Movements and mortality rates of Great Skuas ringed in Scotland

The dispersal and philopatry of great skuas from foula, Shetland

“…If the probability of recovery was constant with age after the first year, then the number of recoveries from each age should show a progressive decline, as can be seen for three year-olds and older. The recovery rates of one and two year-olds were significantly lower than that of three year-olds (x 2 2 = 13.3, P < 0.0001); this may be due to the mainly pelagic existence of one and two year-old Great Skuas (Furness 1978). These results will be slightly influenced by reduced fledging success after 1986, and the lack of opportunity for Great Skuas that fledged in the last few years to be included in the recovery rates of older ageclasses.…”

“…Ring recoveries prior to 1963 were excluded from this study because the aluminium rings used then were lost extremely rapidly, thereby not allowing an accurate determination of the movements of adult Great Skuas. However, Furness (1978) found that excessive wear or loss of monel rings on Great Skuas was 'most unlikely' to occur during a bird's lifetime. Only Great Skuas ringed as chicks were included in this study, thus all recoveries were of birds of known age.…”

“…the interpretation of reported recoveries. For example, the particularly high proportion of birds shot in Greenland and the Faeroes (Arctic) and birds caught in fishing nets in Spain and Portugal (Iberia) will overemphasise the occurrence of Great Skuas in these regions, as the chances of a ringed bird being reported are very much higher with these causes of death than for individuals which do not die in association with man (Furness 1978). Ring recovery analysis also requires the birds to be recovered within a short time and distance of death, yet the lag times of auk recoveries were experimentally determined to be often of several weeks duration, although mostly less than a month (Jones et al 1970).…”

“…As a large proportion of Great Skua recoveries are of birds found exhausted, killed or caught, the average time lag between death and recovery will be very short in this species. Furness (1978) considered the differences in recovery rates from different areas to reflect the dispersal patterns of Great Skuas from Shetland, at least in part, and were only partly obscured by recovery chances.…”

“…The movements of Great Skuas ringed as chicks have been described previously by Furness (1978), based on recovery data up to 1977. Since then, the breeding population on Foula has been steadily declining.…”