New discoveries in the migratory, as
well as spatial and temporal,
behaviour of the
Swift in large
cities, at last enables precise
counting of their populations. It is now known that there are set
periods, or "time windows", when one can count separately
the potential breeders and the non-breeders. Armed with the exact
arrival times of the Swifts in their breeding areas, one can now set
precisely the times for a successful census.
The following analysis of Swift
behavior permits census-taking using a time window method.
Swifts arrive in their breeding areas
in four waves. The mature birds arrive in the first three waves at the
start of the breeding season. The immature yearlings arrive in the
fourth wave, about mid-way through the breeding season.
In the mornings and evenings all Swifts
fly within a set colony territory, usually extending from about 90,000
m² to 120,000 m² . Within square territories the borders are
approximately 300 m x 300 m; within triangular territories they are
approximately 450 m 450 m 450 m.
Breeding Common Swifts
Potential breeding Swifts may be
counted from seven to fourteen days after the arrival of the second or
third wave. At this time all the birds are within the breeding areas,
and in general they have not yet become intensely occupied with
courtship and egg-laying.
The time window for counting depends on
the time of twilight, and in turn is related to the geographical
location. In Jerusalem (31º 52' N, 35º 13'E) the time window is between
0700 and 0800 hours Israel Standard Time (IST) or else between 1730 and
1800 hours IST.
In Berlin (52º 28' N, 13º 24' E) the
time window is 0900 to 1000 hours Central European Summer Time (CEST) or
between 1900 and 2000 hours CEST.
At these times the Swifts are out of
their breeding holes, and circling the colony territory in flight.
At least two counts should be made,
preferably more, so that birds missed the first time (either because
they are in their nest holes or out of sight) can be picked up on the
Non-breeding Common Swifts
The census of non-breeding Swifts
should be taken in the last third of their stay. The best time for this
is in the 5th and 6th weeks before departure. In
Berlin this is between July 1st to 15th. The
census should be timed for the periods when the birds are either
returning to the colony, after being out all night, or else prior to
assembling at the colony for the night out. In Berlin the appropriate
time windows would be 0800 to 0900 hours, and 2000 to 2100 hours CEST.
At these times the breeding Swifts are
occupied exclusively with the search for food for their young, and do
not participate in the territorial and social flights of the
non-breeding colony members. The only breeding Swifts likely to be
encountered during this census would be those bringing food to the nest,
and leaving to fetch more.
Taking a census in a city
Swift populations within well-defined
areas may be counted, if a reasonable number of helpers can be
assembled. Since Swifts are exclusively aerial, (apart from when they
are nesting, and sometimes roosting), and perform territorially at
certain times of the day, they may be counted by making a succession of
observations at pre-set points within the area to be covered, using a
progressive wave technique.
The helpers should be positioned at
exact points set on a vertical axis, about 200 m to 300 m apart from
each other, depending on the availability of suitable positions (e.g.
road crossings). The census should take place at fixed times, and all
Swifts in flight in the marked area must be counted.
As it is quite feasible that either the
whole colony, or parts of it, are flying in a different part of the
territory, the helpers must make successive counts from the same
positions 5 and 10 minutes after the first.
Then the helpers move forward to the
next counting points, set laterally about 200 m to 300 m from the first
set, and the counting recommences. Using this technique quite large
areas can be covered reasonably quickly.
(Source: Tel Aviv Map and Guide 1997, changed)
In the mornings and evenings Common
Swifts establish their colony territories by circling above them
continuously. To count them one establishes a vertical chain of set
points about every 200 m to 300 m. At the same fixed time the Swifts are
counted at each set point. In addition one makes a note of their
direction of flight. Successive counts are necessary, 5 and 10 minutes
after the first. Then the counters proceed laterally to the next set of
counting points, and the counting procedure starts again. The map above
shows an example census area and the positions needed by a counting team
Taking a census in larger cities
Within bigger cities the best way of
counting Swifts may be to use the spot check method. To do this one
divides the area by building type / use (e.g. areas of old buildings,
areas of new buildings, apartment blocks, terraces, semi-detached and
By making a random selection one
chooses areas for the census. The results from these randomly selected
areas are used to generate data for the whole city. This method was used
successfully in the Berlin Census for 2002.
Taking a census in smaller towns
Initial studies suggest that the Common
Swift population in smaller towns and villages behaves in the same way,
both spatially and temporally, as in the larger built-up areas. However,
as precise research has yet to be conducted, more work is needed before
counting in such areas can be performed in confidence using the methods
suggested in this article.
In evaluating the results of the counts
the following need to be taken into consideration:
Care should be taken to conduct
counting only in fine weather. Do not count during, or just after, a
period of cold or wet weather, as the Swifts may have been driven away
from their usual areas by the bad weather.
Breeding birds 45 %
Mature non-breeders 20 %
Immature yearlings 35 %,
but not all the birds in the colony
could be examined.
Ulrich Tigges 2003
0060, 0061, 0067, 2741, 2856, 2867