Swifts display two different roosting strategies. While non-breeders fly high into the sky at late dusk to spend the night on the wing (Weitnauer 1960), the breeding pairs roost in the nesting sites. They only spend the nights flying when their presence with hatching is no longer essential or if access to the nest is foiled for any reason, (cf. Lack & Lack 1952, Weitnauer 1960, Keller 1977, personal observation; Kumerloeve (1954) reports nocturnal hunting). 

Rising into the sky as well as descending at dawn has been described and discussed for 200 years (Spallanzani 1797 in Weitnauer & Scherner 1994, Guérin 1923, Weitnauer 1952, 1954, 1960). More general statements about the roosting times have earned numerous references in the literature. They describe the end of the flying patterns in the sky (e.g. Hoffmann 1917, Schuster 1921, Sehlbach 1926, Haartman 1940). 

Auel 1917, Haartman 1940, Daanje 1944, Haartman 1949, Vleugel 1947, Church 1956 and Arn-Willi 1960 relate the last descent to the position of the sun and cite the number of minutes the swifts stay on the wing beyond sunset.

Table 1. The farther north the nesting sites are, and consequently the longer dusk lasts, the later swifts continue to fly after sunset.

                                               Minutes after                                                   Geographical
Author                                     sunset                          Location                      latitude_____ 
Daanje 1944                            10-15                         (Netherlands)
Vleugel 1947                            30                              Den Haag                    52,20° N
Sehlbach in Haartman 1949      17-38                         Rinteln                         52,2° N
Auel 1917                                24 (mean)                   Potsdam                      52,40° N
Church 1956                            9-45                           Berwick-on-Trend       56° N
Haartman 1949                        33-60                         Lemsjöholm                 60,5° N

Despite their proximity to mankind and their open way of life, direct observations of the last evening entering of breeding pairs into the nest are relatively rare. On July, 23^rd 1917 Quantz (1917) saw two enterings in Göttingen at 9:45 and 9:50 p.m., Swaen (1922) reports, after observation of different breeding places in the Netherlands, that swifts fly into the nest sites between 9.30 and 9:45 p.m. summer time. Keller (1977) recorded the activities at a nesting site for three years in Dresden with the help of a automatic timer. He calculated an average time for returning between 6:15 and 9:54 p.m. (extremes 5:00 and 10:10 p.m.). Tigges (1994) published the times of the last evening enterings of a late brood in Berlin from 11^th May through 7^th September 1993. Gory (1992) registered the presence of the adults on the nest with the help of heat-detectors in Nîmes in southern France. By this method only the last entering of the first incoming bird is noted in cases where both adults had previously exited the nest.

Only Haartman (1949) quoted the brightness of the roosting time in the unit of measurement Lux (cf. note). The values he published are not natural measurements though, but are calculated. This certainly is due to the technical possibilities in his time. As a result of his investigation he stipulated that the swift maintained a certain level of light intensity, no matter what the season, geographical latitude or question of an overcast sky, „corresponding to about 4-15 Lux.“ He presupposes the swift would continue its flight until midnight or beyond if the prevailing light did not diminish into this range. According to his observations the last return to the nest takes place about 10 minutes earlier when skies are overcast, which ought to correspond to the same light intensity as the unclouded sky at this latitude. 

Technological progress means that anyone today can obtain a light meter which permits us to publish the previously missing data. Furthermore maximum and minimum temperatures as well as additional information during the evening hours can be noted during the period of observation. In addition, biological factors concerning breeding patterns which appeared relevant during this study were noted. By means of these observations it was possible, to investigate behavioral influences on the last diurnal return.

 2. Materials and Methods

It can be stated with certainty that the breeding pair observed in this study was experienced, since one of the birds had been ringed as early as 1992. The nesting site is 16 meters above ground level. It is in a wooden box under the window sill of a four-flat old building in a central heavily built up area of Berlin. The distance to the opposite row of houses is 20 meters. The top of the row of linden trees does not reach the level of the nest, leaving a free space under the entrance. The pair is breeding solitarily, not under the influence of other breeding swifts in the neighborhood. The next breeding pair is probably located on the rear side of the house facing. 

Immediately after the second bird entered (apx. 10 - 30 seconds) I measured the light intensity at a location about 2 meters horizontally from and 1 metre below the nest box. I used a standard Digital Lux Meter (according to the manufacturer, the accuracy has been tested by a standard parallel light tungsten lamp of 2854° K temperature; accuracy ± 5% + 2d). To gain comparable measurements I always placed the light sensor at the same point and registered the highest light value there. (For more complete understanding it is useful to know that the Lux is a very small unit. A tiny change in the position of the sensor registers different intensities of lightness. To lower this uncertainty factor I always tried to register the highest light level at the same place.)

