Estimating adult sex ratios from bird mist netting data

Abstract

Summary1. It is increasingly acknowledged that skewed adult sex ratios (ASRs) may play an important role in ecology, evolution and conservation of animals. 2. In birds, published estimates on ASRs mostly rely on mist netting data. However, previous studies suggested that mist nets or other trap types provide biased estimates on sex ratios, with males being more susceptible to capture than females. 3. We used data from a Constant Effort Site ringing scheme to show how sex ratios that are corrected for sex-and year-specific capture probabilities can be directly estimated by applying capture-recapture analysis, for example, in a Bayesian framework. 4. When capture data were pooled from the 19 years of study, we found that in the blackbird (Turdus merula) and the blackcap (Sylvia atricapilla), the observed proportions of males were 57% and 55%, respectively. However, when the observed annual proportions of males were corrected for the sex-specific capture probabilities, the proportions of males did not clearly differ from 50% in most study years, and thus, the apparent male-bias in the ASRs almost completely disappeared. 5. We propose that published estimates on ASRs in birds should be re-evaluated if based solely on observed sex ratios from mist netting studies. 6. We further propose that data from national bird ringing schemes and in particular from Constant Effort Site ringing programs can provide valuable information on ASRs, if analysed using capture-recapture models. We discuss important assumptions of those models; for example, movements that may differ between sexes should be taken into account, as well as the occurrence of transient individuals that do not hold breeding territories within a study site.

Adult sex-ratio distortion in the native European polecat is related to the expansion of the invasive American mink

“…However, the two major methodologies currently used to fit SSR to ASR are: (i) capture-mark-recapture methods (Casula and Nichols, 2003;Alho et al, 2008;Ewen et al, 2011), that allow researchers to account for individuals that are present but not sampled in a certain session. Recently, some authors suggest that estimates of ASR still need to incorporate unequal detectability/catchability between sexes into the analyses to get more realistic approaches of ASR (Amrhein et al, 2012;Pickett et al, 2012;Arendt et al, 2014); alternatively, (ii) noninvasive methods (reviewed in Waits and Paetkau, 2005; see also Palomares et al, 2012;Baumgardt et al, 2013) allow less laborious, and, likely, more accurate sex-ratio monitoring, as differences in catchability are minimized. These new methodologies will certainly help to estimate ASR in an unbiased way in future studies.…”

One cost of being gold: selective predation and implications for the maintenance of the Midas cichlid colour polymorphism (Perciformes: Cichlidae)

“…Inference was drawn, based on the 95% credible interval (CrI), the Bayesian analogue to the confidence interval. Conventionally, if zero is not included in the Bayesian 95% CrI, an effect is considered as 'clear' (Amrhein et al, 2012). We also tested for the influence of the prey fish weight ratio as a fixed effect, although this did not reveal any effect of weight on the attack probability (estimate: 0.93; 95% CrI = −0.87 to 2.74).…”

Skewed adult sex ratios inColumbinaground doves from Venezuela

“…ASRs uncorrected for sex‐specific capture probabilities estimated from mist‐netting data might not reflect the true population ASR (Amrhein et al. ). Sex‐specific capture probabilities are based on recapture rates that are likely biased toward the more dispersing sex.…”

“…However, within a study year, sex differences in dispersal should normally not strongly bias sex ratios (Amrhein et al. ). Concerning shooting, overestimation of males frequently results from their greater detectability because they are often more ornamented or vocally more active (Ancona et al.…”