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Topic : Land Birds

Article 14 bullet 01 January 2007

Common Swift – Oxford breeding project

by George Candelin and Roy Overall


Much has been done to learn about Common Swift Apus apus in the Northern Hemisphere but we know very little of what these swifts do in Africa. Following the Tanzanian landfall article posted earlier this month data presented there shows some of the pioneering work that has been done in Oxford, England. There are a few nestbox schemes in Europe now, the Freiberg study has sites with Alpine Swift Apus melba, this is also a museum sponsored study that has run for years. There are many Swift Towers in Italy, these are now used for study but in medieval times were used as a supplementary source of food. A new scheme is just being created in Cambridgeshire to install nestboxes into church belfries.

Common Swift photographed by Derek Brown

Common Swift photographed by Derek Brown

The Oxford Swift Project

When Dr David Lack was appointed as Director of the Edward Grey Institute of Field Ornithology at Oxford in 1947, he realised that the colony of Common Swifts Apus apus breeding in the University Museum would make an ideal study project. Initially the project consisted of installing one new floor level and 40 nesting boxes with glass viewing panels, one box behind each ventilator cowl.  These nesting boxes were a step in the right direction but were difficult to work as they could only be accessed using long wooden ladders; the upper one of which had to be moved from box to box when examining the contents; the facility did however, lay the foundation for Lack’s studies from 1947 to 1956 and this research formed the basis for the book ‘Swifts in a Tower’; this was the first comprehensive book on the life and breeding of the enigmatic Swift.

Oxford University Tower; behind each vent is a double nestbox in which the swifts breed

Oxford University Tower; behind each vent is a double nestbox in which the swifts breed

The layout of the tower study site changed in 1965 when the twin type of nestbox design that we use today replaced the single nestboxes; thus allowing the expansion of the project to 80 nesting boxes; At the same time a further expansion took place by the addition of nesting boxes of a simpler design that were installed on top of the walls behind the small ventilation cowls. The result of this was that we achieved a total of 147 boxes, a valuable asset that is current today. The boxes are made from 12mm thickness plywood. The box tapers from 200mm wide at the entrance to 400mm wide at the observation window and has a dividing wall down the centre; they are 400mm front to back. The glass windows (they slide out for cleaning) are 175x110mm. The box is 150mm deep. A hinged section in the roof gives access for ringing. The entrance hole in the floor at the front of the box is 40mm square and is 300mm from the window. The 67 simple boxes mounted on the wall are 200mm wide, 150mm deep and 400mm long; these have a hinged lid 150mm by 200mm for inspection and ringing; they do not have a glass window. These are not well used with only about 5 to 10 occupied in any year.

Common Swifts brooding chicks in twin type nest boxes placed at each entrance cowl

Common Swifts brooding chicks in twin type nest boxes placed at each entrance cowl

Dr Derek Bromhall made a documentary about the birds for Anglia television, commencing in the long hot summer of 1976 and finishing the following year. He then wrote the book ‘Devil Birds’ published in 1980. Roy Overall meanwhile had become custodian of the Swifts, and in the years since 1962 has ringed over 4000 young swifts. As a result of his 40 years in this role he co-authored a book with Dr Andrew Lack, one of the sons of David and Elizabeth, this book, ‘The Museum Swifts’ was published in 2002. The Oxford Swift study is therefore well documented and the best book titles have already been used! During the last 5 or 6 years I have been sharing responsibility with Roy pending my eventual takeover of the project when he decides that he has had enough of studying and ringing these fascinating creatures.

Breeding cycle

Swifts use ‘natural’ sites in the roof of old buildings under the tiles. This replaces the original caves, crevices in cliffs and hollow trees that were the historical nesting sites. The annual cycle begins for us during the period when the Swifts are on the way back from Africa. Each April the nesting boxes must be checked for wear and damage, cleaned of loose debris and the viewing glasses cleaned and polished. It is necessary to remove any loose nests as they can obstruct the entrance/exit hole and trap a bird inside. Nests that are still firmly fixed are left in place.

May sees the birds arrive back at the boxes. They do not arrive as pairs but seem to re-bond in the box with much mutual preening. Territorial disputes do occur, but ringing studies have shown that a pair will usually occupy the same box as they did in previous years; If the pair bond is broken by the loss of a partner, we have observed that the new pair will try to occupy a box close to one previously used by one of the pair. Unfortunately we cannot at present confidently sex these birds, even in the hand, so we cannot confidently state that there may be differences in box occupancy depending on which member of a pair fails to return. Juvenile Swifts that have been ringed in the Museum Tower have rarely if ever returned to the natal site; but adults that are ringed in the nesting box almost invariably return to the same box year after year.