Daily temperatures were measured with a maximum/minimum thermometer. It was fixed on the door of the balcony about 90 cm parallel to and 40 cm below the nest box. The measured values tend to be about 2° C more moderate than the values published by the authorities, but correspond to those at the nest site. The hours of sunset were provided by the German Meteorological Service and refer to the meteorological office at Tempelhof Airport, which is about 2.7 kilometers due west of the nesting site.

The fertilised eggs were laid on May 28^th and 30^th 1995 which is within the range for this latitude (Litzbarski, in Rutschke 1987). The 1995 weather was cool and rainy until June 24^th. From June 27^th on a stable period of very warm dry weather started and did not end before the swifts left on their autumn migration. It is well known that swifts feed on air born insects and therefore the conditions of weather were highly favourable for reproduction. On rainy days there was invariably enough time without precipitation to permit the birds to feed. Regular observations of flying activities to and from the nest box took place on the balcony or from the adjacent room. Monitoring was continuous during the expected time for entering the nest in the evenings, correspondingly earlier in times of bad weather. In general I was even able to record the time before the last entering of the second bird in 1995, so that all sensitive phases, such as egg laying or at the end of the breeding period could be registered completely with the exception of five specific days. The last enterings of the first inflying bird were registered 69 times. After watching two attempts to enter another nest box at the rear of the building, at only 1 Lux illumination at 9:45 p.m. on May 17^th, I prolonged the time of observing the breeding pair until it was completely dark, despite my experiences from previous years. I observed no further flight activity however.

The lightness is of different intensity on just about every point in the area. Because of the buildings the light values just in front of the nest box are definitely lower than several meters higher. The data which are listed here do not attempt to represent the intensity of light even a few feet away from the nesting site. This means that the values of the immediate inflight surroundings are registered and not those which provide signals to the bird in flight to return to their roost. These values must be considerably higher. The bird has to calculate in advance when to go back to the nest to pass through the entrance hole successfully. Mistakes are made but these appear to be of extremely rare occurrence (Weitnauer 1990).

3. Results

In the year 1995 the times of the latest entering the nest of a breeding pair of swifts were noted and analyzed together with the intensity of light, weather conditions as well as certain biological data. The result is drawn in Figure 1. The weather conditions had been good so the behavior of the parent birds in this breeding season could be taken as exemplary. 

The seasonal sojourn of the swifts in our part of the world can be divided into four periods which have an effect on the nest entering times: 

1. the time before breeding, 
2. the breeding period, 
3. hatching and 
4. the phase after the fledglings have gained flight. 

The differences of the entering times of the different periods of the breeding cycle show clearly that the intensity of the light, although dominant is not the only relevant factor leading to the timing of the evening return. Furthermore there are another two factors which play an important role in the decision as to when to return to nest: the weather and social factors. 

Figure 1. Times for the final evening return of breeding pairs, giving the time of sunset and light-intensity (Lux) for the second bird, with some weather and breeding data (W wind, R rain)

4. Discussion

If one postulates the ideal situation of last entering, e.g. under warm weather conditions and a cloudless sky, one would obtain a smooth graphical curve. Another possibility to obtain the sense of a behaviour is by smoothing out the data. This is portrayed in Figure 2. In these curves the value of each day is the average of the 11 preceding and 11 following days. With this method one can clearly see the tendency to a later return with the progress of the growth of the young. In reality of course there are deviations (Figure 1) which can be explained by the following factors. 

Figure 2. Smoothed data showing changes in the time of last return to nest. The flight activity of breeders increases with dawn as hatching progresses (Legend see table 1; smoothing factor K 23)

4.1. Light Intensity

When the weather is clear and bright, with the exception of the following cases, the return of the second bird takes place exclusively at or immediately after sunset. From 155 returns to nest observed, 40 took place before and 115 after sunset. So sunset is not a fixed quantity for the birds. The times of flying activity are adapted rather to prevailing conditions. Before the eggs were laid the breeding pair flew in within a range of light intensity between 77 and 10900 Lux, with a tendency towards values between 77 and 176 Lux, during the period of breeding values between 17 and 360 Lux, with a greater tendency to lower values between 17 and 73 Lux. During the feeding period values between 8 and 159 Lux were measured, the majority between 17 and 20 Lux (35 % ). On the days of flight of the fledglings neither parent returned for the night. After the fledglings had achieved flight the values fell to between 9 and 18 Lux.

4.1.1. Weather factors

The influence of the weather is clearly noticeable. In 14 cases rainy weather, rain or thunderstorms (all R in table 1) were the reason for an early return to the nest. In comparison with rainless days, the return was between 9 and 59 minutes (average 25 minutes, n=14) earlier and was in fact irrespective of the light intensity. One reason for the early return can be found in the fact that the density of airborne insects is low when the weather is rainy and a feeding flight for swifts is not suitable. This observation is quite interesting and suggest that social behavioural patterns are less important for late flying than the need to conserve energy when adverse insect availability conditions prevail. 

