La ricerca in provincia di Latina - Gruppo Inanellamento Limicoli (GIL, Napoli)

Timing of the spring migration of the Song Thrush Turdus philomelos  through southern Italy

Abstract from: The Ring (2014) 36: 23-31.

The project was based on standardized mist-netting from the second 10-day period of January to the first/second 10-day period of March over three years, 2012-2014. Approximately 100 m of nylon mist-nets (3,75 m high x 18 m long, mesh size 28 mm) with five shelves were operated. The mist-netting took place in seven sessions per 10-day period carried out in the afternoon, at least three hours before sunset until dark. In total 43 capture sessions were conducted in 2012, 44 in 2013 and 49 in 2014. Futhermore, to locate the main area of origin of the marked population, we examined a sample of 152 foreign recoveries that occurred in Latium. During the study period regular observations of one hour for each capture session were carried out, counting all the thrushes that passed within about 30 m from the nets. Fieldwork was carried out at Pianara (41.21N/13.27E), a place in the southern part of Agro Pontino, not far from the coast line (7 km from the Gulf of Gaeta) about half way between Rome and Naples. In total, 341 Song Thrushes were captured from 2012 to 2014. The trend of the catches in standardized 10-day periods is shown in Figure 1, which also shows the trend for each year.

Figure 1. Trapping patterns (%) per 10-day period during the period 11 January/20 March, 2012-2014.

In the three years the number and distribution of catches show quite different trends. In 2012 the recorded catches in the first three 10-day periods of investigation (January 11 to February 10) represented only 14% of the entire sample; starting from the second 10-day period of February there was a slight but steady increase until it reached a sharp peak in the first 10-day period of March, which corresponded to 59% of the total sample. In 2013 a rather high number of catches was registered in the last two 10-day periods of January (50.9%) and a number of recoveries occurred in subsequent 10-day periods, from a minimum of 15 to a maximum of 41 days after the first ringing. Throughout the month of February there was an almost constant number of catches, with percentages around 11-13% and recoveries occurred until the last days of the month. From the first days of March there was a sharp decline, although a modest increase in catches was observed again in the second 10-day period with the latest ringed March 17. In 2014, a fair percentage of catches (28.5%) was concentrated in the last two 10-day periods of January and a number of recoveries occurred several days after ringing. From the first 10-day period of February there was a modest increase in catches that reached its peak in the first 10-day period of March and then decreased drastically in the second in which the percentage of catches was only 4.8%.
The observations made in the area surrounding the nets during the working hours of the ringing station basically followed the trend of catches. The number of sightings is indicative of the wintering individuals passing through the ringing station at sunset to reach roosting sites. Throughout the study period 7,255 birds were counted: 1,009 in 2012, 5,082 in 2013 and 1,164 in 2014.
Of the 341 thrushes ringed over three years, 39 were controlled of which 10 were one year and two were two years after first ringing; in addition, 12 recaptures were made after a period of 1 to 8 days and 20 after a period of 13 to 56 days. The examination of the body mass of these individuals showed, for all birds ringed by the end of February a decrease of 1.0 to 5.8 g or a minimal increase of 0.7 to 1.0 g while individuals taken in March showed an increase in weight of 0.7 to 4.6 g (Figure 2).

Figure 2. Changes in weights (g) of individual Song Thrush, years 2012-2014.
Weights of the same birds measured on different occasions are connected with a line.

Figure 3 shows the average body mass trend per 10-day periods. It should be noted that the lowest mean weight was recorded in the first ten days of February, while from the following 10-day periods there was an increase of the mean with the highest values recorded in the last three 10-day periods of activity.

Figure 3. Variation of mean body mass, per 10-day periods, years 2012-2014. Sample sizes for each 10-day period are given.

Since the increase of the body mass depends on fat deposit, figure 4 shows the percentage of subjects with a score of fat greater than or equal to 3, calculated per 10-day periods.

Figure 4. Fat expressed as percentage of birds with amount of visible fat scoring at least 3, per 10-day periods,
during the period 11 January/20 March, years 2012-2014. Sample sizes for each 10-day period of birds with high fatness index are given.

