Ornithologists have long maintained that Northern
Mockingbirds are continual learners, adding new imitations to their songs year
after year. The assumption has been that imitations of different sounds
indicate the breadth of experiences a male has lived through, and that female
mockingbirds are drawn to the most experienced males.
The American
Ornithologists’ Union and Cornell Lab of Ornithology’s Birds of North Americamockingbird account, revised and updated in 2011, states, “A male’s repertoire
often contains more than 150 distinct song types which change during its adult
life and may increase in number with age.” The account emphasizes that individuals learn
new sounds throughout life, citing several studies, and cites both Don Kroodsma
(one of my ornithological heroes) and another researcher who listed four different
types of evidence consistent with song learning in mockingbirds: “(1) vocal
imitation in the laboratory of conspecific, heterospecific, and nonavian
sounds, (2) interspecific vocal imitation in free-living birds, (3) conspecific
vocal imitation among free-living birds, and (4) abnormal vocal development
under acoustic deprivation in the laboratory.”
The Birds of North America also mentions that
a minimum of 35%–63% of song types in a given spring repertoire occur again the subsequent spring; the rest are new (Derrickson 1985, unpubl. data). Finally, spring repertoire size (the total number of distinct song types recorded from an individual as determined from analyses of extensive recordings) increases with age (Derrickson 1987b).
The male mockingbird’s many different vocalizations,
indicating all the things he has experienced over his lifetime, are what win
him a mate, which is why I have often characterized the mockingbird as the
Othello of the bird world—Desdemona fell in love with Othello for his story-telling
about his adventures.
Now a recent study is questioning whether mockingbirds
really do keep learning new songs throughout life. I find myself in the uncomfortable position of questioning
that recent study, which was summarized on Cornell’s All About Birds website. So
far I haven’t been able to find the original paper, but I did find some papers
by the scientist, Dave Gammon of North Carolina’s Elon University. He’s made
one very interesting finding:
Mockingbirds mimic birds whose songs are similar in pitch and rhythm to their own vocalizations. "When a Tufted Titmouse sings, it already sounds similar to something a mockingbird would sing," Gammon said. The Mourning Dove is too low and slow, and the Chipping Sparrow is too high and fast.
That fills an important gap in our understanding of what
species mockingbirds are likely to mimic.
But then Gammon’s research gets a little dicey, unless
there’s more to it than the article notes. For six months, he broadcast eight
novel songs from four outdoor speakers on campus for two hours a day. Half were
recordings of birds that don’t live in North Carolina and half were computer
generated. More importantly, two of the exotic bird songs and two of the
computer-generated songs were similar in pitch and rhythm to
mockingbird-specific vocalizations. Gammon expected that the campus
mockingbirds would imitate the real and computer-generated songs similar to
theirs, but not the ones that were dissimilar.
In fact, they didn’t imitate any of the songs. Gammon combed
through hours of recordings from 15 adult banded mockingbirds from that year
and several years afterward and found not a single imitation of any of the new
songs, though on two occasions he did hear a Brown Thrasher of unknown age mimicking
one of the new songs. But it sounds like no mockingbirds at all picked up the
new sounds, including young ones.
I don’t know if this was because the sounds
came from loudspeakers rather than actual natural events or what, but I don’t
think his conclusion that older birds don't acquire new songs is supported, and for now I can continue to believe that at least some mockingbirds may indeed keep learning songs as they get older. But I'll keep looking for the original paper in case I'm wrong.