Researchers at the University of California Davis (UC Davis) are collecting data regarding feeder visitations by hummingbirds, with the help of RFID readers stationed at bird feeders and 134.2 kHz LF RFID tags embedded in the backs of the small pollinators. By identifying when specific birds visit particular feeders, and in the company of which other birds, the researchers are gaining an understanding of the contact activity and network of hummingbirds at feeders and how it might related to disease transmission.
Traditionally, hummingbirds—some of the world’s smallest birds—have been tracked via leg bands, but those can require that a bird first be captured with a net so researchers can physically examine the band. That makes RFID a good alternative, the team explains.
The study, conducted by the UC Davis School of Veterinary Medicine and the university’s EpiCenter for Disease Dynamics, is not the first to use RFID to track hummingbirds, says Lisa Tell, an avian veterinarian at the UC Davis School of Veterinary Medicine and the director of the Hummingbird Health and Conservation Program. However, she believes it is the first to solve a problem that faced previous studies: how to track more than a single bird at a feeder at once, using LF RFID tag reads.
“RFID has been used before at feeders to monitor the presence of hummingbirds,” Tell says. A reader can detect an LF-tagged bird at a distance of about 6 inches. In fact, because hummingbirds are so small, RFID is a technology well suited for tracking them. GPS units, Tell explains, would be too heavy for the birds to carry. However, the miniaturized version of animal tags, known as passive integrated transponders (PITs) and commonly used to track pets, can be carried by a hummingbird without impacting its flight. The devices weigh approximately 0.01 gram (0.0004 ounce) and measure 8 millimeters (0.3 inch) in length.
Attaching the tag non-invasively has been challenging for researchers in earlier studies, Tell says. They have tried adhering the tags to the birds’ back via eyelash glue, as well as inserting them under the skin and then sealing the skin with glue. However, the glue can be hazardous for hummingbirds if any trace gets onto the animals’ wings. Therefore, the UC Davis researchers embedded the tags under the birds’ skin on their dorsal area, then closed the incision via a single stitch. The team tagged 230 birds of two species—Anna’s hummingbirds and Allen’s hummingbirds—in this way.
Developing a system to track more than one bird at a time with the RFID reader posed another challenge. LF readers can interrogate one tag, Tell explains, but when a second one is concurrently introduced, the devices typically only read one of those tags. This presents a problem since hummingbirds often feed in groups, potentially five at a time.
UC Davis solved this problem by using Biomark HPR Plus RFID readers that interrogate every tag ID number every 10 seconds, rather than continuously interrogating transmissions from the nearest tag. In that way, as each new bird arrives at the feeder, the reader captures that animal’s ID, then seeks and captures another read of that specific ID 10 seconds later. It can conduct this 10-second interrogation for numerous tags, thereby eliminating the problem of reading more than one specific tag at a time.
In 2016, the UC Davis researchers began tagging the birds, and installed RFID-enabled feeders at three sites in California: two in the northern part of the state and one in the southern portion. Each site had between one and three RFID-enabled feeders, for a total of seven. The standard, off-the-shelf feeders are each surrounded with a netting material that includes a single entrance where birds pass into the netting area in order to access sugar water. The reader antenna is built into the entrance. The reader is powered either by a wall outlet connection or batteries. In the long term, Tell notes, the system could use solar panels for power.
When a bird enters the feeder, and for as long as it remains there, the reader continues to capture the tag’s unique ID, which is linked to details about that bird in a metadata file. Researchers analyzed the data harvested from the readers to identify which bird was at which feeder, along with which birds were there together. “If the bird is anywhere near the antenna, we can read the tag,” Tell states. The collected data is stored in the reader until a researcher goes onsite and uses a USB drive or Bluetooth connection to download that information.
Between September 2016 and March 2018, a total of 118,017 tag reads were captured. The goal, according to Tell, is to continue monitoring feeders going forward. The birds were tagged more than two years ago, though the life expectancy for a hummingbird can be considerably longer than that, and there will be no effort to remove the tags.
In the future, the team would like to find what Tell calls a “consumer-affordable” lower-cost reader so that the units can be installed in greater numbers—even in backyard feeders, to enable individuals to track the activities of hummingbirds in their own yard. Researchers also imagine eventually using a tag and a reader with a longer range to identify birds at wild locations, such as on flowers during pollination. This need for a very small tag has made that challenging, Tell reports.
The researchers are continuing to learn about hummingbird activity at feeders, Tell says, based on the data collected during the tag reads. The goal of the existing study is not to prove that bird feeders are a benefit or detriment to the health of hummingbirds, she notes, adding: “This study won’t specifically answer whether feeders are good or bad. We’re interested in trying to determine ways to look at feeder usage and identify best practices for those using feeders, as well as look at how hummingbirds congregate.”
The study, however, is helping the researchers to gain an understanding of how and when the birds feed. While Anna’s hummingbirds have been previously tracked via RFID, Tell notes, this is the first time that Allen’s hummingbirds have been, due to their smaller size. Thus far, the researchers have found that males are more likely to visit feeders at the same time as other males, rather than with females. Individual hummingbirds also visited on specific feeder more often than others. Night activity was discovered, too, as several birds fed throughout the night.
The study is ultimately aimed at determining feeder visitations by hummingbirds that would further allow scientists to evaluate disease transmission, based on the visitation patterns of hummingbirds at feeders, as well as examining how bird behavior may be changing and what those changes might mean for the animals’ health. In fact, the researchers can ascertain whether some birds remain in the area. The team’s future goal is to evaluate which conditions influence resident populations, including their genetics or environmental factors in the area.
Approximately 61 percent of the birds tagged were detected in the system based on an RFID tag read at a feeder. The others may have moved on to feed in another location, Tell explains. She says the group hopes for partnerships with RFID technology providers or consultants, to help them modify and enhance the capabilities of an RFID system for the hummingbirds.