Territoriality and sex roles of breeding South Island pied oystercatchers Haematopus ostralegus finschi

Abstract

Defence of resources (e.g., food or nest sites) or mate guarding (preventing rival males gaining access to a mate) have been proposed as reasons why monogamous pairs of birds defend a territory. Reasons for the territorial behaviour displayed by South Island Pied Oystercatchers, Haematopus ostralegus finschi, which breed on farms in mid-Canterbury, New Zealand were studied in 1996 and 1997.

Positive correlations were found between food (dipteran numbers and densities) and South Island Pied Oystercatcher (SIPO) breeding success (number of eggs in first clutch, number of chicks hatched and number of chicks fledged). SIPO breeding success was also inversely proportional to time spent foraging. SIPO pairs in larger territories were more successful than pairs in smaller territories.

Behaviour of SIPO was measured using point sampling. Successful pairs (fledged one or more chicks) spent less time walking and less time feeding than unsuccessful pairs. Analysis of SIPO sex roles showed that females returned to the breeding territories first. Males spent more time than females in territory defence, incubation, preening and walking while females spent more time sitting than males. Despite these behavioural differences, females expended more energy than males over the breeding season and weighed less later in the breeding season.

Productivity of a pair's last year together was positively correlated with duration of the pair bond.

Mate guarding by SIPO was tested using a mounted SIPO skin. Pairs were tested before eggs were laid, while incubating eggs and once chicks had hatched. Although SIPO responded aggressively to the decoy, no significant differences in the response rates at each stage of the breeding season were found. Likewise time before reacting to the decoy and time spent near the decoy did not differ at each stage of the breeding season suggesting mate guarding is not the reason for SIPO territoriality.

Resource defence (food) is the reason why SIPO are territorial. No evidence is found supporting mate guarding in SIPO.