Facultative and non-facultative sex ratio adjustments in a

15 If parental allocation to each offspring sex has the same cost/benefit ratio, Fisher's 16 hypothesis predicts a sex ratio biased towards the cheaper sex. However, in 17 dimorphic birds there is little evidence for this, especially at hatching. We 18 investigated the pre-fledgling (i) sex ratio, (ii) body condition and (iii) sex19 differential mortality in a population of the glossy ibis (Plegadis falcinellus), in 20 southern Spain between 2001 and 2011. We defined two age groups for the period 21 between hatching and fledging. We also compared pre-fledgling with the autumn sex 22 ratio. Metabolic rates were estimated by the Doubly Labeled Water (DLW) 23 technique to establish that sons (the bigger sex) were 18% more energy demanding 24 than daughters, and to compute the predicted Fisher's sex ratio (0.465). As population 25 size increased between years, body condition decreased in both sexes, and mortality 26 increased more for daughters than sons prior to fledging. At the same time, the 27 proportion of males among chicks close to fledging increased (average sex ratio: 28 0.606) while the proportion close to hatching decreased (average sex ratio: 0.434, in 29 line with Fisher's prediction). Furthermore, the proportions of males at fledging and 30 the following autumn were negatively correlated across years. We suggest that, as 31 population density increased and conditions worsened the larger sex had relatively 32 higher survival. These differences in survival produce a shift from a facultative 33 female-biased sex ratio at hatching into a non-facultative male-biased sex ratio of 34 fledglings. Additionally, the excess of males at fledging was counterbalanced by sex35 related dispersal during the autumn. Overall, glossy ibis sex ratio is a product of a 36 combination of facultative and non-facultative adjustments triggered by 37 environmental conditions, driven by rapid population growth, and mediated by 38 highly interrelated life-history traits such as body condition, mortality, and dispersal. 39