Speed and duration of journeys

There must come a point at which the new facts that have been collected are felt to be both too raw and too numerous, and it is at this point that the need for coordinating principles begins to be felt. (Charles Elton 1930.)

In most birds, migration consists of periods of flight, interspersed with periods of feeding, when body reserves are replenished. Both activities need to be taken into account in assessing the speed and duration of the overall journey, which might be important for several reasons. First, travel through unfamiliar areas carries costs and risks which could be minimised if migration were completed as quickly as possible. Second, rapid migration can lead to early arrival, ahead of most other individuals, which in turn facilitates access to the best habitat. Third, the time taken for migration could in some species restrict the time available for other activities, such as breeding and moult. It could thus limit the total distance that can be covered on migration, and hence constrain the geographical ranges of certain species.

The times spent on migration by different bird populations are enormously variable, depending partly on features of the birds themselves (such as body size, wing shape and flight speed), but largely on the distances travelled and the conditions encountered en route. At one extreme, some birds can complete their migration in less than one day (such as a radio-tagged Bald Eagle Haliaetus leu-cocephalus that flew 435 km between its wintering site in Michigan and its nesting place in Ontario, Grubb et al. 1994). At the other extreme, some landbirds take more than three months to reach their distant winter quarters, and a similar period to return, so that more than half of every year is spent on migration. Long journey times are also shown by some marine species, including shearwaters and petrels which have a fixed base only during the breeding season, and are effectively on migration for the rest of the year, pausing to feed wherever food is available en route. Whatever the advantages in migrating as quickly as possible, external conditions provide severe constraints, notably the rate at which food can be obtained and converted into body reserves to fuel the flights, and also the weather at the time, which can speed or slow the journeys. Rain or snow, cold or ice, or unfavourable winds can delay migration for days or weeks at a time.

A broad idea of the overall speed of migration in some species has come from noting the dates when birds leave one area and arrive in another. Such information could in theory be collected for many bird populations, but only for a few is it likely to yield accurate estimates. One problem concerns the lack of certainty that the same population of birds is involved throughout. Another is that individuals from the same population often vary greatly in their dates of departure and arrival, making mean dates hard to estimate. Nevertheless, such records have provided useful indications of the duration of flights by large conspicuous birds, such as cranes, geese and swans, which use only a limited number of traditional stopping sites. Confidence in such records increases if some of the birds are marked in a way that they can be recognised individually. While mostly based on observation, some useful estimates of this type have been based on museum skins, from the dates that migrants were collected in different areas (e.g. Byrkjedal & Thompson 1998).

Other information on the overall speed of migration has come from birds ringed and recovered on passage. Such recoveries carry the risk of error because it is seldom known precisely when a ringed bird leaves one place or arrives in another. The temptation is therefore to use only the fastest records, as an indication of maximum possible migration speeds. Not only do such records then come from extreme individuals, they mostly also cover only a small part of a migratory journey.

More representative data have come from calculating the mean geographical positions of ring recoveries obtained from a defined breeding population at successive dates through a journey. Thus, if recoveries of birds already on migration were centred in mid-October at latitude 30°N, say, and those in mid-November due south at latitude 20°N, then an average of one month would have been needed to cover the distance spanned by 10° of latitude. Similar estimates can be made from the dates that particular populations pass through different watch sites or trapping stations along a route. But many species break their autumn journeys for weeks at a time if conditions are favourable, and for much longer than is needed merely to replenish their body reserves. Some species, notably shorebirds, may moult during a several-week break in their autumn migrations (Chapter 11). Moreover, young birds often migrate more slowly than adults, and because juveniles form a larger proportion of any migratory population in autumn than in spring, they may also have more effect on the population's average migration speed in autumn.

The most reliable data on migration speeds come from individual radio-marked birds tracked by plane or satellite over their whole migration. With daily records of position, it is theoretically possible to separate the flight periods from stopover periods during the entire journey (but in practice short breaks of a few hours are easily missed). As yet, such data are available only for birds big enough to carry a radio-transmitter or geolocation tag, and their interpretation depends on the (probably justified) assumption that migration behaviour is unaffected by the attachment (Chapter 2).

One problem with all four types of data is that they provide no measure of the duration of the initial fuelling period before birds leave their breeding or wintering areas. Strictly speaking, this period is part of the migration, yet can be determined only by separate study. To judge from recorded rates of weight gain, small passerines take from a few days to three weeks to accumulate the required body reserves, while long-distance shorebirds and waterfowl can take up to two months or more, depending on the length of the subsequent flight.

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