Sammendrag
In nature, an unequal sex-ratio in wild populations is a common occurrence. Many factors could result in skewed sex-ratios, for instance, selective hunting, sex-specific mortality, sex-biased dispersal, etc. A biased sex-ratio could affect offspring production and survival of individuals and consequently growth rate. In this study, I investigated how the female-skewed population of house sparrows (Passer domesticus) affected population dynamics.
The house sparrow populations were monitored from 2013 to 2015 in eight study sites, including six islands and two populations in the mainland, along the coast of mid and northern Norway. Fieldwork was carried out during winter every year. Each year, all adult sparrows were captured with mist nets, marked with a numbered metal ring and a unique combination of three colored plastic rings, and sampled for blood. During the winter of 2014 half of the males, randomly chosen, were removed from each population. This allowed me to examine the effects of female-biased population on the survival of adults, number of recruits produced, population growth rate, and whether parents are able to adjust the sex-ratio of recruits toward the rarer sex.
The result showed that one year after the manipulation of the sex-ratio, all populations recovered approximately to the same population sizes as before the manipulation with a roughly equal sex-ratio. In 2015 the average number of recruits produced by female parents did not differ significantly from 2014. However, in 2015 male parents produced on average more recruits and the number of males parents who had zero recruits decreased dramatically in compared to 2014. The higher average recruit production among males maybe because fewer males must serve the female segment with copulations of reproduction females. Also, as some of males did not have the opportunity to produce offspring when the sex-ratio was 1:1 but after removing half of the males most of them have a chance to have recruits. Since the number of recruits only increased for male parents, not females, the total number of recruits only increased slightly; this indicated that the population might recover mostly as a result of immigration. Also, I found out that the survival of parents was not affected by our experiment, but female parents generally had lower survival than males in both years. Accordingly, the study revealed that the adult sex-ratio in the house sparrow populations did not affect the sex-ratio of recruits produced. For future studies, more attention could be focused on the proportion of mated males as well to understand how the experiment affected polygyny, and how polygyny could affect the survival of recruits and adults.