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Turning to the redstart, we find here a bird which leaves its winter quarters in North Africa after the main swallow and willow-warbler migrations have left that area, and yet by the time Denmark is reached, it has caught the main surge of both these birds. The redstart's rate of progress is 33 miles a day in the west, and 41 miles a day in the east. This bird also greatly outstrips the spring (as given by the 480 F. isotherm) from April 15th onwards. Once again, the movement northwards of these birds is probably correlated with food supply. Wood-warblers migrate in the spring at a rate of 22 miles a day in the west and 44 miles a day in the east, whilst the red-backed shrike, which seems to have a main migration trend from Asia Minor across the Balkans to England, progresses at a rate of 55 miles a day. Both the wood-warbler and the red-backed shrike are later migrants than the swallow and willow-warbler, and there is a tendency for the later migrants to have a more southerly limit to their northern breeding range, or in other words, the tide of their migration stops at lower latitudes than does that of the earlier migrants. This may well be due to the fact that the total amount of daylight is already adequate at lower latitudes later in the spring, whereas, earlier in the spring, the early migrants must go farther north to find the equivalent length of day. Since the northern tip of Norway is almost exactly at 700 latitude we can see that this latitude represents the highest at which European passerine birds normally breed. When the redstart reaches there on May 15th, it will encounter about 19 hours of daylight, whilst the wood-warbler, following the redstart about a fortnight later, will then encounter 19 hours of daylight farther south, at about the 6o° latitude mark. The wood-warbler range therefore tends to be no farther north than the 60° latitude. |
