The sex ratio is an important parameter affecting population demographics. This is a study of the mechanisms and causes of changes over time in the sex ratio of the glossy ibis (Plegadis falcinellus) in the main breeding colony in Doñana. As colony size increased from seven pairs in 1996 to 4000 in 2011, a worsening of conditions seems to have increased mortality of female chicks (smaller than males), increasing the proportion of males at fledging. At the same time, the proportion of females increased at hatching, as expected under Fisher's hypothesis related to the costs and benefits that offspring of different sexes incur to the parents. The excess of males among fledglings was compensated by a greater dispersal to other populations. Changes in sex ratio are shown to be a complex process involving both adaptive and non-adaptive mechanisms. informacion[at]ebd.csic.es Santoro et al(2015) Facultative and non-facultative sex ratio adjustments in a dimorphic bird species Oikos DOI: 10.1111/oik.01889