comparison of the dates for the arrival and departure of the Common
Swift in summer showed that the species stays about 109 days in the
breeding areas, and about 95 days on the nest. The stay seems to
slightly by a not yet known factor from north to south. I created a
calendar for the Common Swift, which is valid for most parts of the
breeding range, and which can be used for orientation in the complete
summer breeding area.
All the key
data (arrival, egg-laying, incubation, hatching, brooding, fledging and
departure of adults) are exclusively median values. A median is the
value in the middle, and is used when there are no mathematical
parameters, such as date specifications. The mathematical data need not
necessarily coincide with median dates.
Swifts arrives in four waves. The non breeders may leave before the
breeders or together with them. The Swifts usually lays two to three
eggs with an interval of two days from each egg. Breeding starts after
the last egg is laid. The hatching starts after 19 days in average. (The
difference of the median values here is 23 days.) Swifts feed
exclusively on airborne insects. When the weather turns cold, the food
supply diminishes or even disappears. This is why the young Swift has
evolved to develop a high body mass (weight). It serves as a reserve for
periods of bad weather when there is little or no food to be had. Before
flying out, the young Swift's weight decreases to its normal or flight
calendar may be used anywhere in the summer range, except possibly
within northern Lapland. If one knows the arrival or departure date in a
given location, one can quickly find out, by using the calendar, what
the Common Swift is doing just now.
knows the date of the arrival of the Main Body. Here are some arrival
dates. Jerusalem 20.2. Advance Guard, Tel Aviv 19.2. Advance Guard, 6.3.
Main Body, Teheran 11.3., Peking 12.4., Moscow 12.5., Irkutsk Region,
21.5., Madrid 12.4. Advance Guard, 25.4. Main Body, Berlin, 24.4.
Advance Guard, 8.5. Main Body, Erzurum 29.4., Forlì 9.4., Tulchin
12.5., Cherkasy 1.5., Simbirsk 9.5., Ivanovo 19.5., St. Peterburg 18.5.,
Helsinki 27.5. (If you want to know the sources, please send me an
Further information on this
topic: Tigges, U. (2006): The Breeding Cycle in Calendar Form of the
Common Swift Apus apus across its Eurasian Breeding Range - A
Testable Hypothesis? Podoces 01: 27-33, APUSlist