Is sex-ratio bias in the Strange-Tailed Tyrant [Alectrurus risora] related to nesting habitat?

KRAEMER, SIMÓN; DI GIACOMO, ADRIÁN SANTIAGO; DI GIACOMO, ALEJANDRO G.; MAHLER, BETTINA; KOPUCHIAN, CECILIA

Gramado

Congreso; II Ornithological Congress of the Americas; 2023

Association of Field Ornithologist - Sociedade Brasileira de Ornitologia - Neotropical Ornithological Society

The reproductive success of males and females in polygenic species is different. Trivers-Willard theory proposes that sex ratio bias in nestlings of polygenic species is related to parental body or genetic conditions due to these differences in fitness. However, it has been proposed that environmental factors may led to a similar evolutionary response. The Strange-Tailed Tyrant [Alectrurus risora] is a globally threaten (VU) bird species inhabiting grasslands from Argentina and Paraguay (see Figure 1).Previous studies in the protected area “Reserva Ecológica El Bagual” located in Formosa province (Argentina, see Figure 1) have found differences in its reproductive success when comparing between two different nestling sites: well-drained grassland predominantly covered with [Imperata brasiliensis], and floodable grassland predominantly covered with [Paspalum intermedium]. We have proposed that these differences in reproductive success might affect the Strange-tailed Tyrant nestling sex proportions.We collected nestling blood samples from 4 different reproductive seasons (2008-2009, 2010-2011, 2013-2014, 2014-2015). We sexed these nestlings using a PCR protocol to sex passerine birds with P2 and P8 primers ( see Figure 2). We tested whether climatic variation between breeding seasons could have an effect over sex-ratios, for this we calculated the Standard Precipitation and Evapotranspiration Index (SPEI). Precipitations were obtained from El Bagual database, temperatures where obtain from World Clime using El Bagual shapefile and Evapotrasnpiration was calculated using Heargraves formula. We used generalized linear models to test for significant sex-ratio bias between the different nestling grasslands, the different reproductive seasons and SPEI. Model were selected using Kullback-Leibler Information Criteria (KIC) and Akaike Information Criteria (AIC). The selected model was the one involving the variables Nesting Grassland and SPEI 3-months comparison at laying date (see Table 1). The interaction between nesting grassland and SPEI-3 months comparison was significant in determining sex-ratio biases in the Strange Tailed Tyrant (P=0.007). Male proportion for each nesting grassland variates according to the SPEI 3 months comparison (see Figure 3 and 4). Significantly male biased clutch have been seen in Imperata grassland with negative (drought) values of the SPEI 3-months comparison at laying date. However, male biased clutch have been seen in Paspalum grassland with positive (humid) values of SPEI 3-months comparison at laying date, but the differences are not significant.Increments in precipitation have been correlated with increments in arthropod abundance and reproductive success. Our results suggest that different climatic conditions (drought-humid breeding seasons) influence different sex-biases clutches in the two nesting grassland studied, probably due to food availability, nest concealment or other nesting-related resources.