Is Sex-Ratio bias in the Strange-Tailed Tyrant Alectrurus risora related to nesting habitat?

KRAEMER, SIMON; DI GIACOMO, ADRIÁN S.; DI GIACOMO, ALEJANDRO GABRIEL; MAHLER, BETTINA; KOPUCHIAN, CECILIA

Gramado

Congreso; II Ornithological Congress of Americas; 2023

Sociedad Brasilera de Ornitologia, Associtation of Field Ornithologist y Neotropical Ornithological Society

Trivers-Willard theory proposes that sex ratio bias in nestlings of polygenic species is related to parental body condition or breeding habitat’s environmental conditions due to these differences in fitness. The Strange-Tailed Tyrant (Alectrurus risora) is a globally threaten (VU) 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 between two different nesting sites: well-drained grassland predominantly covered with Imperata brasiliensis, and floodable grassland predominantly covered with Paspalum intermedium. We propose that these differences in reproductive success might affect the Strange-tailed Tyrant nestling sex proportions. We collected nestling blood samples (n=105) from four different reproductive seasons (2008-2009, 2010-2011, 2013-2014, 2014-2015). We sexed these nestlings using a PCR protocol to sex passerine birds. We used generalized linear models 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 nestling environments. However, when analyzing each season separately, we only found a significant bias in the 2008-2009 season, with a greater proportion of males in the well-drained grassland where reproductive success had been higher. Our results do not show for significant sex ratio bias in the Strange-Tailed Tyrant related to the nestling environment. However, either climatic or environmental factors, not included in our firsts analysis, may influence the appearance of sex-ratio bias through the different reproductive seasons.