When white-necked jacobin hummingbirds (Florisuga mellivora) are young, they sport blue heads and throats. When they grow up, males keep this shimmery, showy look, but females take on a drab appearance. Their heads turn olive green and throats become splotchy white and green. Not all females, though, researchers report. Some females sport bright colors to blend in with the guys. That helps them avoid bullying birds and may give them more time to feed.
Not keen on sharing, hummers chase other birds away from feeders. Jacobins are no exception. They’re very aggressive, says Jay Falk. Jacobins are “like the show-off jocks of the hummingbird world,” he says. An evolutionary biologist at the University of Washington in Seattle, Falk studies how diverse creatures evolve.
Falk was working at the Cornell Lab of Ornithology in Ithaca, N.Y., and the Smithsonian Tropical Research Institute in Gamboa, Panama. While in Gamboa, he and his colleagues studied jacobins there. The researchers found that about 30 percent of female birds boast bright blue noggins as adults. To figure out why, the team set up dummy birds at nectar-filled feeders. The researchers could then compare whether blue or green dummy females were more likely to get harassed.
Bullying birds always picked on green birds first, the scientists observed. Attacks came not only from jacobins, but also from other hummers. “These other species are paying attention to the differences between males and females,” Falk notes.
In another experiment, the team implanted tiny tracking tags into wild jacobins. Feeders detected the IDs, allowing the researchers to tally birds’ time at feeders. Brightly colored females got more time to sip nectar, the researchers found. That blue color may help those females fly under the radar and gain greater access to food. But it’s not clear whether this advantage extends to wild nectar sources.
- Figure A shows the likelihood of blue and green birds visiting a feeder with high-sugar nectar. How do these probabilities compare?
- Figure B estimates how often differently colored females visit feeders. Which birds visited feeders more frequently? What is the frequency for the two colors of birds? Roughly how much more frequently did blue birds visit feeders in this study?
- These graphs’ error bars represent 95 percent confidence intervals. A confidence interval depicts a range of values. This range, calculated based on how data are spread out, likely contains the actual value. So “95 percent confidence” means that 95 out of 100 tests would yield a result in the range. For figure B, the true frequency of green birds’ visits would lie roughly between 1 and 2.5 feeds per day. What is the 95 percent confidence interval for the blue birds’ visits?
- Look at Figure C. Which birds spend more time at high-sugar feeders? How do these values and their error bars compare?
- Which birds spend more time at low-sugar feeders? What does it mean that the error bars overlap?
- Why do you think the scientists used electronic tags to capture this data? What are other ways they could have observed birds’ feeding patterns?
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