Upon their arrival in early to mid February, the orange-crowned warblers of Catalina Island found a landscape thirsting for rain. Only two inches of precipitation had fallen since the previous June (as opposed to Santa Cruz's six inches), making it the driest winter on record in southern California. Nearly two months have passed since their arrival and the warblers have yet to begin breeding activities. Sure, they have established territories and acquired mates, but they have not even started to build nests. We suspect this may be due to food supply.
Here on Catalina, orange-crowned warblers feed primarily on small moth larvae, which they find in the foliage of island scrub oak. This food item is currently available in old oak leaves, but later in the season, when they have nestlings to feed, they will require larvae from this year's batch of leaves. In mesic (moist) areas, pockets of oak trees have started to grow new leaves. Most of the oaks in our study area, however, are either barren or are covered in old, parched leaves.
This has left us wondering:
While they wait to breed, many orange-crowned warblers seem to be behaving rather oddly. We have observed them congregating in incredibly high densities in spots where oaks have leafed-out. For instance, on the morning of March 31 we observed 20 to 25 orange-crowned warblers foraging in the same large oak tree! We have found a few such areas of leafed-out oaks and in each of them the birds forage close together and only occasionally chase each other.
Although most of those birds are unbanded (the communal spots are outside our main study areas), we suspect they are males that were unable to acquire females, as there seem to be far fewer females than in previous years. So instead of continuing to sing on territories devoid of food, some unpaired males may have chosen to abandon their territories in favor of sharing a more productive site. It will be interesting to track warbler densities at these communal spots as more oak trees start to leaf-out, and when (if?) the breeding season gets underway.
The events of this year stand in stark contrast to those of 2005, far and away the wettest year since the study began in 2003. The oaks were fully leafed-out when the field crew arrived in early March, and the birds followed a similarly early schedule. Nesting began in mid February and continued into July, with many pairs attempting to raise two broods over the course of the season. The incredible productivity of the 2005 breeding season led to densely packed study plots the following year. In that year the birds weren't so lucky.
Despite the high densities, the 2006 field crew found fewer than half of the nests they found the previous year—a drop from 140 to 60. The preceding winter had been unusually dry, which led to a late spring and a shortened breeding season: nesting didn't start until mid April. From the looks of it, this year nesting may be even later. To find out how much later, we will continue catching warblers and monitoring their daily activities, anxiously awaiting the day when a female comes into view with a load of nesting material in her bill!