In many bird species, the migrants from particular breeding areas can winter over a wide span of latitude and daylength regimes. For example, Siskins Carduelis spinus breeding in the northern boreal forest of western Europe may winter anywhere between mid-Sweden and Morocco, a latitudinal span of about 30 degrees, and the same individuals may winter at widely separated places in different years (Chapters 17 and 18). The general pattern in such species is that return migration begins earliest from the most distant (most southern) parts of the wintering range, and latest from the most northern parts. This sequential withdrawal from lower to higher latitudes can be spread over many weeks. For example, in the Red-breasted Nuthatch Sitta canadensis in North America, the last birds withdrew from the southern parts of the wintering range, in northern Florida (30oN) as early as 21 February, from North Carolina (350N) by 24 March, from West Virginia (380N) by 28 April, and from Pennsylvania (4FN) by 15 May (Harrap & Quinn 1996).
Figure 12.4 The median dates (±5 days) of onset of spring pre-migratory fattening in White-crowned Sparrows Zonotrichia leucophrys gambelii wintering at different latitudes in western North America. Regression analysis indicates a delay of 3.3 days, on average, for every additional degree N of latitude. From King & Mewaldt (1981).
Progressive withdrawal in spring from a latitudinal span of 11 degrees was thus spread over a period of about 12 weeks, or 7.6 days later per degree northward. In wintering White-crowned Sparrows Zonotrichia leucophrys in western North America, the date of onset of pre-migratory fattening varied linearly with latitude (and hence with solstical daylength), averaging 3.3 days later for each degree of latitude northward. The start of withdrawal was thus spread over seven weeks from the 14 degrees of latitude involved (Figure 12.4, Table 12.2; King & Mewaldt 1981). Mean rates of fattening were the same in all areas, regardless of latitude.
In many bird species, wintering entirely within the northern hemisphere, the timing of spring fattening and departure is therefore latitude-specific and, at any one wintering latitude, fairly consistent between years. Regardless of any endogenous influence, therefore, individuals must react appropriately to whichever daylength regime they find themselves under at the time. This is consistent with the experimental finding that longer photoperiods advance migratory fattening and restlessness in captive birds (because before the spring equinox, daylength at particular dates is longer at lower than higher latitudes).
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