Sex differentiation in Whimbrels from Churchill, Manitoba, Canada

Many shorebird species exhibit sexual size dimorphism, a trait that can be used to assign sex in the field based on morphological measurements. Sex determination in the field can be a more cost-effective and less invasive method than molecular sexing. Whimbrels Numenius phaeopus show female-biased size dimorphism, and for this species a discriminant function analysis (DFA) was conducted in the mid-1970s. However, shorebird morphology may change over time, and current measurement guidelines differ from those used in the mid-1970s. We aimed to assess the extent of change in sexual dimorphism in Whimbrels over time, and to revise the DFA using molecularly sexed individuals. We used morphological measurements of Whimbrels N. p. hudsonicus breeding in the vicinity of Churchill, Manitoba, Canada, from two time periods: 1973–1976 and 2018–2019, and molecularly sexed the individuals measured in 2018–2019. Wing length measurements were consistently longer in 2018–2019 than in 1973–1976 due to a change in technique from ‘natural’ wing length (i.e., unflattened) to flattened and straightened wing length in 2018–2019. Although measurement technique did not differ between study periods, culmen length was approximately 3% shorter in 2018–2019 than in the mid-1970s, though only significantly so for males. For the molecularly sexed individuals from 2018–2019, we used a linear discriminant analysis (LDA) to update the DFA developed in the mid-1970s, including characteristics that showed significant female-biased size dimorphism (i.e., culmen, wing length, and body mass). The updated DFA had an accuracy of 0.926 ± 0.001 SE (females: 0.890 ± 0.002; males: 0.950 ± 0.001), thus providing a practical alternative to molecular sexing for N. p. hudsonicus.