Population Dynamics and Breeding Success of the Shag, Phalacrocorax aristotelis, on the Farne Islands, Northumberland

Promiscuity in the Shag as shown by timelapse photography

“…Another marked male and its mate which had reared young, and the last pair to occupy the second nest (below) had also returned to their sites by 14 October. This is in marked contrast to the situation reported from the Fame Islands (100 km away), where even the oldest birds did not re-occupy their nest-sites until February or March (Potts et al 1980).…”

Experimental evidence that age-specific reproductive success is independent of environmental effects

“…In addition, two-year-old shags, the youngest recorded breeding age, can be recognized from plumage characteristics (Potts 1971). Previous correlative work on shags has suggested that the major di¡erence in age-speci¢c reproductive success occurs between two-year-old and older males and that there is no independent e¡ect of female age (Potts et al 1980;Aebischer 1993). However, pairs involving two-year-old males also breed signi¢cantly later in the season than older males (Potts et al 1980).…”

“…This procedure gave a matched design in which old controls and young experimentals were both given eggs laid by old pairs, which hatched early in the season, and young controls and old experimentals had eggs laid by young pairs, which hatched late in the season. The clutch exchanges were carried out within ten days of the hatching of the old eggs and, thus, at least one-third of the way through incubation in all cases (total incubation period 36 days; Potts et al 1980). The experimental protocol ensured that (i) there was the same spread of laying dates for birds in all treatment groups, (ii) no pairs reared their own brood, and (iii) pairs hatching chicks at the same time had eggs of a similar quality (i.e.…”

“…Furthermore, comparisons of breeding performance generally span the whole breeding event and the capacity to rear chicks has not been separated from the capacity to produce and hatch eggs. Therefore, using the shag (Phalacrocorax aristotelis), a seasonally breeding bird with marked di¡erences in reproductive success and timing of breeding in relation to age (Potts et al 1980;Aebischer 1993;Aebischer et al 1995), we (i) manipulated young birds so that the hatching date of the chicks they reared was brought forward in the season to coincide with those of control older pairs, and (ii) manipulated older birds so that they reared chicks hatching late in the season at the same time as control younger pairs. Our protocol allowed us to test whether, when breeding in the same environmental conditions, young parents perform less well during chick rearing than do older pairs.…”

Differential responses of GuillemotUria aalgeand ShagPhalacrocorax aristotelisto a late winter wreck

“…A similar increase in the numbers of birds breeding at this age occurred after many adults died on the Farne Islands in 1963 and 1968. 16 Sandeels are always the dominant food of Shags on the Isle of May 13 and 1994 was no exception. Sandeels occurred in 30 (88%) of the 34 pellets with food remains present which were collected in March and April and made up 97.9% of the 4347 fish otoliths extracted from these pellets.…”