copyright Christopher Majka
all rights reserved. Reproduced from New Brunswick Naturalist, 1992
Few sounds bring as much joy to my heart as the song of the Winter Wren, with its exuberant, cascading warble ringing out in the springtime forest. As ornithologist Stanley Cramp (1988) wrote, it sings "as if [the] bird [were] trying to burst [its] lungs. "
The Winter Wren (Troglodytes troglodytes from the Greek meaning "one who creeps into holes") is one of fifty-nine species of wrens. Of these, fifty-eight are found exclusively in the New World. The one unusual member of the group, found from north-eastern Siberia through southern Asia, throughout Europe (including Iceland) and north Africa and in eastern and western North America, is the Winter Wren. This has over the years prompted some interesting speculation as to how and when and why this representative of a New World family travelled to the Old World. The departure point for this month's article is a recent paper by Donald Kroodsma and Hiroshi Momose in the Condor (1991, Vol 93: pp 424-432) entitled "Songs of the Japanese Population of the Win ter Wren."
Kroodsma and Momose have done some sophisticated analysis of Winter Wren songs from Nagano (about 150 km north-west of Tokyo). This little bird has what ornithologist C. Hartshorne (1973)1 has called "the longest definitely reiterated" song pattern among North American birds. They examine the different song types each male employs, the length of his songs and their complexity as measured by internal microstructure.
Those of us who are bird watchers, amateur or professional, know that bird songs are often useful in recognizing many species. As Robert Payne (1986)2 has pointed out, for many birds, including all non-passerines, songs are learned not from other individuals but are in some unambiguous way encoded in their genes. Therefore they are excellent indicators of avian phylogeny and evolution. Even for passerines, whose songs are often influenced by imitation and cultural evolution, there are some aspects of their songs which have a genetic basis. It is for this reason that the songs of the Japanese Winter Wrens provide a valuable clue which helps in unravelling the history of their distribution.
In other research done by Kroodsma (1980)3 he has shown that Winter Wren's in North America fall into two groups. The eastern population have songs of relatively simple organization with individual birds having a repertoire of only 1-3 songs. In the west Winter Wrens sing songs of greater complexity and each male may have a repertoire of 30 or more songs. One male in Oregon sang 66 different tunes! In Europe, by contrast, the birds sing 6-7 songs which are organized similarly to the eastern N.A. population but are slightly more complex. This pattern seems uniform from Iceland to Morocco. What, then, would the Japanese wrens show? Would they be similar to their European or western North American cousins?
The results of Kroodsma and Momose's analyses show that the Japanese wrens have songs almost indistinguishable from their European cousins: 6-7 song types of medium complexity. The conclusions they draw from this data are intriguing.
According to one of the great pioneers of avian zoogeography, Ernst Mayr (1946) 4, the wren family originated some time during the Tertiary era (beginning 65 million years ago) in tropical regions of North America. Sometime in the mid-Tertiary this group spread to South America and then perhaps in the late Tertiary or early Pleistocene (2 million years ago) the Winter Wren spread across the Bering Sea land bridge or across the Aleutian Islands to Eurasia. These birds then continued across the continent to reach Europe, finally ending up in Iceland -- not that far from their ancestral North American home.
On the basis of songs as well as other information, Kroodsma and Momose propose that there was an "aboriginal" stock of Winter Wrens similar to the eastern North American group from which the Eurasian birds descended. Subsequently, during one of the Pleistocene glaciations (ice ages) the western North American birds became isolated from the rest of the species and evolved separately from their parent stock giving rise to the current population with its unusually complex songs.
Interestingly neither Kroodsma and Momose nor Mayr believe that the spread could have occured in the opposite direction, namely from eastern North America to Iceland, Europe and then across Asia. From fossil evidence it seems that most of the zoogeographical traffic between New World and Old on the part of birds, mammals, plants, insects, etc. was across the Bering Sea. Amongst birds it appears that the Wheatear is one of the few to have come to North America via Iceland and Greenland.
There are still many unanswered questions. What selective advantage do the wide repertoire of songs of the western wrens confer to them? What is the exact relationship of the Winter Wren to the central American Timberline Wren (T. browni)? On the basis of song there seems to be a close affinity. Could further research elucidate this? There are five other species in this genus -- the House Wren (T. aedon), the Tepui Wren (T. rufulus), the Mountain Wren (T. solstitalis) the South ern House Wren (T. musculus) and the Ochraceous Wren (T. ochraceous). What evolutionary relationships might their songs reveal?
All this is wonderful food for thought and yet another example of how evidence of ancient events is embedded in the humblest aspects of our natural surroundings. Next time you hear what Donald Kroodsma calls "the pinnacle of singing complexity," reflect for a moment not only on the poetic beauty of its love song, but also on the remarkable ancestry of its notes.
On a tree by a river a little tom tit
Sang "Willow, tit-willow, tit-willow."
And I said to him, "Dicky bird, why do you sit
Singing Willow, tit-willow, tit-willow?"
-- W.S. Gilbert (The Mikado)