Abstract
Sex differences in adult mortality have usually been explained as a result of differences between males and females in the costs associated with their reproductive investment. Investigating sex-biased mortality is important because it shapes mating opportunities, reproductive strategies and parental care. Here, using known fate models implemented in the RMark package, we estimate annual and monthly or seasonal adult survival rates in a sample of 339 great bustards (Otis tarda) radio-tagged in 1985-2013 and monitored up to 2020. We found that annual survival was lower in males than in females, and lower in Madrid, a highly anthropized region (males: 0.874, females: 0.931), than in Villafáfila, where very good habitat conditions still exist (males: 0.948, females: 0.973). The maximum ages reached by marked individuals were also higher in females (17.3 years) than in males (13.8 years). The seasonal survival pattern was similar in both sexes, with maximum mortality at the end of the mating and incubation period, and lower survival in males, suggesting a direct effect of reproductive cost in both sexes and a higher cost in males. These results are consistent with previous comparative studies, which attribute male-biased adult mortality to the cost of mating competition in males and egg productivity in females, although these hypotheses could not be tested in the present study. Anthropogenic mortality was considerable (39.5% of deaths) and also male-biased, with power line casualties standing out and affecting 3.5% of males and 1.9% of females each year in Madrid. Anthropogenic mortality thus contributes to aggravate the naturally biased survival rates and could be a major cause of the declines observed in most great bustard populations over the last decades.