The Common CrossbillLoxia curvirostrain the Pyrenees: some observations on its habitats and on its relations with conifer seeds

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Abstract
Our study aimed to discover the major characteristics of the Pyrenean forests frequented by the Common Crossbill, the seed requirements of the birds, and the response of the birds to the variability of the seed supply in time and space. To realize the first aim, we carried out censuses and used literature data. To realize the second one, we fed captive birds with Mountain Pine cones and simulated seed demand through a bioenergetic model. To realize the third aim we gathered, over a few years, data on crossbill abundance and on seed production in 2 pine forests, surveyed crossbill densities in 2 stands in the same forest but with different seed supplies, and investigated the relationship between seed production and seed consumption in groups of trees. Common Crossbills are essentially birds of Scots Pines or Mountain Pines in the Pyrenees, these 2 species appearing equally preferred. Within pine forests, crossbills are more abundant at higher altitudes. From our samples, there is no great difference in abundance between open and dense forests. The estimates of daily individual seed needs obtained from the model agree fairly well with those obtained in captivity, though they usually are higher. The total annual requirement corresponds to the production of 80–280 Mountain Pines. We observed a decrease in crossbill numbers following a poor cone crop but the fluctuations in numbers observed in 6 years in the study forest were generally slight. Crossbills move from stand to stand according to the variation in the time of the seed supply and seed consumption in groups of trees was related to their seed production.