Acrocephalus 28 (133): 69-73, 2007 Yellow-legged Gull Larus michahellis in the Se~ovlje saltpans (SW Slovenia) – abundance, age distribution and habitat preference in post-breeding period in July 2003 Rumenonogi galeb Larus michahellis v Se~oveljskih solinah (JZ Slovenija) – {tevil~nost, starostna sestava in izbor habitata v pognezditvenem obdobju v juliju 2003 Linus Blomqvist Sardal Lyckebo 119, SE-31042 Haverdal, Sweden, e-mail: linus.blomqvist@gmail.com A large number of Yellow-legged Gulls, Larus michahellis, gather annually after the breeding season in the Se~ovlje saltpans, Slovenia. The abundance, habitat choice and age distribution were registered on several occasions by counting the gulls and marking their position on maps. The population reached a maximum of nearly 13,700 individuals on July 19, making it one of the largest in the Mediterranean, after which it decreased to approx. 10,000 on July 30. The majority of the gulls in the area were non-juveniles, i.e. birds in their second calendar year or older, which probably came there to moult. The percentage of juveniles was quite high at the end of the month. Most of the gulls spent the days outside the area, gathering a few hours before sunset to roost in the saltpans. The gulls roosted mainly on frm and dry ground providing clear views. Key words: Yellow-legged Gull, Larus michahellis, Se~ovlje saltpans, abundance, age distribution, habitat preference Klju~ne besede: rumenonogi galeb, Larus michahellis, Se~oveljske soline, {tevil~nost, starostna sestava, izbor habitata 1. Introduction Large numbers of Yellow-legged Gulls gather every summer in the Se~ovlje saltpans in coastal Slovenia. The The population of Yellow-legged Gull, Larus michahellis, few previous studies include counts of the population is distributed throughout the Mediterranean, parts of made every summer from 1983 to 1997 ([kornik the Aegean Sea, the Black Sea and in central Europe et al. 1998). A maximum of 4,000 individuals were to about 52°N. It increased substantially during the counted in August 1996 and 1997 (Makovec et al. last century, now numbering 150,000–200,000 pairs 1998). Rubini~ (in Polak (ed.) 2000) states that the (Blomdahl et al. 2003, Cramp & Simmons 1983, del largest number of Yellow-legged Gulls recorded in the Hoyo et al. 1996, Malling Olsen & Larsson 2003, entire saltpans area was 15,000, registered in August. Mullarney et al. 1999, BirdLife International 2004). A considerable proportion of the Yellow-legged 2. Methods Gulls stay close to the breeding sites year-round, but the populations in south-western Europe, the Adriatic The Se~ovlje saltpans are situated by the Slovenian Sea and the eastern Mediterranean migrate northwards coast in the Bay of Piran in the northernmost part to an increasing extent after the breeding season of the Adriatic Sea and cover an area of 738 ha. They (Blomdahl et al. 2003, Cramp & Simmons 1983, del consist of two parts: Fontanigge, where commercial Hoyo et al. 1996, Malling Olsen & Larsson 2003). salt production was abandoned in the 1960s, and Lera 69 L. Blomqvist: Yellow-legged Gull Larus michahellis in the Se~ovlje saltpans (SW Slovenia) – abundance, age distribution and habitat preference in post-breeding period in July 2003 14000 12000 10000 8000 6000 4000 2000 Method 1 Method 2 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Date / Datum Figure 1: Total abundance of Yellow-legged Gulls Laws michahellis on Se~ovlje saltpans in July 2003, using methods 1 and 2. Method 1 involved successive sub-area counts (5-7 days) with the median date shown, and method 2 involved counting all sub-areas on one day. Slika 1: Skupna {tevil~nost rumenonogih galebov Larus michahellis v Se~oveljskih solinah, z uporabo metode 1 in 2. Metoda 1 pomeni zaporedno {tetje na podobmo~jih (v razmiku 5-7 dni), prikazana je mediana datuma. Metoda 2 pomeni {tetje na vseh podobmo~jih naenkrat. Prikazani so dnevi v juliju 2003. (263.5 ha), where salt is still being produced (Beltram 1996). This study focuses on Lera, which consists of shallow basins intersected by channels and dikes. The following roosting habitats have been defned: water-flled basins, wet mud in basins, dry mud in basins, low dikes, high dikes without vegetation and high dikes covered with vegetation. The population was monitored using two methods, both carried out in evenings from 1-30 Jul 2003, using telescope and binoculars. The total number of feld days was 20. The area was divided into ten sub-areas and in each one of these the population was monitored on three occasions with a method involving marking the position and activity of the gulls on a made-to-scale map. Water and weather conditions were also recorded. To calculate the abundance of Yellow-legged Gulls in the whole area from this data, the numbers of gulls in the sub areas in one monitoring round (when all areas were monitored over a 5 to 7 day period) were added (referred to as “Total A”). The median date was then chosen as the date for the total number. On four occasions, the number of individuals in all sub areas was counted during one evening (referred to as “Total B”) and, on 19 Jul all individuals in all sub areas were counted on four occasions during the day. Here, a second monitoring method was used, involving counting the gulls in groups of ten, recording only total numbers in the different sub-areas and thus not their exact position or activity. The numbers of juvenile individuals (i.e. birds in their frst calendar year) in groups of Yellow-legged Gulls ranging from 48–356 individuals (mean 136) were counted regularly, allowing calculation of the proportion of juveniles, as defned above, and non-juveniles, i.e. birds in their second calendar year or older, in the population. 