The influence of windy or squally weather was recorded only three times. On the evening of July 14^th both swifts returned 67 and 54 minutes earlier in comparison with a cloudy July 17^th, despite a 36 Lux higher light factor. On the two other days (24.06. and 22.07.) the early return time was diminished by about 10 minutes.

4.1.2. Clouding over

The effects of weather, described in the preceding section, are caused, with the exception of the wind factor, by a clouding over and thus are linked to reduced light intensity. Early returns which were exclusively caused by clouds were registered only twice and occurred on 16.05. and 06.07. I failed to measure the Lux on May 16^th because I did not expected the last return at the early hour of 20.34.

4.1.3. Social Factors 

For further nine deviations from the postulated ideal line there are no obvious physical causes. These took place on May 5^th and 19^th with fine weather (1050 res. 374 Lux), May 21^st, 22^nd and 23^rd with blue sky, May 25^v and 27^th (cloudless, 10900 Lux, res. clouded and 4240 Lux) and July 2^nd and 4^th (each day cloudy, 36 and 20 Lux). The temperature range on those days lay between 8 and 19° C minimum, and between 18 and 28° C maximum and so were not unusual. With the exception of May 5^th, which was the day of return from Africa, social factors offer possible explanations. On May 26^th and 28^th the eggs were laid, lending credence to an early return to nest the evening before. Naturally this observation of only two dates lies within the limits of statistical error. But it is interesting that this behavior was noted on the two evenings before egg laying and was exhibited by both partners. Moreover the return on May 25^th took place at the same time. On the evening before the third egg was laid, the weather was poor so that this early return can not be taken as a sole reason for this behavior. But the same behavior can be noted in the graphics presented by Gory (1992). In five of the breeding pairs he observed that the first bird entered earlier on the evening before the eggs were laid.

The following events also speak in favor of social factors playing a role in early return to nest. May 21^st was day five before the laying of the first egg. This is the usual time for the gestation of swift eggs (Lack 1956, Weitnauer & Scherner in Glutz von Blotzheim 1994), so it can be assumed that mating occurred on the respective evenings. The same is true for May 19^th and the following days. In Gory’s graphics one can find at least one early return on the fifth day before egg laying. Finally July 2^nd and 4^th were the evenings before the chicks opened their eyes. The parents reacted to this incident in both cases. Because this occurred during the period of permanent presence in the nest Gory’s methods of measuring could not record it. 

The behavior of entering is remarkably similar with both members of the pair. On ten days they entered the hole consecutively with a lapse of under a second. Sometimes I was able to watch them flying so close behind one another towards the hole that one bird had to break the flight just to let the partner enter. The distance in those cases must have been even closer than in previously described cased. The human eye cannot differentiate with such precision. On 13 days both partners returned within two minutes of each other from their last flight. The biggest synchronisation was noted in the period without brood duties, such as before breeding and again four days before the fledglings took first flight. Around this time the phase of feeding the young by the parents is concluded (Schulz 1881, Weitnauer 1947, Lack & Lack 1951, Wendt 1988, Polaschek 1992). In the period between the egg laying and the eleventh day after hatching both parents entered the nest box together only once. This is the phase of the nearly continuously brooding (Cramp 1994, Gory 1992). It can be stated that swifts demonstrate impressive social contact.

In this context some answers to the question concerning a possible differentiated role played by the two sexes with relation to the last return to nest would be of great interest. While there are occasionally some statements about the bird’s sex in the literature (Weitnauer 1947, Zimmermann 1956, Flath 1980, Bezzel 1985), it is not made clear how the sexes are identified and were recognised. Such remarks are normally without value since live males and females can only be identified when copulating or , a recent discovery, when screaming (Kaiser 1997). To answer the above question the birds would have to be marked in a manner that which would make verification possible throughout the season without repeated catching, which is impossible under current protection laws.

In the investigations presented here the sex of the birds was not determined. The swifts were numbered by the sequence of entering. The last home-comer to the nest might have been the male or the female. So far - apart from time when copulation occurs and the calling pattern - it is not yet known whether there is a difference in behavior in the male and female swift, particularly concerning the sequence of the last evening entering of the nest.

Acknowledgments

I wish to thank Dr. Henry Hahnke, Museum of Natural History, Berlin for valuable criticism and Prof. Brian Hawkins, Berlin for the linguistic revision of the English text.

Note. Scheer 1949 published values of measurements of sun rising and morning flight in Darmstadt. The values are calculated again and are between 18-29 Lux under good and between 103-165 Lux under bad weather conditions. 

References
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© APUSlife 2000, No. 2340
ISSN 1438-2261

The German version “Das täglich letzte Einfliegen der Mauersegler (Apus apus) in Abhängigkeit von Brutphänologie, Umwelt- und sozialen Faktoren“ (APUSlist No. 0067) is to be published simultaneously in Berliner ornithologischer Bericht 09.