I risultati - Gruppo Inanellamento Limicoli (GIL, Napoli)

Sex ratio and biometrics of the European Swallow (Hirundo rustica) on autumn migration in southern Italy

Sergio Scebba, Giuseppe La Gioia, Silvia Capasso & Gabor Lövei

SUMMARY

Since 1991, the Gruppo Inanellamento Limicoli (G.I.L., Napoli) has been working on a long-term study of migrant species in order to describe migrant flows in southern Italy. This paper gives a brief summary of the Swallow ringing activity between 1996 and 2002 at the Lake of Lesina (Apulia region), the location of one of the biggest migratory Swallow roosts.
In total, 36,222 Swallows were mist-netted during the autumn periods, among which 30,758 were juveniles (85%) and 5,410 adults (15%). Of the adults, 1,042 (19%) were classified as males and 2,910 (54%) as females, whereas 1,458 (27%) remained unsexed.
Measurements of wing length, third primary length and body mass indicated that young Swallows were smaller than older birds. Most birds had medium fat reserves; the proportion of Swallows with high fat scores (5 and 6) tended to be higher in adults than in juveniles.
Only two birds carried rings from elsewhere, ringed in Germany and in Slovenia; eleven of the Swallows ringed at the Lake of Lesina have produced long distance recoveries: in the Central African Republic, Congo, Cameroon, Libya, Malta and Greece.

INTRODUCTION
The European Swallow (Hirundo rustica) is distributed over the whole of Italy, although in areas under modern agricultural cultivation and in urbanized centres is on the decrease (Niederfriniger 1993). A similar situation has been documented in several other countries: Finland (Saurola 1997), Great Britain and Ireland (Gibbons et al. 1996, JNCC 1996), Switzerland (Weggler et al. 2000) and Kazakhstan (Gavrilov et al. 2002). The main causes of decline seems to be the high intensity of agricultural cultivation and the modernization of animal husbandry (Jenni 1997; Weggler et al. 2000, Møller 2001).
To find out about the extent and causes of this general decline, the Italian Ringing Centre started the "Swallow Project" in 1994. In 1997 this became an EURING project and was extended to most of other European countries. The project has two important aims: 1) to describe the variability of survival rates and dispersal; 2) to study the impact of human activities on Swallow populations (Jenni 1997).
Since 1991, the Gruppo Inanellamento Limicoli (G.I.L., Napoli), the first such group in southern Europe, has been working on a long-term study of migrant species in order to describe migrant flows in southern Italy.
This paper gives a brief summary of the Swallow ringing activity between 1996 and 2002 at the Lake of Lesina (Apulia region), the location of one of the biggest migratory Swallow roosts of southern Italy.

STUDY AREA AND METHODS
The study was done near the mouth of the Lauro canal system (41.54N 15.34E), on the eastern side of the Lake of Lesina (near Foggia, Southern Italy). The Lake of Lesina is a shallow brackish water lake of 5328 ha; the eastern third of which is covered by extensive reedbeds. The reedbeds are dominated by Phragmites australis, with some Arundo donax and Typha angustifolia. The surrounding area, made up of pastures and cultivated fields, provides an excellent feeding-area for Swallows.
Roosting Swallows were tape-lured and mist-netted between 24 August - 1 September and 6 - 8 September 1996; 25 August - 31 August and 12 - 14 Sept 1997; 25 August - 5 September 1998; 24 August - 3 September 1999; 24 August - 3 September 2000; 22 August - 1 September 2001; 24 August - 3 September 2002. Mist nets were 9-12 m long, 3 m high and had four shelves and a 16 mm mesh size. Seventy-eight metres of mist-nets were used in 1996-2001 and 159 m in 2002. Loudspeakers with Swallow song were switched on at 18:30 and switched off at variable times, usually 20:30 in the years from 1996 to 1998 and 21:30 in later years.
All Swallows were ringed but only adults were all measured. Due to the large numbers captured and in order to release birds as quickly as possible, only the first 100 juveniles were measured every evening. The following data were collected: body mass at 0.1 g accuracy, fat score (following a scale ranging from 0 (no fat) to 8 (maximum amount of subcutaneous fat) as proposed by Kaiser 1993), wing length (maximum chord method, Svensson 1992) and third primary length (measured to the nearest 0.5mm, Berthold & Friedrich 1973). Age and sex were established following Svensson (1992): individuals with a tail fork ?59 mm were classified as males, and those with ?50 mm as females. Individuals with intermediate values, or those with broken streamers were unsexed.

