Thursday, April 24, 2014
By Herb Wilson
In today's column, I'll provide a summary of some recent scientific articles on birds that can be found in Maine. All of these articles appeared in the most recent issue of the Wilson Journal of Ornithology.
Christina Masco describes research she did in Maine on Appledore Island about interactions among nesting Great-blacked Gulls. These gulls nest at high density and a pair aggressively defends its small territory (about 350 square feet) throughout the breeding season. Real estate on Appledore is hard to find, so most territories abut several other territories.
If an intruding gull enters the territory of another, aggressive interactions ensue. But some of these fights are avoided by the use of threat displays, warning another gull that it should leave or prepare for a fight.
One of these threat displays is a distinctive vocalization that sounds like "yeow." This call tells an adjacent bird that an attack is imminent. Masco used play-back experiments to determine if Great Black-backed Gulls can recognize individuals based on their "yeow" calls. She found that a territorial gull will hold alert postures longer in response to unfamiliar "yeows" compared to the "yeow" given by a mate or neighbor.
This finding is an example of the dear enemy phenomenon. Animals get used to adjacent enemies and tend to respond less vigorously to them than to strangers.
THE EASTERN WOOD-PEWEE is a fairly common breeding bird throughout our state. Felicity Newell and colleagues wrote an article describing surprising reproductive traits of this species in Ohio.
Before this research, ornithologists had presumed that Eastern Wood-Pewees were monogamous, with a pair-bond cementing one male and one female together. Newell and colleagues present research that challenges this assumption of mate fidelity. They color-banded 79 pewees with unique color combinations. Much to their surprise, they found one male that was simultaneously feeding young at two nests. The same male was polygynous (mated to more than one female) the following year as well.
Polygyny is advantageous for a male because he can father twice as many young as a monogamous male. Polygyny can be disadvantageous for females because their presumed faithful mate is devoting half of his time to a different family.
ASIDE FROM HABITAT destruction, bird mortality from bird-window collisions represent the greatest human-associated source of bird deaths. Bird deaths from window collisions number in the billions.
Dan Klem of Muhlenberg College has been studying bird-window collisions for over 30 years. Klem and Peter Saenger wrote an article in which they evaluate the effectiveness of selected visual signals to deter bird-window collisions.
Their work was based on a carefully controlled experiment where they placed wood-framed picture windows in an area of mowed pasture in Pennsylvania. They placed a platform feeder 10 meters from each window.
Experimental treatments included using clear glass, reflective (mirrored) glass, ORNILUX Mikado glass, and Acopian bird savers (vertically hung parachute cords spaced three or four inches apart). The ORNILUX glass reflects more ultraviolet light than other types of glass, as well as the Acopian bird savers. Unlike humans, birds can see a portion of the ultraviolet spectrum.
Klem and Saenger recorded the number of bird collisions (some of which were fatal) with the picture windows equipped with the three different types of glass. They also tested the effectiveness of the Acopian bird savers.
In one experiment testing the three types of glass, no difference in total collisions was found. However, fatalities were lowest against clear glass (10 percent) and highest against the ORNILUX glass (58 percent). The ultraviolet-reflecting glass had the opposite effect expected.
A second experiment showed that parachute cords, hung outside of either clear or reflecting glass, were very effective in preventing collisions.
Visit www.birdsavers.com/ to learn more about Acopian bird savers.
Herb Wilson teaches ornithology and other biology courses at Colby College and perches in South China. He welcomes reader comments and questions at: