Swift natural history
The common swift (Apus apus) is a member of the Apodidae family, with 92 species that are distributed throughout the world, though most are tropical. They spend nine month in Africa, arriving for the breeding season to the south of Europe in April. Common swifts reach more northern latitudes in the first and second week of May. There are other three species that breed in Europe, although in the southern Mediterranean area; pallid swift (Apus pallidus), alpine swift (Tachymarptis melba) and white-rumped swift (Apus caffer).
Common swifts are long-lived birds that spend almost their entire life on the air, gathering their food and material for nesting in flight. They drink by skimming over the surface of still water. They do not show evident sexually dimorphic characters, they mate both in flight and on the nest, and may even pass the nights without roosting. Swifts are altricial birds, which do not exhibit post-fledging care and breed once per season.
They have long wings, with a body shape adapted to rapid and efficient flight. Swifts with a wing span of 40 cm need the ability to brake suddenly and fold their wings before entering nest holes of less than 8 cm. Swifts never foot on the ground except by accident, this aerial life limit their choice of nesting sites to places above the ground with a clear drop to allow a fast take off. Even though their long wings and short legs, they can take off from flat surfaces. They have characteristic sticky saliva used for building nest. They salivary glands are greatly enlarged in the breeding season. Swift saliva have an essential function in shaping and lubricating the food balls that they give to the young, also transferring beneficial flora to the chicks and establishing their immune competence in the first days.
Swifts are dependent on food captured in the air, being abundance or scarcity of insects important in the breeding success. Warm temperature, sunny and little rain conditions (fine weather), increases insect abundance and it is favourable to this success. Cold, rainy or windy weather conditions (poor weather) are detrimental as they encounter great difficulties in finding enough food as insects are scarce.
Normally swifts pairs previous years’ mate, although they arrive in different days. Even the two members do not arrive together, they rejoin because return to the same nest of previous years. This may suggests that pairs are separated out of the breeding season. It has been recorded from on data from a colony controlled for longer than 20 years, pairs being together longer than 10 years.
Some authors denoted that swifts breed in their second year; however others observed that pairs did not breed successfully before third or four years of age.
Eggs are white, like those of other hole-nesting birds. Swifts usually lay two or three eggs, with remarkably differences depending of the latitudes related to the average number of young that parents can rise. The most frequent incubation period is 20 days, laid with some degree of asynchrony, from two to three days.
Both parents share incubation. It has been observed that eggs and hatched naked chicks can be left unattended in cold conditions for long periods meanwhile both parents are out collecting food. Swifts are the only birds that nest in cool regions which can leave eggs unincubated and young unattended for long periods without harm. Although the cold temperature, it has been denoted that eggs hatch normally. The ability of the developing embryo to cool for some hours have an important advantage for an species breeding in areas of unpredictable summers, although this ability is unusual among birds.
Swifts as altricial species are blind, naked and helpless at hatching. Parents care for the chicks until they fledge.By the second week after hatching, both parents are much of the time on hunting, leaving the nestlings unattended. Nestlings have a particular thermoregulation and usually from four days old they may not be warmed by their parents even during the night, despite the temperature, assuming also certain effect of mutual warming between siblings.
In almost all latitudes it is described an average nestling period of 42.5 days, being 37 days in fine weather and the longest 56 days in poor weather, suggesting that poor weather may considerably prolong the nestling period.
Many authors sustain that swifts have particular adaptations related to the scarcity of food, the so-called facultative lethargy. Swift nestlings have evolved to revert their metabolism to a cold-blooded condition, using their fat stores for vital functions. Other birds are known to have these facultative hypothermic responses, for example the common poorwill (Phalaenoptilus nuttallii) during hibernation and many hummingbirds’ species during cold nights. One researcher, Koskimies, conducted physiological studies related to this adaptation to the poor weather and concluded with several affirmations: a swift chick is able to survive up to 12 days without being fed, being same capacity in adults much lower; the fat accumulated along nesting period is used for vital functions being able to use more than 50% of its body weight along the fasting period; the feathers continue growing along this fasting period and finally that the chick can enter a torpor status with its metabolism decreased to environment temperature and its fluctuations. It has been recorded that nestling temperature may drop to 21ºC at night (41ºC normal adult temperature), and recover it during the day even without food. ALso recorded in extremely unfavourable years, adult swifts in hypothermia state during the night, which seems to reduce the energy loss and increase survival in years of extreme food scarcity.
Young are fed by both parents. Whereas most of passerine insectivorous birds bring one or few insects to their nestlings every few minutes, the feeding intervals in swifts are much longer. In fine weather each parent returns to the nest every 45 minutes, but in poor weather they may return once every three hours or still longer. Feeding frequency is higher in broods of larger size; however it does not increase proportionally. Thus, nestlings in large broods receive fewer feeds than in small broods.
A single ball food may contain from 90 small to as many as 1500 very small insects and spiders, ranging from 0.7-1.8g and very occasionally up to 2.5g. Some researchers has recorded 42 meals brought to a brood of two, meaning about 50g of insects in a single day. This large number of insects are kept in the back of the parent throat, alive and entangled in saliva. Although it is an obvious part of the insect ball, swift saliva composition has not been determined.
It has benn observed the adults passing the food to very small nestlings in several successive portions, fed that may last three or four minutes. When the nestlings were older, the ball was passed quickly and swallowed by just one nestling.
