Roberto Carbonell

HISTORICAL DIVERSIFICATION OF MIGRATION PATTERNS IN A PASSERINE BIRD

Abstract Migratory strategies of birds require complex orientation mechanisms, morphological adaptations, and life‐history adjustments. From an evolutionary perspective, it is important to know how fast this complex combination of traits can evolve. We analyzed mitochondrial control‐region DNA sequences in 241 blackcaps (Sylvia atricapilla) from 12 populations with different migratory behaviors. The sample included sedentary populations in Europe and Atlantic archipelagos and migratory populations with different distances of migration, from regional to intercontinental migrations, and different heading directions (due to a migratory divide in central Europe). There was no genetic structure between migratory and sedentary populations, or among populations from different biogeographic areas (Atlantic islands, the Iberian Peninsula, or the continent), however we found evidence of a genetic structure when comparing populations located on either side of the migratory divide. These findings support an independent evolution of highly divergent migratory strategies in blackcaps, occurring after a postglacial colonization of the continent along western and eastern routes. Accordingly, mismatch‐distribution analyses suggested an expansion of blackcaps from a very small population size, and time estimates dated such an expansion during the last postglacial period. However, the populations in Gibraltar, located in a putative Mediterranean refuge, appeared to be independent of these processes, showing evidence of restricted gene flow with other populations and demonstrating insignificant historical changes in effective population size. Our results show that the interruption of gene flow between migratory and sedentary populations is not necessary for the maintenance of such a polymorphism, and that even the most divergent migratory strategies of a bird species are susceptible to evolution in response to historical environmental changes.

Bird conservation in fragmented Mediterranean forests of Spain: effects of geographical location, habitat and landscape degradation

Abstract The effects of habitat fragmentation on forest bird assemblages were analysed in 214 holm oak ( Quercus ilex ) remnants spread across the northern and southern plateaux of central Spain. Bird richness was highly dependent on fragment area for all species regardless of isolation, and barely affected by habitat traits. Geographical location was associated with high differences in richness of bird assemblages, which included 17 species exclusive to northern remnants and one exclusive to southern remnants. This supports the hypothesis that habitat suitability deteriorates sharply from north to south for forest birds in Spain. The species-area relationships of bird assemblages sampled in fragmented forests along a broad continental gradient (from Norway to southern Spain) showed that true forest birds only nest in woodlands >100 ha in southern Spain, whereas the full complement of forest species occurs in much smaller fragments in central-western Europe. Loss of species that are particularly sensitive to habitat fragmentation accounts for these differences between dry Spanish and mesic European woodlands. These results are explained by the low habitat suitability of Spanish woodlands, associated with the restrictive conditions for plant regeneration in the Mediterranean climate and long-standing human usage. There is, therefore, a particular need to develop management strategies that conserve birds, and probably other forest organisms, in Mediterranean regions by preventing habitat deterioration and decreases in fragment size, and by conserving all woods >100 ha.

Morphometric variation of five Iberian Blackcap Sylvia atricapilla populations

This paper analyses the variation of several morphological traits in five populations of Blackcaps Sylvia atricapilla distributed along a latitudinal gradient in the Iberian peninsula. The northern and central populations differ from the southern ones in their longer and more pointed wings, narrower bills, shorter tarsi and smaller body size. These features define two morphological groups and correlate with differences in their migration and feeding habits. Birds from northern and central Iberia breed in habitats with harsh winter conditions, which they abandon in autumn when they migrate to their wintering grounds. Birds from the mild, southern sectors remain there throughout the winter. Their migratory behaviour, and a stronger specialisation for feeding on foliage invertebrates, could explain the morphological differentiation of northern Blackcaps relative to southern ones. Our results suggest that the Iberian migratory populations might have descended from ancestral, southern-like ones, that have become adapted to exploit their seasonal breeding grounds.

A method for differentiating between sedentary and migratory Blackcaps Sylvia atricapilla in wintering areas of southern Iberia

We used the morphological differentiation of southern, sedentary Iberian Blackcaps Sylvia atricapilla to distinguish them from migratory Blackcaps during the winter. To do so, we studied three morphometric traits (wing length, wing shape and tail length) of three migratory Iberian populations and two sedentary ones during the breeding season. We obtained a discriminant function which allowed us to differentiate the two population types (>90% correct classifications). We discuss the utility of the method for differentiating between sedentary and migratory Blackcaps in their sympatric wintering areas, as well as the implications of our results for the study and conservation of sedentary, perhaps differentiated Blackcaps in southern Iberia.

Feather traits and ptilochronology as indicators of stress in Iberian Blackcaps Sylvia atricapilla

This paper compares feather traits to explain diferences in stress between three Blackcap Sylvia atricapilla populations distributed along a N–S gradient of increasing drought in the Iberian Peninsula. Indices based on the width of growth bars, length, mass and fluctuating asymmetry (FA) of rectrices were examined for age efects and variation between populations along the gradient. The growth bars of juveniles (but not adults), feather mass, mass/length ratio and FA varied between populations (narrower bars, lighter feathers and increasing FA in the southern populations), providing evidence of increasing stress of moult in Blackcaps of drier Iberian regions.

The importance of northern Spanish farmland for wintering migratory passerines: a quantitative assessment

Migratory birds are critically dependent on adequate wintering habitats for their long-term survival. Cantabrian farmland, a mixed agricultural landscape extending across the coastal lowlands of northern Spain, constitutes an important wintering area for many short-distance migrants coming from central-western and northern Europe. Unfortunately, the Common Agricultural Policy of the European Union and national afforestation schemes have resulted in a massive replacement of farmland by pine Pinus spp. and eucalypt Eucalyptus sp. plantations. This work assesses the importance of Cantabrian farmland as wintering grounds for short-distance European migrants and for wintering species that originate in nearby woodlands. We examined the seasonal changes in passerine bird populations in the Cantabrian region and used winter ringing recoveries obtained in the area to evaluate the contribution made by European migrants to winter populations. Bird communities were surveyed along 299 500-m long transects distributed between 67 farmland patches, 67 lowland forests and 14 upland forests. Winter assemblages were more diverse and species more abundant in farmland than in lowland or upland forests, whereas these differences were smaller in the spring. Bird numbers in farmland tripled in winter, numbers increasing by about 6.9 million birds compared to breeding populations. Most of this increase was accounted for by species that also bred in the region and that considerably increased their abundance (65.6% of all wintering birds, with the Chaffinch Fringilla coelebs responsible for 31.4% of the total increase) and by five exclusively wintering species (34.4%, with the Meadow Pipit Anthus pratensis accounting for 25.2% of the total increase). The main bulk of this increase is caused by the influx of European migrants. The importance of halting the current spread of eucalypt plantations (which increased over 400% over the past 30 years) and of applying more effective agri-environment schemes to achieve appropriate farmland conservation is discussed.