In the first half of the twentieth century, much attention was given, with the help of observer networks, to recording the northward advance of various bird species in spring (Figure 14.7). In general, earlier migrants took longer over
(/cont.) than those from point localities, such as bird observatories. In species which undergo marked changes in numbers over the study period, first dates tend to be earlier in years of greatest abundance, possibly because of the statistical effect on the chance of observation (Loxton & Sparks 1999, Sparks 1999). The same points apply to last observations in autumn. Because by definition first or last birds are atypical, a more representative picture is obtained by using median or mean dates, such as those based on captures at bird observatories, but this greatly reduces the numbers of localities from which such records are available.
the journey, and spread north at a slower rate per day. Five African-European migrants, namely the Barn Swallow Hirundo rustica, Willow Warbler Phylloscopus trochilus, Common Redstart Phoenicurus phoenicurus, Wood Warbler Phylloscopus sibilatrix and Red-backed Shrike Lanius collurio, arrived in southern Europe on the progressively later dates of 13 February, 5 March, 15 March, 1 April and 1 April respectively. They spread north through western Europe at average speeds of about 40, 46, 66, 70 and 88 km per day respectively, and took 109, 88, 61, 45 and 45 days to get from the southern to the northern parts of their breeding ranges
(Southern 1938-41). Their northward progression generally kept step with particular isotherms (different isotherms for different species), but like the isotherms, their migrations tended to accelerate with distance northwards. Of these species, the Barn Swallow bred over the widest latitudinal range, and its period of spread over this range (109 days) more or less fitted expectations from the different measures of the northward advance of spring conditions, mentioned above.
More recent estimates of migratory progression, based on first arrival dates at different localities within Britain, gave mean arrival dates in the south of the country of 22, 26 and 19 April and 14 May, and mean rates of progress of 42, 43, 52 and 145 km per day for Common Cuckoo Cuculus canorus, Common Nightingale Luscinia megarhynchos, Barn Swallow Hirundo rustica and Spotted Flycatcher Muscicapa striata respectively. Yet again, the last species to arrive made the fastest progress (Huin & Sparks 2000). Remember that these rates of spread refer to the dates that birds first appeared in successive localities, and not to the movement speeds of individuals, which can be faster, as revealed by ringing (Chapter 8).
Similar trends were found in North America (Lincoln 1935a). In some species, such as Canada Goose Branta canadensis, the birds push north 'on the heels of winter' and keep step with the 35°F (3°C) isotherm, as advancing warmth melts the ice on lakes and rivers and creates bare ground for feeding (Lincoln 1935a). In other species, such as the Blackpoll Warbler Dendroica striata, the northward movement occurs much later in spring and much more rapidly, often with increasing rapidity towards the northernmost breeding areas (Figure 14.7). Mean rates of advance in different North American species were found to vary from about 30 km per day in the earliest migrants to 300 km per day in the latest, towards the ends of their journeys. This again reflects the facts that the rate of spring warming gets progressively more rapid with advancing date and increasing latitude.
These relationships with isotherms presumably hold because temperature gives a good indication of the date at which each area normally becomes suitable for the species in question to settle. The fact that the migratory advance is slower in colder springs is further indication of the importance of the development of a food supply. It does not necessarily mean that the species migrate in direct response to temperature, or that individual migrants travel north at the speed of 'isothermal lines'. Rather the average rate of northward spread keeps step with particular isothermal lines, different lines in different species, in accordance with the development of their particular food supplies.
The advance of species from lower toward higher latitudes each spring is therefore linked with warming conditions, and the ecological changes they set in train. For example, the northward advance of geese in spring is closely linked not only to ice melt, but also to plant phenology. The birds arrive at successive latitudes just as plant growth begins (usually above 3°C), and follow a 'green wave' northwards (Owen 1980). After this spring flush, food quality and digestibility decline as the plants age, but on their well-timed northward journey, geese are able to benefit from successive latitudinal peaks in plant growth and digestibility. From each site, most birds leave as the food passes its peak, and arrive at the next site as food reaches its peak. The geese therefore move according to the phenology of their food plants (van der Graaf et al. 2006, Hubner 2006, Hupp et al. 2006). Unlike geese, which feed mainly on land, swans obtain most of their food from water, so also track the melt line, as it advances northward. Naturally, in many species the speed of migration varies from year to year with the temperatures encountered en route. This was shown, for example, for 15 species studied over a 40-year period in North America (Marra et al. 2005). By comparing the passage dates of these species between a trapping site in coastal Louisiana and other sites 2500 km to the north, birds were found to cover this distance most rapidly in the warmest springs. Species that nest at high latitudes are in some years detained some distance short of their breeding areas, as they wait for a thaw to set in and expose their food.
If one watches passage migration at low-latitude localities, species are found to differ not just in their mean passage dates but also in the spread of their passage dates, with some taking much longer to move through than others. Typically, species with short passage periods are those that breed over a narrow span of latitude (with relatively small breeding ranges), whereas those with long passage periods breed over a wider span of latitude, giving a wider range of dates at which different localities become fit for occupation (for shorebirds see Nisbet 1957, for raptors see Leshem & Yom-Tov 1996a).
With increasing latitude, as the annual warm season becomes shorter, many species spend progressively shorter periods in breeding areas, arriving later in spring and leaving earlier in autumn. The first individuals to arrive at successive locali ties in the breeding range are normally those that nest there, these settlers being followed by others destined for even higher latitudes. By the time when individuals arrive at the highest latitude breeding areas, perhaps in late May or June, other individuals of their species at lower latitudes have already started nesting, and may even have young.
This low-to-high latitude settling pattern is particularly apparent in species in which different races breed at different latitudes. On passage migration, the different races move through in sequence according to the latitude at which they breed. For example, among Yellow Warblers Dendroica petechia migrating north through Arizona, the first birds to arrive in March and early April belong to the local breeding race D. p. sonorana. Races breeding further north do not arrive until late April, and the Alaskan race D. p. rubiginosa until May-June (Phillips 1951). Similar differences occur among populations of White-crowned Sparrows Zonotrichia leucophrys (Blanchard 1941), Swainson's Thrushes Catharus ustulatus (Ramos 1983), Yellow Wagtails Motacilla flava (Curry-Lindahl 1963, Moreau 1972), Blackcaps Sylvia atricapilla (Klein et al. 1973) and many others. Such temporal differences extend back along the migration route, and where several populations winter in the same region, those that nest at lower latitudes depart first, and those that nest at the highest latitudes depart last (Chapter 12).
Because of the shortness of the favourable season at high latitudes, populations that breed there remain for relatively short periods. This can be illustrated by the migration records from the Alaska Bird Observatory situated at 64° 50'N, where the average frost-free period each year spans only 105 days (Benson & Winker 2001). The six species of passerines that migrate there from within North America were present on their breeding areas for an average of 119.8 days (standard error (SE) 3.4 days), or 33% of the year. The 12 species of passerines that migrate there from Central or South America were present for an average of only 90.6 days (SE 4.4 days), or less than 25% of the year. These various estimates were based on the intervals between the median spring and autumn migration dates, as assessed from the numbers of birds caught each day at the bird observatory. Extreme values were provided by the American Robin Turdus migratorius at 129 days (35% of the
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