The graph shows the percentage of birds with fat deposits scoring at least 3 including those recaptured in each 10-day period. The birds with large fat deposits (scores 3 and 4) represent only 11.4% of the total sample and are concentrated between the second 10-day period of February and the second 10-day period of March, while the remaining 88.6% had fat reserves that were very small or even absent (scores 0-2).
Mean wing lengths, third primary and tarsus lengths calculated for the three years and for all years combined are presented in Table 1. The samples of the years 2013 and 2014 also including data of birds ringed during the previous year.

Values obtained for the three morphometric variables did not differ from year to year. The biometric measurements suggest that the population of thrushes visiting the area is fairly homogeneous in size. In particular the mean wing length recorded for birds captured during this study is consistent with the values obtained in the period from January to March in the province of Salerno in Campania (118.6 mm during 2013 and 118.7 mm during 2014) (Scebba 2014) and in the years 1997-1999 (118.6 mm, d.s. 2.80, n = 647) (Scebba e Moschetti 2000) and 2003-2005 (118.5 mm, d.s. 2.48, n = 1.340) (Scebba 2006) on birds ringed in autumn on the Volturno Plain (Caserta), about 50 km away from the ringing station of Pianara. It is therefore possible to think that these three capture sites intercept the same northward migratory flow along the south-Tyrrhenian coast.
The spatial analysis of data of birds ringed in other countries during the breeding season and recovered in Latium indicates one main area of origin of the marked population centred in central-eastern Europe. Some 69% of recoveries of foreign-ringed birds are from Hungary, Slovakia and the Czech Republic.
The data collected indicate that the study area is mainly visited by wintering thrushes that concentrate at the site of capture by moving from pastures to night-time roosts. The analysis of the catches and observations confirms that from year to year there may be more or less marked fluctuations in the number of birds visiting the area during the winter, probably due to erratic movements in relation to food availability and weather conditions.
When the autumn migration is over, the birds wintering in the area remain during the months of January and February and sometimes until March, as confirmed by the recoveries which occurred in early March of thrushes which were ringed in January. In addition, recovery data occurred after a period of one or two years showed that several individuals were faithful to the wintering area.
The analysis of average body mass and fat of birds examined during the present study showed that only from the end of February did the birds show significant increases in weight, due to the deposit of fat necessary for the migration, consistent with the onset of the hyperphagia phase. Remarkable in this respect is the weight change of the thrushes recaptured in the same season: the majority of subjects taken until the second 10-day period of February showed a reduction of body weight, while all individuals taken in early March had significant weight increases. In agreement with this fact, the analysis of the accumulated fat suggests that birds with large fat deposits (scores 3 and 4), then have sufficient energy to undertake migration.
These data also differ from what was found by Licheri and Spina (2002), which showed, instead, an increase in fat levels as early as the third 10-day period of January and up to the third 10-day period of February, but it must be considered that the latter referred to a rather heterogeneous sample as it included data from birds captured throughout Italy in the years 1982-1999.
On the basis of information gathered during the three years of investigation it is possible therefore to assume that, for Latium, in the study area, and for southern Italy in general, by January 31 the pre-nuptial migration is not yet started but only starts from the second to third 10-day period of February.

REFERENCES

Licheri, D., Spina, F. (2002) Biodiversità dell’avifauna italiana: variabilità morfologica nei Passeriformi (parte II: Alaudidae – Sylviidae). Biologia Conservazione Fauna 112: 1-208.
Scebba S. (2006) Tordo bottaccio Turdus philomelos: studio della migrazione autunnale in Campania. Uccelli d’Italia 31: 26-49.
Scebba S. (2014) Studio per la determinazione della data d’inizio della migrazione prenuziale del Tordo bottaccio in provincia di Salerno. II Resoconto. Ambito Territoriale di Caccia Salerno 1: 1-17. Relazione Tecnica non pubbl.
Scebba S., Moschetti G. (2000) Biometria e rapporto giovani/adulti nel Tordo bottaccio Turdus philomelos: analisi preliminare della migrazione autunnale in Campania. Uccelli d’Italia 25: 18-24.

Scebba S., Soprano M., Sorrenti M. (2014) Timing of the spring migration of the Song Thrush Turdus philomelos through southern Italy. Ring 36: 23-31.