Male-biased Sex Ratio and Body Size of a Wintering Roost of Brambling, Fringilla montifringilla Linnaeus, 1758

Bramblings arrived at our study site by late November and started to leave by early April. We captured birds randomly using mist-nets from 10 December of 2019 to 24 March of the year 2020 at the campus of Longdong University (35°43′47″N, 107°41′04″E, 1367.3 m), eastern Gansu province, central China. Once the birds were captured, their sex was determined by plumage coloration, male with black head crown, while female no. Age was classified as first-year birds and adults according to the feathers of wing (Arizaga et al., 2012). We measured six morphological parameters (culmen length, beak width, beak height, tarsus length, tail length, and wing length) with digital calipers (to 0.01 mm), body mass was measured with electrical balance (to 0.01 g). All measurements were taken by the same person, each bird was banded with a unique combination of colored leg rings and released after measurements completed.

We used chi-square to analyze the sex ratio, t-tests to investigate sex differences in all body measurements. We also calculated an index of dimorphism using the formula: dimorphism index = male mean/female mean ×100 (Wagner, 1999) and a coefficient of variation as: CV = SD/Mean×100 (Fletcher and Hamer, 2003) for each sex and all variables to indicate the variability of each measurement (Sokal and Rohlf, 1995). All statistical tests were performed with SPSS software package for Windows 22.0, and all probabilities were 2-tailed with a significance level of P= 0.05, results were given as Mean ± SD.

We captured and measured 122 individuals (75 males, 47 females) in this work.

This wintering roost with 61.5% male birds, sex ratio was male-biased (Chi-square = 6.426, P = 0.011), 59.8% were first-year birds, more than adults in both sexes (Table I). The culmen length, wing length, tail length of males were significantly longer (ranged 2.8–6.5%, depending on body measurement definite; all P < 0.001 (Table I) than females, males (23.00 ± 2.83 g, n = 75) were heavier than female (20.73 ± 2.69 g, n = 47) in body mass (t = 2.883, P = 0.020). The most dimorphic measurement was tail length (107, Table II).

Sex ratio in our study roost was male-biased, more than 60% of the flock were males, this is consistent with the results of study in northern Europe, where females migrate further south than males (Cramp and Perrins, 1994; Arizaga et al., 2012). Compared to the latitude of these study areas (above 43°N), our study site was located at a relatively lower latitude (35°N) than those mentioned above. This may suggest that our sample size was small or the origin of wintering bramblings was different from study areas in Europe and may even be the global warming affect and the wintering ground selection by bramblings. More than 59% captured birds were first-year birds; this is in agreement to the reported results from southern Europe (Arizaga et al., 2012), while contradictory to those from northern Europe (Cramp and Perrins, 1994). Some researcher argued that this contributed to the first-year birds migrating further south than adults (Cristol et al., 1999). Although the juveniles with little different body size to adults, they are lacking in foraging ability, migrant more lower altitude to wintering is a better strategy for their survival (Cramp and Perrins, 1994; Cristol et al., 1999; Arizaga et al., 2012).

Table I. Number and frequency of each age-sex class of brambling, Fringilla montifringilla captured in a wintering population at the campus of Longdong University, eastern Gansu, central China.

We found that bramblings exhibited slight male-biased SSD, a pattern shared with many other songbirds, such as plain laughingthrush (Garrulax davidi concolor, Liu and Sun, 2018), Elliot’s Laughingthrush (Trochalopteron elliotii, Liu and Sun, 2016) and tree swallow (Tachycineta bicolor). This result especially the sexual dimorphism in wing length and body mass is consistent with other studies reporting on Brasmblings (Jenni, 1993, Arizaga et al., 2012), suggesting that the SSD widely exist in different geographical populations of brambling. SSD is variously considered to be either the outcome of natural selection for minimizing intersexual competition for limited food resources or linked to other life history traits. SSD resulted from males and females foraging for food items of different sizes, reducing intersexual competition (Shine, 1989). In wintering bramblings, males and females foraging together on the same substrates, males often forage in center of the roost and dominate females in their access to food (Jenni, 1993), small difference in food items may result in difference in the body size. Sexual selection theory proposes that intra-sexual contests offer the larger individual a selective advantage to get mates, food and defend breeding territory (Andersson, 1994; Haggerty, 2006), larger males have the potential advantages to process resources and mate successfully. A study conducted in a bramblings winter roost found that males were always foraging in the center and dominated females to access food (Jenni, 1993).

Table II. Morphometric data, dimorphism index and coefficient of variation of male and female brambling, Fringilla montifringilla captured from a wintering population at the campus of Longdong University, eastern Gansu, central China.

Measurements differed significantly between sexes was in bold (P < 0.05).

In the present study, the sex ratio of wintering roost in our study site was biased towards males, age composition cline to first-year birds. This is related to different migration strategies of sex and age in this bird species. Slight sexual size dimorphism in this bird mainly results in sexual selection, because the male and female always forage together.

We are grateful to postgraduate students Dong Wang and Jiaxin Hang for help in the field work. Funding for this project was provided by the Doctoral Program of Longdong University (XYBY1914).

The authors have declared no conflict of interest.