Why so blue?
Days are getting longer, and the grays and whites of winter are fading. Montana is welcoming the return of spring, and along with it migrant birds. Mountain Bluebirds (Sialia currucoides) are one of the most vibrantly-colored birds that we see on their breeding grounds here in Montana. Bluebirds arrive late-March to early-April to start raising their young of the year. The brilliant UV-blue ornamental plumage makes this bird easy to spot in the green-less landscape of early spring. It’s easy to marvel at their beautiful blue coloration, but have you ever wondered why bluebirds are so blue? Sara Berk, a current PhD student at the University of Montana, has spent a lot of time thinking about why bluebirds are so blue.
Scientists have found that the blue color is a structural color (as opposed to a pigmented color) because it is produced by the interaction between light and a feather’s 3-D arrangement. However, bluebirds are sexually dimorphic, meaning that males (bright-vibrant blue) are different from females (muted blue and overall gray coloration). The blue color of males is a sexually selected trait. Through evolution, sexual selection acts on traits that increase mating success and favor the selection of the “blue trait” in bluebirds. In other words, bluer males have higher mating success and then pass on their genes. “Females pay attention to these sexually selected traits and can have either indirect or direct benefits,” Berk explains. A direct benefit increases female or offspring fitness during that same breeding season. For example, a male bluebird behaving aggressively to defend a nest territory could directly benefit himself, his mate, or his offspring.
Male bluebirds need to be very aggressive to acquire and defend their territories during the breeding season. Bluebirds are secondary cavity nesters, which means they don’t create their own cavities and instead choose to nest in existing cavities. They will use man-made boxes or cavities made by other species, such as woodpeckers. Therefore, each season the number of nest sites are limited. Males have to compete to acquire these sites not only with other bluebirds, but with other secondary cavity nesters such as Tree Swallows (Tachycineta bicolor), House Sparrows (Passer domesticus), and House Wrens (Troglodytes aedon).
How do you measure aggression in birds?
In order to test if bluer males were more aggressive, Berk set an experiment to examine this in detail. “I simulated territorial intrusions [in the nesting territory] to examine the relationships between coloration and male aggression,” said Berk. Male birds will attack other birds coming into the territory that they are trying to defend. Berk used a fake bluebird (using a preserved specimen) to simulate an “intruder” to test how aggressively a male reacted. Males would vary in their response, from simply monitoring the “intruder” within eyesight, to attacking the bluebird specimen and forcefully plucking out its cotton fillings. She then quantified the male behaviors by scoring each aggressive advance and found that bluer males were, in fact, more aggressive.
In addition to looking at how aggressive males were, she studied whether bluer males were associated with better territories. She assessed territory quality from low quality (bad for nesting success) to high quality (good for nesting success) and compared territory quality to male aggression. Competition for these territories continue throughout the breeding season. Bluebirds start nesting early in the breeding season at a time when they also competing heavily with other bluebirds for nest sites. Later in the season, birds like tree swallows become their main competitor and bluebirds are still working to defend their territories while feeding their young. Through a creative approach to a rather complex subject, Sara Berk found that blue coloration signals higher aggression to both interspecific (among bluebirds and other species) and intraspecific (just between bluebirds) intruders competing for nests, and that bluer males also defend higher quality territories. So in the case of Mountain Bluebirds, having bluer feathers is anything but sad.
*This work was performed as part of Sara Berk’s PhD research. She just successfully defended her dissertation under the advisement of Dr. Creagh Breuner in the Division of Biological Sciences’ Organismal Biology, Ecology, & Evolution program at the University of Montana. This research as well as her other chapters will be published in peer-reviewed scientific journals in the near future.