Breeding and wintering Mediterranean Gulls do not mix
A new breeding population of Mediterranean Gulls Larus melanocephalus in the species’ main wintering area maintains independent spatial dynamics. Carboneras, C. & Dies, J. I. 2016. IBIS. DOI: 10.1111/ibi.12324. View
Studying colour-ringed gulls
As part of my PhD research, I carried out a detailed monitoring of a population of Mediterranean Gulls Larus melanocephalus wintering at the species’ main stronghold off NE Spain. I compiled a database of colour-ringed individuals occurring in the area over 5 seasons 2005-2010, totalling 625 individuals – a notable 2.5 % of the entire number of Mediterranean Gulls ever to be colour-ringed. This provided a good basis for the study of bird demography and migration strategies from the point of view of the wintering areas, which was the main objective of my investigation.
My work was touched by serendipity when a breeding colony of the same species – established near Valencia, a mere 60 km from the southern limit of the wintering area – started to grow exponentially at the same time of my study. The breeding population started with a single pair in 2001 and grew to 447 pairs in four colonies in 2014. Several of the birds present at the main breeding colony in the Albufera de València natural park were wearing colour-rings that indicated they had been born outside Spain, so I decided to investigate their origin. Seabird colonies often grow as a result of immigration from other colonies, and this possibility raised a number of interesting questions. If the colonies were indeed attracting immigrants, which could be the likely origin of those recruiters? Had they spent the winter locally and stayed on to breed? What was the relationship between the breeding and the wintering populations? How many birds did they have in common?
Figure 2 Views of the beach in Vilanova i la Geltrú, Barcelona, a typical locality in the wintering area with wide, vegetated beaches near an important fishing harbour. In winter, Mediterranean Gulls invariably roost at sea, but during the day they often loaf on beaches and can tolerate up to medium levels of human disturbance.
Living close, doing differently
Given the proximity between both areas, it would make sense to expect a number of individuals following the short-distance strategy, i.e., wintering locally and staying on to breed. This could have great implications for the future distribution of the species in the western Mediterranean, where the wintering population is in excess of 40,000 individuals. However, unexpectedly, the data showed that breeding and wintering birds followed different dynamics altogether, and probably did not even meet. The databases of breeding and wintering colour-ringed birds had no individuals in common, and the composition of each population was different.
Wintering areas differ in the proportion of birds born in the Atlantic colonies (mostly in Belgium, the Netherlands and NW France) as opposed to Mediterranean/Black Sea colonies. And, while Mediterranean/Black Sea birds dominate in the wintering population in NE Spain, the breeding population in Valencia consisted mostly of birds born in Atlantic colonies. The Atlantic : Mediterranean ratio pointed at the population wintering in Malaga (S Spain), 700 km away, or in SW Portugal, 1200 km away, as the most likely source of new breeders recruiting to the Valencia colony.
Mediterranean Gulls’ continued expansion
The Mediterranean Gull is widely known for the large-scale expansion of its breeding range across Europe since the 1940s, from the original stronghold in the Black Sea westwards to the Atlantic, Baltic and North Sea. What was possibly the same movement led to the establishment of new breeding colonies in the Mediterranean, e.g. in the French Camargue since 1965 and the Italian Po delta in 1978, where regular wintering also occurs. However, in the Spanish Mediterranean, which hosts the bulk of the global population in the winter months, breeding had only been sporadic or in small numbers before the colonies in the Valencia region took off.
Figure 3 Distribution of breeding and wintering Mediterranean Gulls plotted according to the European Ornithological Atlas 50 x 50 km grid, updated from Bekhuis et al. (1997). The dotted line indicates the likely migratory route of the western European (Atlantic) population southwards to Iberia. Map: S. Requena.
The new breeding population grew by attracting immigrants, but these did not come from nearby. Instead, a more likely origin was the Atlantic population, which now forms the majority of the birds wintering in Portugal and Malaga, and which migrates along the Atlantic coast. A small number of birds from that population have established to breed near Valencia and probably return every winter to their traditional wintering areas in southern Iberia. They behave as if the important wintering area to the north of their colony did not exist because, at the time of their arrival, the winter visitors have all left. Likewise, the birds forming part of the wintering population, which has existed for several decades, do not seem to take advantage of the emergence of new breeding opportunities nearby and consistently migrate to their distant breeding grounds.
In conclusion, the Mediterranean Gull continues to expand its breeding distribution but the mechanisms may be different from expected, and are not determined by distance. The breeding and wintering populations of the same species may be made of different individuals, and they may tend to maintain their independent momentum and spatial strategies. This indicates that shifts in distribution (e.g., those expected as a result of a changing environment) may not necessarily be simple, linear processes.
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Featured image: Breeding Mediterranean Gulls, adults with chicks in the Albufera Natural Park colony in Valencia © J.I. Dies.
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