Fledgling Common Swifts

Fledgling Common Swifts

A note of caution must be sounded here; adult Swifts are highly prone to abandoning or deserting the nest if they are disturbed during the nest-building or incubation phases of breeding. Experience has taught us that it is safe to handle adults once the young have reached the ‘fluffy’ stage; this is reached about three weeks after hatching. We do not attempt to ring the young Swifts until they reach the well-feathered stage (FM in IPMR parlance), (i.e. about five weeks after hatching), normally this is in early or mid-July. Doing this allows the soft legs of the pulli to harden a little and ensures that its tarsus is properly grown making the fitting of the ring a little easier. Young Swifts generally fledge after about six to seven weeks in the nest. We therefore have a window of about 3 weeks for the capture and examination of the adults. To minimise disturbance we initially visit only on Saturday mornings, we wear dark clothing, work in the dark and talk very quietly. There are red background lights available and these do not seem to disturb the birds. Noise can be a factor in the early stages and sudden movements should be avoided. Later in the season we have found the birds to be less sensitive and even tolerant of flash photography.

Swifts lay 2 or 3 eggs in May or early June, but are capable of replacing a damaged egg if necessary. It is common for Swifts to eject one or more eggs from the nest cup; we have marked these eggs and replaced them. The usual outcome of this is that an egg is again rejected by the following weekend; this is not necessarily the marked egg. We have found a slight correlation between the weather conditions at the time of incubation, and the total number of eggs ejected but despite several theories it is not clear yet exactly why or how the eggs are rejected. Approximately 20% to 30% of eggs are ejected depending on the prevailing conditions during the incubation period (late May/early June); of the young hatching however, over 95% usually fledge. This is because the young have few predators in the nest and they are capable of going into a torpid state for days on end in adverse conditions. In our study we have found a small number of cases of mortality in which young are not only ejected from the nest cup but sometimes completely absent from the box. This can only have been the result of an avian intruder, probably another Swift, but this has not been observed so it is speculative.

We do not have weasels, rats or mice in the tower unlike some Swift nest sites; also there is no evidence of mammalian activity i.e. chewed boxes, droppings, etc. There is however, documented evidence of an insect predator in the form of a wasp that we have observed chewing on the back of a living but semi-torpid pulli, this has been seen on three occasions and has definitely caused the death of 4 young. We are still attempting to capture one of these wasps for species identification. The Swift has some other insect associations. The largest of these is a host specific parasitic flat-fly with a pair of vestigial wings, Crataerina pallida; this particular species of blood-feeding louse-fly is host specific to the Swift. An interesting feature of this insect is that it does not lay an egg but broods internally before laying a fully formed larva that immediately starts to pupate; when the pupa hatches into an adult it seeks to associate itself with a bird or seeks a nest of young Swifts. Other insect pests of the Common Swift consist of feather lice (Mallophaga sp), ticks, fleas and mites.

5The host specific blood-feeding Flat Fly Crataerina pallida

The host specific blood-feeding Flat Fly Crataerina pallida

Brooding swift showing a parasitic flat fly crawling over the wings

Brooding swift showing a parasitic flat fly crawling over the wings

Of the 147 boxes in the study site, we would expect about 50 to 60 to be actively used for breeding each year, although some of the others are used for loafing or roosting. The average number of fledglings is circa 95 per annum from between 115 to 160 eggs laid. The maximum number of fledglings lost in a normal year was 5 but 2 of these 5 may have been due to miscounts as we do not disturb incubating or brooding birds until the young are well feathered. During the particularly cold wet summer of 1997 however, 13 of the 100 young hatched were lost as were 5 adults that were found dead in the nest.

Despite more than half a century of study we still do not have the answers to many questions about Swifts; they gather food and nesting material on the wing, they feed, sleep, preen, moult and even mate on the wing, they drink rain water but where do they get calcium for the egg shell and for the development of bones in the growing young? Why do the young not return to the natal site and where do they go? At what age do they breed for the first time? Do they land at all in Africa or en-route? What is the annual survival rate for young birds and adults? We know that one of the birds ringed in the Oxford Museum Tower lived for a further 16 years after being ringed as an adult of indeterminate hatching date, thus being at least 19 years old, so how long can they live? We have little or no information on how or when they moult, although it seems to occur in Africa or perhaps en-route during migration?

We suspect that only DNA testing or isotopic studies will provide some of the answers, although others may be found through observation and ringing but the Swift will hold many of its mysteries for a long time to come.



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