Some reserachers suggested that chick mortality and growth rates were influenced by temperature, rainfall, wind speed and sunshine. However, Also stated that only the temperatures during the period of hatching and early days were found to have any significant influence in the number of chick surviving, and that brood reduction soon after hatching was the reason of breeding success. Brood reduction is a strategy where parents may induce a selective removal of most weak offspring in a way to assure breeding success when food is scarce.The effects of weather, apart of the brood reduction, include plasticity in laying dates, the period of incubation and nestling growth rates.
The growth curve shows a typical sigmoid pattern of rapid growth to a peak, followed by a period of weight recessio. A description on a nestling growth registered example was as follows: the young swift weights 2.7–3.1g at hatching. The daily increment weight from broods of two was 1.8g from the 1st and 25th day of age, and 2.3g in broods of one young, with a maximum increment of 4.8g in one single day. Until the 34th day of age it was in both a daily increment of 0.5g, with peaks of 53-63g, exceeding remarkably those of adults. From the 34th day to departure body weight decreased an average of 0.7g every day, with a maximum loss of 3.4g in only one day. The maximum gain between one day and the next recorded was 10g, being the maximum loss 7g in a fast day.
Whereas young swifts have evolved to withstand starvation, other altricial nestlings such as passerines (e.g. songbirds) increase gradually in weight as they grow and a fast period of even 1-2 days leads to death. The growing rates of swifts may be interrupted by sharp drops in weight, particularly in northern latitudes, with periods of poor weather. In these periods, nestlings use their stored fat for its vital functions. However, variations in food availability, unless are severe enough to cause starvation, do not influence nestling growth rates,
Many authors describe the growth of the wing. The young is 22 days old on average when the wing reaches 100 mm, growing an average of 4 mm per day and fledging with a wing around 167 mm. Exceptionally some birds may had short wing-length (e.g. 146 mm), suggesting a permanent stunting. Average wing-length on adults is 174 mm; hence the wing may continue growing along the post-fledging period. The wing may increases in length at the first moult.
The average age at fledging is 42 days, with remarkably ranges from 36.5 to 56.5 days. Researchers did not find difference in duration of parental care on relation to clutch size, even observed young that hatched the earliest stayed in the nest the longest. Also described some young with same weight and wing length than other birds already fledged, remained in the nest for not particular reason for several days longer.
Young body weights at fledgling are normally slightly over the adult averages weights, being the parents usually somehow underweighted due to the breeding effort.
Researchers suggested the parent-offspring relation, when parents influence young to leave then nest whereas nestlings try to stay longer to maximize its survival chances. One way could be the parents reducing the provisioning, thus reducing parental investment. It was observed as the nestlings reached the weight peak how the parents reduced significantly the number of offered feedings. Also, it has been denoted that nestlings, after reaching peak weights, ceased and refused for a while to take food from the parents. It has been observed that fledge was preceded for a short period where young were not fed by the parents, losing the excess of weight and completing feather growth before fledging.
Swifts may be classified as species as a long-lived bird with a low annual reproductive output.Being described an average 1.63 fledglings per breeding attempt, suggesting that long reproductive success appeared more influenced by the breeding lifespan than the reproductive performance.
Reearchers described the growing development of aerial insectivores and pelagic seabirds, in a form of sigmoidal curve where initially weight increase gradually, reaches a maximum much above the average adult weight and then decreases before fledging, with fledgling weights that tend to exceed those on adults. In other species, weight reaches adult averages before fledging (e.g. starlings), weight growths substantially below the adult weight at fledgling (e.g. pigeons); or in others, young fledge completing their growth outside the nest (e.g. warblers).
It is of great importance the food supply related to the growth pattern. When food levels fluctuate along nestling period, young nestlings are at risk of dying. An adaptive modification of the growth pattern is to store fat to use them as reserves in shortfalls in food supply. This strategy has been termed as the resource storage strategy, which also includes other adaptive characteristics: a labile growth to compensate poor weather spells during the fine weather spells; fat deposition progressing in step with metabolic rates to assure survival prospects at any stage of development if an interruption in food availability occurs; thermoregulation achieved at early development stages and finally ability to brood size reduction at any period of sustained food shortage.
A researcher developed a model predicting that altricial species do best if they grow at the maximum physiologically possible rate rather that at some limits due to availability or scarcity of food. This author stated the two types of limitations that growth rates may encounter: The first is based on the rates at which energy and nutrients are distributed, assuming no limits to this distribution about the body. Since the young is responsible for their own thermoregulation, the energy ingested for this function may limit the energy available for growth. Nutrition acts a whole in this constrain. The second is supported by the idea of the cell function and physiological design, which may limit functionality and continue proliferation and growth.
Amongst aerial insectivores, it was premised that excess nestling weight was due to high water content of growing tissues, being lost as the tissues matured. An author analysed weight recession in barn swallows (Hirundo rustica), doing also an extensive literature research related to weight recession and concluding that this phenomenon was restricted only to some avian groups (Hydrobatidae, Procellariidae, Hirundinidae and Apodidae). Ricklefs, concurring with O’Connor, concluded based in body constituents of the barn swallows, that weight recession was accounted entirely by a decrease in water content, mainly from the integument but also important from the body and liver.
It has been emphasised that species showing weight recession exhibit well develop flight capacities and aerial foraging habits when fledging. Also suggested that the reduction in body weight and the increase in wing length in swift would influence the flight efficiency by increasing the lift: drag ratio, in other words, a reduction in the cost of flight and improvement in manoeuvrability. Some researchers found by experimental variation in nestlings’ body weights and wing length that prior to fledging young always achieved similar and appropriate wing loadings, concluding that young were able to make facultative adjustments in their weight loss. In addition suggested that exercise could be an important tool used by fledglings to asses their optimal wing loading before fledgling.