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Individual study: The effects of investigator disturbance upon breeding success of Leach's petrels Oceanodroma leucorhoa on Kent Island, New Brunswick, Canada

Published source details

Blackmer A.L, Ackerman J.T. & Nevitt G.A. (2004) Effects of investigator disturbance on hatching success and nest-site fidelity in a long-lived seabird, Leach's storm-petrel. Biological Conservation, 116, 141-148

Summary

The effects of investigator disturbance upon a long-lived, non-threatened procellariiform, Leach's petrel Oceanodroma leucorhoa, was examined to see if such disturbance influenced hatching success, and if this disturbance affected hatching success and nest-site fidelity in the subsequent breeding season.

Study area & study species: This study on the effects of investigator disturbance upon Leach's petrel Oceanodroma leucorhoa was conducted in 1999 and 2000 on Kent Island, New Brunswick, Canada. Leach's petrels can live for over 30 years and pairs exhibit high fidelity to nest burrows. A single egg is laid and both parents incubate the egg for around 43 days. The chick is brooded for the first six days, after which it is left alone in the burrow for 55–65 days and is fed during nocturnal visits by the parents.

Methods: Most pairs in the study had received no investigator disturbance, those that had, had not been disturbed during the previous 6-13 years. Active burrows were located in mid-June 1999 after egg-laying. Standard research procedures used to monitor Leach's petrels nesting in other parts of the Kent Island colony (see original paper), were employed.

Each pair was assigned to one of six treatments (see below) based on the time of day (morning or evening) when disturbed and the frequency of investigator disturbance i.e. control, weekly, or daily. Pairs were systematically distributed among the treatment groups thus ensuring that pairs within the treatment groups were not clustered. Morning treatment burrows were visited between 08:00 and 12:00 hrs, evening treatment burrows at dusk between 19:00 and 21:00. Control pairs received the minimum disturbance possible, whilst procedures for the weekly and daily treatments simulated investigator disturbance that might be experienced by incubating birds during routine monitoring in a scientific study.

Control treatment (morning or evening): Each pair was visited only once during incubation, and only one member of a control pair was ringed (banded). After determining that a burrow was active by briefly feeling for the presence of an adult and egg, at least 43 days (i.e. the average length of the incubation period) were allowed to elapse before revisiting the burrow to determine hatching success.

Weekly treatment (morning or evening): Each bird of a pair was disturbed about four times a week. During the first visit, the egg was removed (to avoid damaging it while removing the parent) and then the incubating bird was ringed, weighed (in a cloth bag using a spring scale) and its right wing and tarsus measured. The egg and parent were then returned to the burrow. A twig lattice was laid over the burrow entrance and the nest was not disturbed again unless the twigs were displaced, indicating that an incubation change-over may have occurred. After change-over had occurred, the second bird of the pair was ringed, weighed and measured. The burrow was then left undisturbed for one week. After this the burrow was visited four times/week and whichever member of the pair was present was weighed (but not measured). The twigs were reset and checked daily until displaced, at which time the burrow was again entered and the other partner weighed, if present. Generally, the displaced twigs reliably indicated a change-over. Occasionally the twigs were displaced but a change-over had not occurred, in which case the incubating bird was weighed again. When an unattended (i.e. cold) egg was found, the nest was checked the following day to see if a parent had returned to incubate (in which case the bird was weighed) or if the egg remained unattended. If unattended, the nest was checked for 15 days, after which the egg was deemed dead if no adult had returned to incubate.

Daily treatment (morning or evening): Procedures were the same as the weekly treatment except that pairs were disturbed once per day throughout incubation. After initial ringing, weighing and measuring of both adults, whichever bird was present in the burrow was weighed daily.

Long-term effects of disturbance: During the following breeding season, each of the 1999 study burrows were monitored. In occupied former weekly and daily treatment burrows the adults were briefly captured and identity recorded. They were not disturbed again until after the egg's projected hatching date, to determine hatching success. Several ringed birds that were breeding in different burrows in 2000 were searched for and located. The distance between burrows was measured.

A total of 58 Leach's petrel pairs were monitored during the 1999 breeding season. Pairs in the control group (n=20) were disturbed during a single visit but not handled. Weekly treatment pairs (n=20) were disturbed on 33%, and daily treatment pairs (n=18) were disturbed on 100%, of the days their burrows were under observation. Of the 76 individuals in the weekly and daily treatments, 80% had never been handled by humans, the rest had been previously banded but not handled for 6–13 years.

Hatching success was significantly influenced by the frequency of disturbance. Weekly and daily handling of adults greatly reduced hatching success - by 50 and 56% repectively, compared to the control group. Most failures (91%) were caused by egg desertion, and all the deserted eggs belonged to pairs in the two disturbance treatments. Hatching success was not influenced by the time of day of disturbance. During the subsequent breeding season, the hatching success of disturbed pairs (i.e. those disturbed the previous breeding season) that continued to breed together returned to normal levels. However, 37% more disturbed pairs than control pairs deserted the nesting burrows that they had used in the previous year. Since most changes in nest site also result in mate change, investigator disturbance may have had long-term negative effects on reproductive success as well.

Conclusions: The results demonstrate that both weekly and daily investigator disturbance of Leach's petrels during incubation greatly reduced hatching success, and also reduced subsequent nest-site fidelity in the following breeding season.


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