Using multiple methods to describe breeding, stress response, and disturbance of Marbled murrelets (Brachyramphus marmoratus)
Laura A. McFarlane Tranquilla
BSc, University of New Brunswick, 1997
© Laura A. McFarlane Tranquilla 2001, Dept. Biological Sciences, SIMON FRASER UNIVERSITY, November 2001
I investigated the breeding biology of Marbled Murrelets using (a) vitellogenin (VTG) analyses (b) brood patch (BP) scores (thought to imply incubating adults), and (c) radio telemetry data. VTG analyses allowed description of the 5-month breeding season for Marbled Murrelets, the timing of which did not vary between years (1999-2000). Of the females caught between April to July (the ‘egg-production period’), 55% were producing eggs. Using brood patches (BP) to infer reproductive status is an approach that should be used cautiously: 53% Marbled Murrelets caught with fully-developed BP never incubated, and likewise, 50% of fecund, radio-tagged females never incubated (failed incubators?). Of a sample of fecund females, 40% started incubation about 15 days later than expected (delayed incubators?). This suggests large numbers of birds that failed to start incubation, for reasons that were not clear. While investigator disturbance explained some cases, seasonal date also had an effect on breeding success. We detected a seasonal decline in breeding success in Marbled Murrelets, with failed incubators occurring later in the season (by 18 days) than successful incubators, and ‘delayed’ incubators initiating incubation later (by 24 days) those not delayed. Thus, while capturing murrelets sometimes affected individual breeding status, later breeders were affected more than earlier breeders. This finding suggests that researchers should aim to capture Marbled Murrelets early in the breeding season. My investigation of capture effects also included an analysis of the stress response to capture, using corticosterone. Like other birds, Marbled Murrelets reach maximum corticosterone levels at 30 min. Corticosterone increased with mass in females (but not males), suggesting that females are more sensitive to stress when they are heaviest, during egg-production.