This study describes the migration of Swifts breeding in Asia (the pekinensis subspecies) for the first time. It involved capturing birds at their breeding site, the Summer Palace in Beijing, and tracking them with tiny geolocators, which record day length and the time of solar noon, allowing their position through time to be calculated.   

After leaving Beijing in the middle of July, the Swifts headed north-west into Mongolia, before travelling west, re-entering China and crossing into central Asia. From there, the birds migrated to north-east Africa, flying over the Red Sea around the middle of August and reaching the longitude of the eastern Congo basin at the start of September. The final leg of their journey took the Swifts to their wintering grounds on the Southern African Plateau, where they remained from early November until mid-February.

Remarkably, their entire migration route tracked arid land, despite this requiring a longer journey. Previous work involving BTO tracking the apus subspecies shows they favour wetter habitats, so this journey is specific to the pekinensis subspecies, and may be a relict of the Swift's population expansion after the last ice age, when the birds spread over a wider region via arid habitat.  

The study also revealed other key differences between the subspecies. Compared to apus Swifts, pekinensis birds had a significantly longer breeding period, a shorter wintering period and a slower migration speed. Swifts breeding in Beijing travel an amazing 30,000 km each year - nearly twice as far as their apus counterparts breeding in northern Europe. As well as tracking arid habitat on migration, pekinensis Swifts also favoured more arid habitats on their wintering grounds than apus birds, suggesting the two subspecies are dependent on different food resources outside the breeding season.

Background

As a widely distributed and aerial migratory bird, the Common Swift (Apus apus) flies over a wide geographic range in Eurasia and Africa during migration. Although some studies have revealed the migration routes and phenology of European populations, A. a. apus (from hereon the nominate apus), the route used by its East Asian counterpart A. a. pekinensis (from hereon pekinensis) remained a mystery.

Methods

Using light level geolocators, we studied the migration of adult pekinensis breeding in Beijing from 2014 to 2018, and analysed full annual tracks obtained from 25 individuals. In addition, we used the mean monthly precipitation to assess the seasonal variations in humidity for the distribution ranges of the nominate apus and pekinensis. This environmental variable is considered to be critically relevant to their migratory phenology and food resource abundance.

Results

Our results show that the swifts perform a round-trip journey of ca 30,000 km each year, representing a detour of 26% in autumn and 15% in spring compared to the shortest route between the breeding site in Beijing and wintering areas in semi-arid south-western Africa. Compared to the nominate apus, pekinensis experiences drier conditions for longer periods of time. Remarkably, individuals from our study population tracked arid habitat along the entire migration corridor leading from a breeding site in Beijing to at least central Africa. In Africa, they explored more arid habitats during non-breeding than the nominate apus.

Conclusions

The migration route followed by pekinensis breeding in Beijing might suggest an adaptation to semi-arid habitat and dry climatic zones during non-breeding periods, and provides a piece of correlative evidence indicating the historical range expansion of the subspecies. This study highlights that the Common Swift may prove invaluable as a model species for studies of migration route formation and population divergence.