Is sex-ratio in the Strange-Tailed Tyrant (Alectrurus risora) related to nesting habitat?

KRAEMER, S.; DI GIACOMO, A. S.; DI GIACOMO, A. G.; MAHLER, B.; KOPUCHIAN, C.

Gramados

Congreso; II Ornithological Congress of the Americas; 2023

The reproductive success of males and females in polygynous species is different, as a male can produce more offspring (both pair and extra-pair) than a female. Trivers-Willard theory proposes that sex ratio bias in offspring of polygynous species is related to parental conditions. Sex-ratio bias has been identified in many different species across the avian phylogenetic tree. The Strange-Tailed Tyrant (Alectrurus risora) is a globally threatened (Vulnerable, IUCN) polygynous bird species inhabiting grasslands from Argentina and Paraguay. Previous studies in the protected area “Reserva Ecológica El Bagual” located in Formosa province (Argentina) have found differences in its reproductive success when comparing two different nesting sites: upland (well-drained) grassland predominantly covered with Imperata brasiliensis, and lowland (flooding) grassland predominantly covered with Paspalum intermedium. We propose that these differences in reproductive success between nesting habitat might affect the Strange-tailed Tyrant nestling sex proportions. To test this idea, we collected nestling blood samples from 4 different reproductive seasons (2008-2009, 2010-2011, 2013-2014, 2014-2015. We determined the sex of the nestlings (n = 105) using a PCR with P2/P8 primers. We used generalized linear models (GLM) to test for significant sex-ratio bias between the different nestling grasslands and the different reproductive seasons. Whole data analysis showed no significant sex-ratio bias between nesting habitats. However, when analyzing each season separately, we found a significant bias during the 2008-2009 season, with a greater proportion of males in the well-drained grassland where reproductive success had been higher. The following seasons neither showed significant differences between nestling grasslands nor showed the same pattern observed in 2008-2009 reproductive season. Our results do not show significant sex ratio bias in the Strange-Tailed Tyrant related to the nestling environment. However, either climatic or environmental factors may influence sex-ratio bias in different reproductive seasons.