Seasonal changes in abundance and flight-related morphology reveal different migration patterns in Iberian forest passerines
Published: Volume 48(1), June 2001. Pages 27-46.
Although the attenuation of seasonality southwards in the temperate region is known to be a major determinant of the latitudinal decline in the migratory behaviour of many passerines, the spread of altitudinal migrations in southern areas as a response to similar changes with elevation still remains obscure, principally because no extensive ringing programs have been conducted in these areas. Here we use spatio-temporal changes in abundance and variations in flight-related morphology to infer differences in the migratory behaviour of forest passerines in the Iberian Peninsula, where the environmental optimum for these species moves seasonally between northern highlands (very suitable for breeding but unproductive in winter) and southern lowlands (less suitable for reproduction but highly productive in winter). According to this pattern of environmental conditions, we predict that migratory behaviour should be more intense in the highlands while sedentary behaviour should be the rule in the lowlands. We studied abundance and morphology of several common passerines (Robin Erithacus rubecula, Blackbird Turdus merula, Blackcap Sylvia atricapilla, Firecrest Regulus ignicapillus, Blue Tit Parus caeruleus and Great Tit P. major). In all species but Blackbirds, winter abundance increased in lowlands but decreased in highlands, despite some European migrants arriving at this later area for wintering. A common pattern of variation in flight-related morphology was found, birds having longer and more pointed wings in highlands. In addition, a transSaharan migrant species that we used as a control (the Nightingale Luscinia megarhynchos) did not differ in these traits between regions. Both the seasonal changes in abundance distribution and the morphological variations between regions indicate a stronger migratory behaviour of birds in highlands as compared to lowlands. This kind of movements, which are probably widespread across the Mediterranean region, are likely to constitute small-scale, altitudinal migration patterns overlapping with the broader latitudinal migratory system in the Palaearctic.