Taille du bec et période de reproduction chez les beccroisés des forêts de pins

Abstract

Within the genus Loxia , it is among the crossbills that feed on the cones of pine trees (Pinus spp.) that the greatest morphological differences occur. Pine Crossbills also present the largest variations in the timing of the reproductive period. The presumed selective force in bill structure is considered to be cone morphology. Cones and scales of various species of pine were measured and a relationship sought with the bill characters of the species and subspecies of crossbills living on the various pines in the western Palearctic and in South-East Asia. Crossbills are renowned for breeding opportunistically and using the food supply in timing reproduction. We gathered data on pine cone predation during their annual cycle and on the phenology of crossbills breeding in relation with the maturation and opening of the cones. For the whole set of samples studied, no correlation was found between the variables describing the pine cones and those of the birds’bills. Principal component analysis (PCA) of the matrix of nine geographic locations * four morphometric cone variables and three morphometric crossbill variables led to a distinction between two subsets. A “cold” subset towards the negative pole included three bird species : Loxia pytyopsittacus and L. scotia (from northern Europe) and L. curvirostra from the Pyrenees, with a single pine species (Pinus sylvestris). A “hot” subset towards the positive pole covered Mediterranean and Asian locations ; it involves one bird species : L. curvirostra and three pine species. So, at one end there are small cones and large birds (Scots Pines and northern European Crossbills) and at the other end there are large cones and small birds (Philippines). In these extreme situations of northern Europe and the Philippines, the structure of the bill does not therefore appear to be a character of adaptation to the size of the cones or the thickness of the scales. Parrot and Scottish Crossbills feed on the cones of the same Scots Pine as the Common Crossbill in the Pyrenees. The larger bill in the two former northern species could result from stronger selection pressure during the winter when the cones are closed and the seeds not yet ripe. On the other hand, in the Pyrenees, the cones ripen and open earlier making available large numbers of high-energy seeds in winter. Extra bill strength would not therefore be as advantageous as in Northern Europe. The Philippines Crossbill has the smallest bill size that is particularly noticeable on comparison with the Vietnamese subspecies, which uses the same Khasya Pine cones. This could be accounted for by the use of alternative food sources by the Philippines bird (mainly insects abundant in this tropical region) when the cones are closed and the seeds hard to reach. Statistical analysis of the central part of the PCA, corresponding to the Mediterranean, the Pyrenees and Vietnam (excluding the extreme locations : northern Europe and the Philippines) reveals a strong positive correlation between the proximal thickness of the scales and the index of curvature of the bill (r = 0.884 ; p = 0.019) in the six regions studied. The crossbills were all subspecies of L. curvirostra and the pines were of all the species studied. This supports the adaptive origin of the bill structure of the subspecies of Common Crossbills for the use of pine cones (excluding the insular form of the Philippines). The crossbills that live throughout the year in the pine forests can present two distinct breeding patterns : 1) strictly seasonal, limited to the period when ripe cones are available, as in northern Europe and the Philippines, 2) much more spread out, with an earlier start when the cones are still unripe as in southern Europe, North Africa and Vietnam. One of the originalities of the present work is to reveal a relationship between the structure of the bill and the breeding period. In crossbill populations where there is a correlation between the index of curvature of the bill and the thickness of the scales of the pine cone, the breeding period starts earlier and so lasts longer. This opportunistic capability of reproduction which becomes superimposed on the pattern of seasonality alone, presents numerous advantages for the population and, with the bill-scale correlation, indicates optimal adaptation to the use of resources in the different contexts : montane, Mediterranean and tropical. For crossbills in which the size of the beak is not correlated to the thickness of the cone scale, the length of the breeding season, and especially its earliness (before the cones are ripe) seems to be restricted by the availability of the food resources. Moreover the accessibility of food supply could have led to particular adaptations which can be morphological (large bill sizes in northern Europe) or behavioural (seeking alternative food supplies in the Philippines).