Swifts are usually gregarious and colonial nesters. Colony size is determined largely by nest site availability. Birds defend their nest sites vigorously.
Pair bonds are often formed when the birds are only one year old. These birds sometimes occupy a nest hole and may even attempt to build a nest. However, they are unlikely to nest successfully until they are four years old.
Swifts pair for life, meeting up each spring at the same nest site. The nest is located high up in the roof space under the eaves of old houses and churches where the birds are able to drop into the air from the nest entrance. The nest is built by both adults out of any material that can be gathered on the wing, including feathers, paper, straw, hay and seeds. It is cemented together with saliva, and renovated and reused year after year.
Young birds looking for a nest site will fly past prospective sites brushing or ‘banging’ the entrance with their wings. Resident birds respond by coming to the entrance and screaming. This continues throughout the breeding season. Swifts often fight over nest sites, grappling with their feet until the intruder is ejected.
Swifts normally lay two or three eggs at two or three day intervals. Incubation starts with the first egg, and lasts for 19–20 days per egg. The adults share all the nesting duties equally. Fertile and infertile eggs are not uncommonly ejected from the nest at any time. The reason for this behaviour remains unclear.
The young hatch a couple of days apart, so in the first week the young are different sizes. This is unusual in small birds, and is believed to be a survival mechanism. If there is plenty of food, the younger birds soon catch up, but if food is scarce the older bird will get the largest share, so at least one will survive.
Swifts grow rapidly on a diet of insects brought to the nest by the parents in food balls. At first the brood will share each meal, but by the time they are two weeks old they can handle a full delivery by themselves.
At two to three weeks of age the young swifts start to move about the nesting chamber and exercise their wings by performing ‘press-ups’ on their wing tips. In a good year the young develop quickly, and are ready to fly at about six weeks old.
For several days before fledging they spend long periods at the nest entrance looking out. They usually leave the nest in the early morning, and will be independent immediately. Within a few days of leaving the nest they will start their flight to Africa in the company of other fledglings.
Premature fledging is not uncommon during particularly poor summers. A bird that has left its nest too early may not be able to remain airborne, and crash lands. Birds like these will have no chance of survival in the wild, but they can be reared with some difficulty to give them a second chance. If possible, this should be left to experts.
Swifts have a remarkable and unique way of surviving bad weather and food shortages. Their eggs survive chilling at any stages of development if adults have to spend more time away from the nest foraging – something that would kill the embryos of any other bird. The development of the egg will simply slow down until a parent returns. Bad weather can extend the incubation for four to five days.
Chicks hatch with reasonable fat reserves, and at any stage of development can survive long periods between feeds. During periods of bad weather, the adults sometimes move a long distance to find better conditions to feed themselves to survive. The young can reduce their metabolic rate, and by becoming semi-torpid can survive for several days by using up their fat reserves, which can halve their weight. Periods of food shortage slow down the development of the chicks, and can delay fledging by two weeks.
Swifts mature and breed when they are four years old. Those that survive the hazardous early years can expect to survive a further 4-6 years. The oldest ringed bird lived for at least 21 years. Because of their mastery of the air, swifts have few predators. Tired, hungry birds are sometimes taken by birds of prey such as hobbies.
You can encourage swifts to nest at new sites or replace sites no longer available by fitting nestboxes either within the eaves of the house or on the wall under the eaves. Because swifts need height to take off, single storey buildings are not suitable. To stop other birds using the nestbox, block the opening until your local swifts return.