Sexual Imbalance in Migrating Blackcaps

One of the things that we have noticed this year is that, during September, there has been a huge sexual imbalance in the number of migrating Blackcaps. So I had a look at the numbers in the Braydon Forest and at Lower Moor Farm in September over the years that I have been ringing there. These are the results:

We don’t work anywhere that could be described as a migration hotspot, so we don’t get huge numbers either in the spring or the autumn. The team has processed 2190 in the 11 years since I started operating at these two site groupings: 199 per annum, one sixth of which are processed in September.

My trainee, David Williams, just happens to be a very clever individual with a first class honours degree in Zoology (I only managed a 2:1) and he had a look at the Blackcap data and did some statistical analysis on it. We don’t have thousands of entries, but we have more than l’Oreal usually claim for their adverts. Anyway, I don’t pretend to understand it all, but this is what David came back with:

• Overall, the observed numbers of female (122) and male (213) Blackcaps were significantly different from those expected under the hypothesis that both sexes are equally likely (chi-square goodness of fit test with Yates’ correction: χ² = 24.179, d.f. = 1, N = 335, p < 0.001).

• By year, there is considerable variation: in some years (e.g., 2018 and 2019), there are about equal numbers of each sex, but in others (e.g., 2021, 2022) there are more than twice as many males as females.

In 2021 and 2022, the observed numbers of female and male Blackcaps were significantly different from those expected under the hypothesis that both sexes are equally likely (chi-square goodness of fit tests with Yates’ correction, d.f. = 1 in all cases):

The mean (± 95 % confidence interval) numbers of female and male Blackcaps processed each September were significantly different from each other (paired samples t-test: t = 2.842, d.f. = 10, p = 0.017).

Back to my thoughts: essentially, over the last three years the sexual imbalance in the catch of Blackcaps moving through the Braydon Forest and Lower Moor Farm in September has become significant. What the reason for that is I don’t have any idea.