Hummingbird Migration

ON CATALINA ISLAND: Hummingbird Migration

Catalina Island is home to several hummingbird species, including the resident Channel Islands Allen’s hummingbird (Selasphorus sasin sedentarius) and the Anna’s hummingbird (Calypte anna). In other parts of California, the Allen’s hummingbird is considered a migratory bird, but the ones breeding on the Channel Islands have decided to call it home all year round. That is what sparked the interest of researchers David and Erin Rankin of the University of California, Riverside. They traveled to Catalina to study hummingbird diets to try to determine what these birds are eating and see if that gives clues to why they don’t leave the Island.

Their research, funded in part by the National Science Foundation, has been underway for four years, primarily focusing on Anna’s and Allen’s hummingbirds and observing foraging behavior. The Rankins catch these birds using a special trap that surrounds a hummingbird feeder. Once caught, samples are collected while the birds are measured and banded.

According to David, standard measurements include the length of the bird’s wings, bill and tail, as well as how much they weigh. For their research in particular, the Rankins also need to collect hummingbird feces samples for genetic sampling.

“We keep filter paper underneath the bird, then they do magical things in the lab,” David said.

Erin added that the technology was adapted from forensic methods used to get DNA off of cigarette butts. “Birds drink so much nectar that there is enough pollen in their feces that we can figure out what plants they’ve been visiting and what insects they’ve been feeding on.”

They found that during months when there aren’t as many flowering plants, the hummingbirds eat a lot of sap-sucking insects and other hemipterans (insects that have sucking mouth parts). Small arthropods, spiders and flies, provide a protein source throughout the year.

“It might be that they are preferentially feeding on these sap-sucking insects for extra sugar when there aren’t other sugary resources available,” said Erin, before adding that more research needs to be done before any determinations are made. “The fact that they are such diverse little predators of really tiny insects, particularly annoying midges or gnats, suggests that hummingbirds can be small biocontrol agents.”

Hummingbirds seem to love native sages, specifically the white sage (Salvia apiana), as well as the Catalina Island manzanita (Arctostaphylos catalinae). Samples also found that 4 native species of monkeyflower and lemonade berry (Rhus integrifolia) were among the varied hummingbird diet, as well as some non-native plant species.

“Catalina Island Conservancy has mapped out the plant diversity on the Island nicely,” said Erin. “We’ve been able to look at what plants are in the diet and get an idea of how far the individual birds are flying.”

Allen’s and Anna’s hummingbirds are both found in the same habitat and are similar in what they eat. According to David, sometimes the two species will be side-by-side defending territories, which is unique because in most cases one species ends up outcompeting the other, but on Catalina they seem to coexist. Their diets have a high overlap, but the Rankins hope to determine whether or not the two species are eating slightly different bugs and nectar, which may keep down competition. Sometimes the two species will be side-by-side defending territories,

The researchers are looking forward to at least one more trip to Catalina to continue their study.