This year's field season on Santa Cruz Island is off to a great start, and the birds are breeding despite the dry weather. We arrived in early March and found that territories had been established and the warblers were paired. Since then, some of the territorial boundaries have shifted a little, and new birds have arrived on the plot.
Unlike long-distance migratory birds, which start breeding right after the females arrive on the breeding grounds, the sordida subspecies of orange-crowned warblers have a fairly long breeding season. The long season gives them a little more flexibility in the timing of breeding than most other temperate birds. This year has been much drier than usual, and they didn't seem to be in much of a rush to get started! Even more interesting, we've had birds build nests then wait up to six days before laying the first egg. We're not sure why they would wait so long, except that food availability might be lower than usual since it's been such a dry year.
We first saw females gathering nesting material in mid-March, and more females have built in the past two weeks. We've noticed that they like to gather material to build their nests in native island buckwheat bushes, which have recently recovered since sheep were removed from the islands. The earliest pairs are incubating eggs, which should hatch in about a week. As of late March, we've found seven nests, but one already got eaten!
Warbler nests can be predated by snakes, small mammals, or the island scrub-jay, which is only found on Santa Cruz Island. These scrub-jays are bigger than the ones on the west coast, and are very long-lived. The scrub-jay breeding season is also getting started; we've seen jays carrying nesting material to build their nests, and more recently we've seen mate feedings between scrub-jays.
Our research is focused on how the presence of jays affects orange-crowned warbler reproductive behavior and success, so we're going to compare nest predation rates on islands with (Santa Cruz) and without (Catalina) the jays. We're also interested in how the presence of the jays affects behavior around their nests, and so far we've learned that the females on Santa Cruz are super sneaky! Besides behavioral differences, this breeding season has been interesting since the warblers on Santa Cruz are breeding while the warblers on Catalina haven't started yet.