The Daily Yomiuri, Japan – 12 hours ago
The Yomiuri Shimbun
A private group that observes swallows has discovered the birds begin building nests simultaneously in three regions each year, a finding that challenges the established theory that their nest-building areas move north as the temperature rises.
The regions are in eastern and western Japan divided by a line connecting Chiba and Niigata prefectures, and Kyushu. Until now, it had been assumed areas where swallows begin building nests go northward across the Japanese archipelago as temperatures rise, and a line called the “swallow front” would move northward gradually.
However, findings of the group, Tsubame Kansatsu Zenkoku Network, or national network of swallow observers, might pull the rug from under this theory.
The network examined the times and places where the swallows began building nests each year. About 630 cases over three years were reported to the network.
According to the network, swallows begin building nests in Kyushu in the middle of February. In the region west of the line between Chiba and Niigata prefectures, nest-building begins between the middle of March and the first half of April. In the region east of the line including Hokkaido, nest-building begins about one month later, or in the first half or latter half of April.
A nest-building front was not observed in any of the three regions. There does not seem to be a relationship between nest-building time and temperatures that influence the emergence of insects the swallows eat, the network said.
“There hasn’t been any reliable nationwide research on the issue. It would be interesting if the [swallow] front doesn’t exist,” said Assistant Prof. Go Fujita at Tokyo University’s School of Agriculture and Life Sciences.
