Brood Sex Ratio Variation in a Colonial Raptor, the Eleonora's Falcon, Falco Eleonorae

Sex allocation theory predicts that species with reversed sexual size dimorphism frequently overproduce the cheapest sex at the brood level. A sex ratio deviation from parity is frequently documented among the offspring of raptors and is largely produced by sex-biased survival or parental adjustment as a response to environmental variability or demographic conditions. The Eleonora's falcon is a long-distance migrant wintering in Madagascar and southeast Africa and breeding colonially primarily in the Mediterranean Sea and the Canary Islands. It feeds on migratory passerines and thus depends greatly on the autumn migration flow and the prevailing winds. Being sexually size dimorphic (i.e. females are larger than males) and depending greatly on the migration, the species is a good case for investigating whether brood sex ratio variation is adaptive. In the present study, we examined the proportion of males and females reared during 2009–2020 in a falcon colony in southeast Crete (Greece) in relation to specific ecological and biological attributes. Female nestlings were shown to be costlier to produce as they are heavier than males. Overall, an unbiased sex ratio was observed across the years, although a consistent trend was recorded towards females early in the breeding season and first-hatched chicks and males late in the breeding season and third-hatched chicks. Our results provide strong evidence that parental condition, habitat quality and a brood reduction effect were the significant drivers for the observed sex ratio patterns. The overproduction of the costlier sex and a high survival of the cheaper one during food-rich years should be regarded as an adaptive evolutionary strategy of the species for maximizing its fitness returns per clutch and for maintaining a balanced offspring sex ratio in the population.