Question on sub adults returning?

[Spiderman]

Has anyone seen a statistic on percentage of sub adults that survive their first year and return?


30%, 50%?

[randyM]

Various publications by BRIDGET J. M. STUTCHBURY from York University in Toronto indicate a first-year survival rate for both male and female martins from fledging to spring arrival to their nesting sites as SY birds equal to about 27%.

Interestingly, in her related studies it is reported that annual survival of one-year-old females (1-year time span from arrival on nesting grounds as SY bird through arrival on nesting grounds as a first year ASY bird) was 48%, while male annual survival of that same cohort was 59%. The publication suggests that physical stresses from young females' first molt, coupled with stresses associated with migration, egg laying and incubation were contributing factors to the lower annual survival rate when compared to males of the same cohort. However, male and female annual survival was similar for ASY males and females from 2-4 years old at about 63%. Males and females at least 5 years of age also had similar expected annual survival rates of about 52%.

Based on this information, the main reason for the higher number of males compared to females in the breeding aged martin population appears to be due to the higher mortality in 1-year-old female martins, as overall annual survival during all other periods seems to be quite similar for male vs female martins.

Randy

[bootjack farm]

Nice science based reply.