Parental sex allocation and sex-specific survival drive offspring sex ratio bias in little owls

• Published in: Behavioral ecology and sociobiology 2019-06, Vol.73 (6), p.1-10, Article 85
• Main Authors: Tschumi, Matthias, Humbel, Jolanda, Erbes, Joscha, Fattebert, Julien, Fischer, Jochen, Fritz, Gerhard, Geiger, Barbara, van Harxen, Ronald, Hoos, Bernd, Hurst, Johanna, Jacobsen, Lars Bo, Keil, Herbert, Kneule, Werner, Michel, Vanja T., Michels, Heinz, Möbius, Leander, Perrig, Marco, Rößler, Philip, Schneider, Dieter, Schuch, Siegfried, Stroeken, Pascal, Naef-Daenzer, Beat, Grüebler, Martin U.
• Format: Article
• Language: English
• Subjects: Animal Ecology; Behavioral Sciences; Bias; Biomedical and Life Sciences; Body mass; Body weight; Breeding; Chicks; Dependence; Ecological conditions; Endangered species; Females; Hatching; Life Sciences; Males; Offspring; ORIGINAL ARTICLE; Proximate causes; Ratios; Reproduction (biology); Sex; Sex ratio; Sexes; Sexual dimorphism; Social behavior; Survival; Zoology
• ISSN: 0340-5443; 1432-0762
• DOI: 10.1007/s00265-019-2694-8

Although biased offspring sex ratios are common in species with sexual size dimorphism, the proximate causes are often unresolved. This is because two general mechanisms operating in different ways and in various periods of reproduction can lead to the bias: sex-biased survival or parental sex-allocation. We examined nestling sex ratio patterns between hatching and fledging, sexual size dimorphism, and factors affecting nestling survival using growth and survival data of 846 individual little owl Athene noctua nestlings with known sex from 307 broods from Germany, the Netherlands and Denmark. Nestling sex ratio was female-biased, mainly due to a significant female bias in the first-hatched chicks. Females showed a higher body weight than male nestlings at ringing and body weight of nestlings decreased with hatching sequence. Nestling survival was higher in females (Φ = 0.91) than in males (Φ = 0.85), and survival rates were positively related to body mass and negatively to brood size. Although the observed lower survival of males can cause an overall female-biased sex ratio, the sex dimorphism and survival patterns found here are unlikely to explain the conspicuous sex ratio pattern with a female bias in the first-hatched nestlings and the increase in female bias across the season. Thus, these results point towards interacting mechanisms of parental sex allocation strategies and sex-specific survival. As the female bias was allocated to the first rank that is most likely to survive, the female bias will increase under suboptimal breeding conditions. We therefore suggest that under suboptimal ecological conditions, higher investment into females is adaptive in little owls.