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Updated in 2020/9/2 下午 09:57:31      Viewed: 12 times      (Journal Article)

Assessing four decades of wintering crane counts in Spain, Portugal and Morocco

Juan C Alonso , Javier A Alonso , Alejandro Onrubia
The Common crane winter counts carried out in Spain, Portugal and Morocco during the last four decades show a extraordinarily increasing trend, from the ca. 40000 birds counted in the mid 1980's to a total of 232298 birds in winter 2013/14. Such a marked increase cannot be explained as an intrinsic demographic growth alone, given juvenile productivity values measured in this species, and reasonable mortality estima­ tes based on values published for other cranes. Additional factors contributing to the trend observed have surely been: (a) an increase in survey coverage; (b) a progressively better knowledge of the surveyed areas, particularty in Spain; (c) a northward shift of the wintering range, from relatively high numbers wintering in unknown sites in Morocco in the past to higher concentrations at present in areas where birds are easier to count, like Gallocanta; (d) a possible shift of an unknown number of cranes from eastern migratory routes to the western route. In sum, an actual demographic increase has unquestionably occurred in the western European population of Common cranes over the last 40 years, which can be explained by a likely reduction in mortality favoured by the shorter migration journey and higher food availability at staging areas, and to a higher availability of nesting sites due to current protection measures at breeding areas. However, here we want to highlight that such increase has not been as pronounced as the available counts suggest. To quantify that population increase more accurately, and to understand its consequences for the species' management, we need to obtain accurate mortality rates from the database of marked birds. Using these marked birds, we can also quantify possible shifts between migratory routes.