3. Results and discussion 3.1. Abundance The number of Yellow-legged Gulls roosting in the Se~ovlje saltpans increased from about 8,000 in early July to a maximum of nearly 13,700 on 19 Jul (Figure 1). The main cause for the increase was probably the increasing numbers of gulls leaving their breeding grounds after the breeding season, which normally ends in late June or early July (Cramp & Simmons 1983). The number thereafter decreased to about 10,000 on 30 Jul (Figure 1). Access to suitable roosting habitat can probably be excluded as a possible limiting factor to the total abundance. The sum of the highest recorded numbers in the different sub-areas was more than 18,000, i.e. much higher than that recorded in the whole area on 19 Jul. This shows that, even on that occasion, not all possible roosting areas were occupied. 3.2. Age distribution In early July the Se~ovlje saltpans were visited by a large number of non-juvenile Yellow-legged Gulls (Figure 2), probably immature birds or failed breeders (Cramp & Simmons 1983). In mid July the numbers were even higher, probably due to the arrival of adults having fnished breeding. The majority of the non-juveniles probably come to the Se~ovlje saltpans mainly to moult intensively over a limited period of time. Moulting consumes a lot of energy and the gulls therefore need abundant food resources which they fnd on nearby refuse dumps. Furthermore, suitable loafng and roosting places are available in the saltpans, making the Se~ovlje saltpans a suitable place for moulting. Large numbers of adults gather similarly after the breeding season for moulting in other places in the Mediterranean (e.g. the Ebro delta in Spain) (Malling Olsen & Larsson 2003). At the end of July, approx. 70% of the Yellow-legged Gulls in the Se~ovlje saltpans were juveniles. 3.3. Diurnal variation of numbers On 19 Jul, the number of Yellow-legged Gulls was 2,810 at 8.00 h, decreasing to 1,660 at noon. The 7C Acrocephalus 28 (133): 69-73, 2007 14000 1 12000 g 10000 ¦k ¦a 4) M 0 >S 8000 "v JS I s 6000 L 4000 2000 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Date / Datum Figure 2: Age distribution of the Yellow-legged Gulls Laws michahellis on Sečovlje saltpans in July 2003. Black bars denote birds in juvenile plumage and white bars denote non-juvenile birds. Numbers are extrapolated from the count in smaller groups (48-356 individuals, mean 136). Slika 2: Starostna struktura rumenonogih galebov Larus michahellis na Sečoveljskih solinah v juliju 2003. Črni stolpci označujejo število juvenilnih osebkov, beli število vseh ostalih starosti. Število je ekstrapolirano iz štetja manjših skupin (48-356 osebkov, povprečje 136). number thereafter increased slowly until 16.00 h and very fast between 16.00 and 20.00 h, when it reached 13,673. The majority of the Yellow-legged Gulls thus spent the day outside the Se~ovlje saltpans, leaving the area in the early morning and returning just before sunset. 3.4. Habitat preference High dikes without vegetation constituted the preferred roosting habitat (occupied by an average 66% of the gulls), followed by dry mud in basins (26%), wet mud (5%), low dike (2.4%) and water-flled basin (0.75%) (Figure 3). No gulls roosted on high dikes covered with vegetation. The concentrations on the different habitats (Figure 4) support this distribution. The insignifcant number of gulls occupying water-flled basins shows that this is not a frequently used roosting habitat, in keeping with the absence of records that Yellow-legged Gulls normally roost or loaf in water. The individuals that stood in water-flled basins were generally more active than those standing on dikes or mud in basins and their activity can thus be considered foraging rather than roosting. The latter birds probably moved to another habitat at sunset. Two factors, apart from disturbance, appear to be important for the Yellow-legged Gulls when choosing a roosting place – the view from the roosting place and its condition (consistency and dampness). The material on which the gulls appear to prefer to roost is dry, solid mud. Earlier studies have shown that the gulls want a good view in order to escape predators (Cramp & Simmons 1983). This conclusion is supported by two observations. One is that the concentration of gull droppings on a broad dike with small hills about 4 dm high was considerably higher on the hilltops than on the surrounding dike. The second is that, in a ditch with dry mud in the bottom, gulls only occupied the sides of the ditch and not the