RESULTS AND DISCUSSION
The number of Swallows using the roost in autumn was estimated at dusk to be between 1,000 and >10,000 birds. In total, 36,222 Swallows were mist-netted during the autumn periods of 1996-2002, among which 30,758 were juveniles (85%) and 5,410 adults (15%). Fifty-four birds were not aged. This ratio between young and old birds was similar to that of 24 other Italian roosts (Pilastro et al. 1998) examined during September 1996. Of the adults, 1,042 (19%) were classified as males and 2,910 (54%) as females, whereas 1,458 (27%) remained unsexed (Table 1).

Table 1. The age and sex class composition of Swallows ringed at the Lake of Lesina, southern Italy, between 1996-2002.

Measurements of wing length, third primary length and body mass (Table 2) indicated that young Swallows were smaller than older birds. This is the case for many other passerine species (Alatalo et al. 1984). The mean body mass of juveniles (19.3 ± 1.82g, n = 12,429) was lower than that of adults (20.1 ± 1.94g, n = 4,478). All the differences between juvenile and adult were statistically significant (p<0.001). These values were similar to those calculated for 24 Italian roosts (juveniles 19.26 ± 1.76g, n = 7,028; adults 20.57 ± 1.93g, n = 1,562, Pilastro et al. 1998).

Table 2. Biometric data for Swallows ringed on Lake of Lesina, southern Italy, between 1996-2002.

Figures 1 and 2 summarise the data for fat scores of juveniles (n = 12,485) and adults (n = 4,451), respectively. Most birds had medium fat reserves (score 3, 39.9% and 49.1%, respectively). The proportion of Swallows with high fat scores (5 and 6) tended to be higher in adults (10.8%) than in juveniles (6.5%), even though this trend was not statistically significant. None of the birds had a fat score 8 and only one adult had a fat score of 7. The mean body mass of birds with fat score = 0 was 17.36g (± 1.28g, n = 86) for juveniles, and 18.15g (± 1.81g, n = 59) for adults. Detracting this from the mean body mass, 1.94g and 1.95g as mean fat load for the two age groups results. When considering only the heaviest 25% of individuals (the value usually considered as mean body mass at departure, see Alerstam & Lindström 1990), the fat load was 4.4g in the juveniles and 4.6g in the adults. This corresponded to 25.3% of the lean body mass of both juveniles and adults. The body mass of the heaviest juvenile Swallow trapped in this study (29.3g) corresponding to 68.8% of the mean body mass of lean birds.

Figure 1. Frequency distribution of fat scores of juvenile Swallows trapped on the Lake of Lesina, southern Italy, between 1996-2002.

Figure 2. Frequency distribution of fat scores of adult Swallows trapped on the Lake of Lesina, southern Italy, between 1996-2002.

Although there was variability among years with respect to sex and age classes, the comparisons among different years is confounded by the different operation schedule, namely the different hours of evening capture. This is suggested to significantly influence age class composition (Scebba et al. 2001). However, with the similar evening operation regime in the last three years, we can cautiously indicate a successful year for reproduction in 2002, as the relative share of juveniles was considerably higher than in the two earlier years (Table 1).
Among these 36,139 ringed Swallows, only two birds carried rings from elsewhere. One was ringed in Germany and another one in Slovenia, both during autumn migration. Eleven of the Swallows ringed at the Lake of Lesina have produced long distance recoveries: seven individuals were retrapped during autumn migration (from 7 October to 22 November) in the Central African Republic (four), the Congo (two) and Cameroon (one). Two birds were recovered in Malta, one in Libya and an other one in Greece during spring migration. Year-to-year stopover site fidelity was demonstrated. Thirty-two birds were recaptured at the Lake that were ringed there in earlier years. Twenty-four birds were recaptured after 1y, five after 2y, two after 3y, and one after 4y.
The scarce recapture rates indicated that Swallows passing through the Lake of Lesina probably originated from areas where there was no massive ringing effort. A more detailed analysis of trends in age class and body mass variability from year-to-year could indicate the general conditions at this site as well as of feeding areas further north.

ACKNOWLEDGEMENTS
This study was partially supported by the Earthwatch Institute, Cambridge, Mass., USA. We thank the field assistance provided by EarthCrops volunteers in years 1998-2002.

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Paper n°. 51 from the Gruppo Inanellamento Limicoli (G.I.L., Napoli)