b. Benefits of migration

Migration can benefit birds in different ways, which all help increase their chances of survival and ultimately, reproduction.

i. Increased food supply

The cold winters of northern Europe experience radical changes in countryside communities: deciduous trees shed their leaves in anticipation, insects hibernate or die. Dwindling food supplies are a fundamental reason why birds seasonally leave their breeding grounds for warmer climes where food remains plentiful. Some birds can be observed to closely track the movement of their prey - for instance, white storks which move over southern and tropical Africa following concentrations of orthoptera (cricket-like insects)⁶.

An increase in population may also cause a decrease in food supplies through competition. This has been seen in populations of crossbills prior to migration.

ii. Decreased food requirements

Higher temperatures also mean that birds need to eat less to stay warm. This alone can reduce their energy requirements by 16%. Birds are also less active in Africa as day-lengths are shorter. As a result, the maintenance needs of Palaeoarctic birds can be at least 17% less in Africa than at their breeding grounds. Thus, birds may have an overall decrease of 33% in energy requirements in Africa compared to northern Europe⁶.

iii. Increased chances of survival and reproductive success.

Migration may benefit an individual by increasing its chances of survival from one year to the next. The advantages of migration also translate into the number of offspring which an individual is able to raise.

In evolutionary terms, the number of offspring an individual has which survive to reproduce themselves, is called its Lifetime Reproductive Success (LRS). In the natural world, each individual is trying to maximise its LRS. Migration has been shown to benefit the LRS of some populations.

The probability of a chick surviving appears to be greater if it is a migrant than if it is a resident. Resident species generally need to raise more than one brood per year in order to sustain the population, whereas migrant species tend to breed only once a year, with relatively few offspring in that brood. Mönkkönen showed this to be true for Palaeoarctic long-distance migrants compared to resident populations. Whilst the multiple-brood strategies of resident species may increase the chances of some offspring surviving until the next winter, they remain at the mercy of the environment even as adults, periodically exposed to cold temperatures and lower food availability in winter.