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known with a fair degree of accuracy, since there are many people scattered over Europe who record the arrival and departure of the birds. From them we may be able to obtain sufficient data to enable us to make a map showing the rate of progress northward of certain species during the spring. This has recently been done for several birds of the summer migrant class, namely the swallow, willow-warbler, redstart, wood-warbler and red-backed shrike, by correlating and collating a vast number of observations of the time of the main arrival in spring of these birds at places all over Europe. Then lines joining points where the average date of arrival is the same (called isochronal lines=lines of " equal time ") are traced across a map of Europe and in this way we can get a series of lines showing the rate of spread of a species northwards in the spring. Such a map is that in Fig. 1, where the spread of the willow-warbler over Europe is shown. The heavy black lines, with dates on the right-hand end, show the position of the main tide of the birds at fortnightly intervals. The birds reach Gibraltar about the 5th of March, and they reach the northern tip of Norway by June 1st. They have therefore covered approximately 2500 miles in 88 days, or a daily rate of movement northwards of 29 miles a day. It will be noticed that, latitude for latitude, the arrival of the birds is earlier where the spring comes sooner, as on the western sea-board of Europe, but that the rate of spread is quicker where the actual rate of advance of the season is most rapid, as in eastern continental Europe. This is merely a reflexion of the fact that spring " comes with a bound " in those countries with a continental climate. We have to decide on a criterion to tell us when the spring has reached a given place, and we do this by considering the date at which the average temperature, over a number of years, reaches a given value. Lines joining places on the map whose mean temperature (corrected for elevation above sea-level), are identical, are known as isotherms (equal temperature). If we draw in on the map the isotherm representing 480 Fahrenheit, as has been done with the map of Fig. 1, by using dotted lines and labelling the dates on their left-hand ends, we see that the general shape of the isotherms agrees very well with the |
