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Wednesday, Feb. 16, 2000
Rambling after migrating bramblings
By MARK BRAZIL
The many seed-bearing plants of the temperate region, the grasses and the herbs, the trees and the shrubs, produce an enormous volume of seed each year. Typically of the natural world, a vast amount of effort is rewarded by very few successes. In the game of chance that is life, relatively few seeds survive to germinate, grow and ultimately replace the adult plants they were shed by.
Some plants invest in rather few large seeds, such as coconuts. Other species take the opposite strategy, producing great numbers of tiny seeds, such as the poppies with their pepperpotlike seed heads. For some, wind is the dispersal agent, while others depend on becoming hooked into the coats of passing animals. Still others must actually be eaten and pass through the guts of birds or beasts which carry them long distances.
Among the seeds that do finally reach the ground, only a proportion land in an area with appropriate soils and growing conditions, and even for those, growth is not assured. There are many accidents that can befall them -- too damp and they may rot, too dry and they won't germinate, and then there are the many seed-eating predators.
Insects and rodents are significant in their impact on seeds. Mice and rats in particular range widely across fields and forests consuming large amounts of seed. They can make massive inroads into seed that has managed to reach an appropriate germination site.
As a countermeasure, many plants put out enormous amounts of seed over a very short time, swamping the seed-eaters with more food than they need. Since the seeds are not available for long, the seed-eaters cannot become dependent on one species as a sole food supply.
Even more sophisticated an adaptation is the way many trees produce very different amounts of seed from year to year: A seed-eater cannot specialize in a single tree, because the amount of food available from year to year is not certain.
While in one part of a forest, then, certain trees may produce heavy seed one year and very little the next, in another part a different species may produce in just the opposite pattern. Seed-eaters in temperate regions, like fruit-eaters in tropical regions, tend to be highly mobile and generalized in their tastes. They travel widely in search of a good supply, feed up when they find it and move on again quickly as the supply becomes depleted.
Among the seed-eaters we can count the colorful brambling (atori, Fringilla montifringilla). This medium-size member of the finch family measures about 16 cm in length. A migrant, it arrives in Japan via Hokkaido each October; most just pass through the island and are gone by the latter part of the month, though a very few remain for the winter. I am not sure exactly why so few stay, but perhaps the temperature falls too low for them here, or the seeds they eat are likely to be covered with snow during the Hokkaido winter.
Bramblings also reach Japan by way of the Sea of Japan, crossing into Ishikawa and Niigata prefectures from late September until November, peaking in mid-October.
During the winter months bramblings keep together in flocks, sometimes of dozens, sometimes of hundreds of birds, wandering in search of food. I have often encountered them while walking in the forest, when a flock takes off suddenly from the ground and flies up into the branches in a great flutter, revealing their narrow white rump patches as they go. The males in particular are attractive with their black, gray and orange coloring, and as spring nears some assume full breeding plumage before they migrate north again.
Most of Japan's wintering bramblings begin to move north in late March, and by the time April is over almost all have returned to their Siberian breeding grounds.
The brambling's breeding range stretches from Scandinavia in the west through an enormous swath of taiga forest as far east as Sakhalin and Kamchatka. They build their small, cup-shaped nests of moss and grasses high in a fir, larch or birch tree. The males sing their simple, nasal songs from a prominent perch nearby, and the endless repetition of their notes early in the morning contributes a refrain to the dawn chorus. Interestingly, during the breeding season they change their diet completely, living for several months on insects.
Since the brambling breeding range is so vast, when all the birds from a given region gather and concentrate the flocks they occur in can be impressive. I have heard of massive flocks wintering in parts of Scandinavia, numbered in tens of thousands, but nothing I had read prepared me for the sight I saw last year in early May in Akan National Park.
I was scanning the sky for ravens a kilometer or so outside the hot-spring town of Kawayu, when I became aware of a continuous murmuring in the trees at the forest edge several hundred meters away. My mind was on ravens, and as I couldn't place the sound in the background it was some time before I really took notice of it. It was a vague, unformed noise, more like a blend of sounds than of individual calls, and yet loud and penetrating like the hum of a machine through a workshop.
Eventually I located the source, and was astonished to see through my telescope huge numbers of finches gathering in the treetops. By their flicking flight I suspected they might be brambling, but it was only when the flock began to cross the road that I could be sure; the white rumps and orange collars were convincing.
What was astounding was their number. Masses of them were swirling about in the trees, alighting and taking off again in a drone of noise. Part of the flock headed south across the road, taking many minutes to pass. There seemed to be thousands.
As the fringe of the flock neared me I could finally make out the "tweak-tweak" calls of individual birds. Another arm of the flock was moving off in another direction, toward the east, and it looked as if the massive gathering might split, but then they all swirled around again, joined forces and moved off deeper into the forest.
It was difficult to estimate how many birds might have been in the flock, and certainly I saw only a portion of them, because the flock was spread through the tops of the trees out of my line of sight, but they must have numbered in the tens of thousands. I had heard of vast numbers of these birds occurring in Japan centuries ago, and occasionally one sees flocks of hundreds, but this was as if all the birds migrating through Hokkaido had decided to do so in one flock.
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