Search

Fecundity, an important demographic variable for wildlife populations, is time consuming and expensive to measure. For seabirds, reproductive success is often estimated from 2 surveys of the colony, 1 during incubation and 1 during chick rearing. Using 33 yr of data on Magellanic Penguins (Spheniscus magellanicus), we compared reproductive success estimated from 2 surveys per season, from infrequent nest checks, and from daily nest checks to answer 4 questions: (1) Is reproductive success estimated from 2 surveys per season accurate enough for population models (within 15% of the ‘true' value)? We found that the answer was yes, if surveys were done on optimal dates and chick counts were adjusted by the percentage of chicks that were heavy enough to fledge. Optimal dates depended on the synchrony and phenology of the species. If penguins bred particularly late or early in a given year, estimates were wrong even if counts were done on the average optimal date. (2) Can counts of occupied nests be ad...

In many species of birds, periods of incubation of eggs within a clutch depend on the order in which the eggs were laid and determine whether the eggs hatch asynchronously or on the same day. Magellanic Penguins (Spheniscus magellanicus) lay two eggs 4 days apart that hatch 2 days apart; first eggs take 41 days to hatch, and second eggs take 39 days. We tested whether temperatures of the two eggs differ and whether delayed onset of incubation caused this pattern. First eggs were cooler than second eggs during their first few days (P < 0.001). First eggs averaged 23.4 ± 0.3 °C in the first 24–48 hours after they were laid. Second eggs averaged 27.9 ± 0.3 °C, warm enough for development. Egg temperature did not stabilize (33.9 °C) until eggs were about 18 days old. We swapped first and second eggs of different nests to determine if parental behavior caused the differences in temperatures and incubation periods. First eggs treated as second eggs developed as fast as control second eggs, and second ...

We analyzed the effect of the shrimp fishery on Magellanic Penguins (Spheniscus magellanicus) in two ways: (1) we determined whether penguins were incidentally killed and the magnitude of incidental take, and (2) estimated the overlap between penguin diet and fish by-catch of the shrimp fishery (total capture excluding shrimp and seabirds). We worked with the fishing fleet operating at Golfo San Jorge with onboard observers over 200 days in 1995-1997, sampling fishery by-catch and entangled birds. Penguins were affected during the austral summer. Estimations of mortality rates showed 0.33% of the breeding population at Golfo San Jorge is incidentally killed by the shrimp fishery every summer. By-catch in shrimp fishery nets was composed of species important as penguin prey (anchovy and hake) in higher proportions during summer. The daily by-catch of these species was higher than the total calculated daily intake for all penguins breeding in the Golfo San Jorge. This could have a significant effect on birds. We recommend that an observer program be implemented to monitor seabird mortality and that fishing gear should be improved to reduce the indirect effect of the fish by-catch on penguins.

We tracked four male Magellanic Penguins (Spheniscus magellanicus) from the beginning of their winter migration for periods ranging from three to six weeks. Upon leaving the colony at Punta Tombo, Argentina where they had bred and molted, all four birds traveled northeast along the coast at speeds consistent with sighting and band-return data which suggest a round-trip winter migration in this species of up to several thousand kilometers. Three of the birds followed a similar route, moved at approximalely the same rate, and were still moving northeastward at a distance of over 800 km from the colony when signals ceased. The fourth bird departed from the northeast route at approximately 300 km into its trip and remained in a large bay for at least 29 days, when the last signal was received. Initially, the birds traveled rapidly and made few deep dives. Later, their rate of travel slowed, their paths became more erratic, and they made more deep dives, all indications of probable foraging. This suggests that location of the penguin migration route and rate of travel along it may be determined by seasonal movement of prey. As all birds were within 250 km of shore for the entire period in which signals were received, our results underscore the importance of controlling pollution associated with petroleum transport along the coast of Argentina, currently a major source of Magellanic Penguin mortality.

Dolphin Gulls (Larus [Leucophaeus] scoresbii) at Punta Tombo, Argentina, aggregated in small colonies of 16-24 pairs to breed. Colony location varied among years. Nesting density was 1.02 to 1.6 pairs/m 2 . Mean clutch size was 1.9 and 2.0 in 1987 and 1989, respectively, and egg volume was similar among years. Incubation took between 24 and 27 days, with chicks hatching from mid to late December. When chicks were two to five days old they followed their parents away from the nesting area and, as they grew older, they began creching in one or more groups. Egg mortality in 1987 and 1989 was 41% and 46%, respectively, mainly through disappearance and presumed predation. Breeding success from 1984 to 1990 was variable, ranging from complete failure to 0.86 chicks surviving to three weeks of age per active nest. After nest losses in 1986 and 1990 birds renested. During the breeding season Dolphin Gulls were specialized scavengers, mainly eating southern sea lion (Otaria flavescens) excrement and scraps of food dropped when Magellanic Penguins (Spheniscus magellanicus) and Imperial Cormorants (Phalacrocorax atriceps) fed their chicks. Dolphin Gulls also fed in the intertidal zone, stole cormorant and Kelp Gull (Larus dominicanus) eggs, and scavenged remains of penguin eggs already preyed upon by other species. They nested with other species and left the breeding area in February after the chicks fledged. They did not return to the breeding area in any numbers until September. The breeding biology of the Dolphin Gull appears to be largely influenced by their specialized scavenging habits and vulnerability to predation.