Coastal Louisiana

Scott Sillett and Brent Horton conducted a week of scouting in southwestern Louisiana, where they looked for wintering orange-crowned warblers. They were guided by Robert Dobbs, a local bird expert. Although about a dozen warblers were seen, none were captured because the bulk of the wintering population had not yet arrived.

At a later date, they will attempt to capture the warblers in order to determine where this migratory species breeds by analyzing stable isotopes in their feathers. They also want to collect blood samples to analyze levels of corticosterone, an avian energy-regulating steroid that promotes behavioral and physiological adjustments for coping with energetic challenges and environmental stressors. In addition, they hope to individually mark birds so that their survival might be monitored.
researchers in scrubby habitat

The scientists found several promising field sites in the coastal woodlands, or cheniers, of Cameron Parish. The chenier woods are recovering after being heavily damaged by Hurricane Rita in September 2005. The outlook for orange-crowned warblers is uncertain there, but it is possible that the dense regenerating vegetation is to their liking.

The most promising field sites contain hackberry trees and live oaks growing on the remains of old sand dunes. This habitat is very important for migratory birds as it is the last stop before migrating over the open expanse of the Gulf of Mexico. It is also ideal wintering habitat for orange-crowned warblers.

Catalina Island Update

Fierce wildfires in late spring swept across the island and defoliated most of the trees. Although the study site, a well-protected canyon, was mostly untouched by the conflagration, few warblers were enticed to nest. This nesting season appears to be a complete wipe-out, with likely no young warblers surviving to fledge.

Catalina Island

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.

caterpillars lined up next to a much larger pair of tweezers

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:

• Are the orange-crowned warblers waiting for the oaks to leaf-out to start breeding?
• Could this visual cue serve as a reliable indicator of food supply during the nestling period?

warbler habitat, stunted growth on hilly terrain

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!

Santa Cruz Update

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.

Orange-crowned Warbler Study

Scott Sillett, researcher at the Smithsonian Migratory Bird Center, received a $20,000 grant from The Nature Conservancy (TNC) to study the endemic and declining Vermivora celata sordida, a subspecies of the orange-crowned warbler, on the California Channel Islands (Santa Catalina and Santa Cruz).

The project will compare the population biology of warblers between the two islands, with particular focus on the response of warblers to the islands’ understory vegetation.

The picture below shows orange-crowned warbler habitat on Catalina Island. Note the lush vegetation on the ground due to the lack of feral pigs—the warblers often build their nests on the ground where the vegetation provides camouflage from predators.

Feral pigs and goats were removed from Catalina several years ago. As a result, the understory on Catalina is regenerating and warblers reach incredible population densities in some moist canyons on the island.

Although cows and sheep have been removed from Santa Cruz, about three to four thousand feral pigs remain and the forest understory is severely degraded. The Nature Conservancy initiated a feral pig removal program in March 2005.

We will collect one season of warbler data on Santa Cruz this year, prior to pig eradication (TNC won’t be removing pigs from study areas until after the warbler breeding season).

Warbler demography, habitat selection, and behavior will be compared to data from Catalina as well as data we collect in subsequent years on Santa Cruz as the understory recovers.