bottom, where the view was very limited. On high dikes without vegetation, both criteria for the view from the roosting place and its condition are met. The fact that no gulls occupied high dikes covered with vegetation highlights the importance of the view. The ji L. Blomqvist: Yellow-legged Gull Larus michahellis in the Se~ovlje saltpans (SW Slovenia) – abundance, age distribution and habitat preference in post-breeding period in July 2003 100% - g 90% - * s m O ë u o S D. v. 50% - 75 i 40% - .a ¦a .E L I 20% - L 10% -0% - Dry mud in Wet mud in basins/ High dikes without Low dikes/ High dikes covered Water-filled basins/ basins / Suho Vlažno blato v bazenih vegetation/Visoki Nizki nasipi with vegetation/ Bazeni z vodo blato v bazenih nasipi brez vegetacije Visoki nasipi z vegetacijo Habitat Figure 3: Average percentages of Yellow-legged Gulls Larus michahellis in different habitats in Sečovlje saltpans. Error bars denote standard deviations. Slika 3: Povprečni procent rumenonogih galebov Larus michahellis v različnih habitatih v Sečoveljskih solinah. Označena je standardna deviacija. i i ! 'S I s o Ü 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 Dry mud in basins / Suho blato v bazenih Wet mud in basins/ Vlažno blato v bazenih High dikes without vegetation/Visoki nasipi brez vegetacije Low dikes/ Nizki nasipi High dikes covered with vegetation/ Visoki nasipi z vegetacijo Water-filled basins/ Bazeni z vodo Habitat Figure 4: Average concentrations (gulls/m2) of Yellow-legged Gulls Larus michahellis in different habitats in Se~ovlje saltpans. Error bars denote standard deviations. Slika 4: Povpre~na gostota (galebov/m2) rumenonogih galebov Larus michahellis v razli~nih habitatih v Se~oveljskih solinah. Ozna~ena je standardna deviacija. 72 Acrocephalus 28 (133): 69-73, 2OO7 assumptions can furthermore be corroborated by the fact that the main predation of the gulls in Se~ovlje saltpans comes from two terrestrial predators – Red Fox Vulpinus vulpinus and Stone Marten Martes foina. Higher bare points offer good views to terrestrial predators but, at the same time, gulls’ exposure to air predators is minimal, since air predators such as Goshawk Accipiter gentilis or Peregrine Falcon Falco peregrinus are rare at this site. This does not, however, account for the observation that the concentrations of Yellow-legged Gulls in several cases increased very drastically on low dikes and wet mud in basins, while the concentrations on dry mud in basins and high dikes without vegetation were more constant. This indicates that, when the preferred habitats are fully occupied, the gulls have to occupy less preferred habitats such as wet mud in basins and low dikes. There is some view over possible predators from the bottoms of the basins, where a considerable proportion of the individuals roosted. What differs between dry mud in basins, wet mud in basins and low dikes is the consistency and the dampness, and therefore dry mud, which is dry and solid, is preferred. Acknowledgements: Many thanks to Andrej Sovinc and Iztok [kornik, Slovenia, and John Strand, Sweden, for invaluable help. 4. Povzetek V pognezditvenem obdobju se v Se~oveljskih solinah zbere veliko {tevilo rumenonogih galebov Larus michahellis. Populacijo je avtor spremljal v juliju 2003 z namenom dolo~iti njene parametre glede {tevi~nosti, izbora habitata in starostne sestave. [tevilo rumenonogih galebov je doseglo maksimum 19.7.2003, ko je na{tel kar 13,700 osebkov, kar pomeni eno najve~jih pognezditvenih skupin v Sredozemlju. Po tem datumu je {tevilo upadlo na pribl. 10,000 dne 30.7.2003. Ve~ina osebkov ni bila mladostnih, procent mladostnih osebkov je narasel le na koncu meseca. Ve~ina galebov se je med dnevom zadr`evala izven solin, kjer so se zbrali pred son~nim zahodom z namenom preno~evanja. Galebi so preno~evali v glavnem na trdnih in suhih tleh z jasnim razgledom. 5. References Beltram, G. (1996): The conservation and management of wetlands in Slovenia in the context of European policy related to wetlands. Ph.D. Thesis. - Vrije Universiteit, Brussels. BirdLife International (2004): Birds in Europe. Population Estimates, Trends and Conservation Status. - BirdLife International, Cambridge, UK. Blomdahl, A., Breife, B. & Holmström, N. (2003): Flight Identifcation of European Seabirds. - Christopher Helm, London. Cramp, S. & Simmons, K.E.L. (eds.) (1983): Handbook of the Birds of Europe the Middle East and North Africa. Vol. III. - Oxford University Press, Oxford. del Hoyo, J., Elliot, A. & Sargatal, J. (eds.) (1996): Handbook of the Birds of the World. Vol. III. – Lynx Editions, Barcelona. Makovec, T, Škornik, I. & Lipej, L. (1998): [Ecological evaluation and conservation of the important bird species in the Se~ovlje Salina]. – Falco 12 (13/14): 5-48. (in Slovene) Malling Olsen, K. & Larsson, H. (2003): Gulls of Europe, Asia and North America. - A&C Black, London. Mullarney, K., Svensson, L., ZeTTERSTRÖM, D. & Grant, P.J. (1999): Collins Bird Guide. - Harper Collins, London. Poiak, S. (ed.) (2000): Important Bird Areas (IBA) in Slovenia. - DOPPS, Ljubljana. Arrived / Prispelo: 23.8.2004 Sprejeto / Acceped: 10.12.2007 73