started the field season wondering when the birds would breed. Last year, after the driest winter on record, only a few of them did and we had to wait until mid-April to see that happen. But this year the island received a few inches of rain in the months that preceded our arrival, and some key storms in late February led to an immediate greening-up of our study site (see photo above). The oaks leafed-out and the birds quickly followed suit, building nests, laying eggs, and providing us with an actual breeding season as opposed to last year. Needless to say that made us happy, and busy at the same time!

I am used to migratory birds, where some birds arrive, you find their nests, and then other birds arrive, and you find their nests. But in this system the birds over-winter on the Channel Islands and the nearby mainland, so they had come to Catalina and had established territories weeks, if not months, in advance of the initiation of nesting activities. So when the environmental conditions were right, they all starting building nests at once! At one point it seemed like there were building females everywhere I looked.

The incredible synchrony that we observed this year was unusual compared to previous years, and created a situation that kept us on our toes for the next few weeks. We found most of our nests during the incubation period, either by following chipping females or by flushing them off their nests. This was a busy time, as we had many nests to find and only 4 of us on the ground trying to find them. Once we found a nest we had to carefully check it every 2 days to see if the eggs had hatched, and to monitor whether it was still or active or had been found by a nest predator.

On Catalina, there are quite a few nest predators in the form of snakes, foxes, mice, and other creatures. But the ones that seem most troublesome from the warblers' perspective are the snakes. For instance, so far this year we have positively identified two predators eating warbler young and both were of the reptilian variety - one a gopher snake and the other a california kingsnake. We started seeing snakes early this year. I spotted my first rattlesnake on March 9th (see photo) and proceeded to see many more in the ensuing weeks (one of which was far too close for comfort). This, after having gone an entire season without seeing a rattlesnake last year. Despite this, the birds had an exceptionally low rate of nest predation this year. Only a few of our nests were eaten in the month of March, and only in the past few weeks have predators been apt enough to find our warbler nests.

Lucky for the birds, and lucky for us. One of the things we are interested in is parental feeding behavior, and we study this by setting up a small pin-hole camera at nests and recording for an entire 24-hour period. But we can only do this if the nestlings survive long enough to be 6 days old. This year, many of them did and we were left running around video-taping nests and, on the following day, taking nestlings out briefly to weigh and measure them.

Through all this we had help from many people. In addition to Jongmin and myself, we had two field assistants actively searching and monitoring warbler nests. We were also fortunate to have our advisors - Drs. Cameron Ghalambor and Scott Sillett - visit in early March, a postdoc - Dr. Brent Horton - visit two times in March and April, and a labmate who is working on Santa Cruz Island - Helen Sofaer - visit in April. They helped to identify birds, to capture and band them, and of course to find their nests. It was truly a team effort.


So what are we up to now, you may ask? Well, believe it or not some of the birds are going a second round. They bred early, experienced an exceptionally high level of nest success, and still have the time (and food) needed to raise another batch of young. So they are giving it a shot at least. But it remains to be seen how successful this attempt will be. The predators seem to be picking up steam in terms of their nest-finding ability, so we are not expecting an incredibly synchronous (or busy) nestling period this time around. Stay tuned to find out!