A new method for ageing Marbled Murrelets and the effect on productivity estimates

Abstract

Accurate knowledge of the number of newly-fledged juveniles offshore is critical to estimates of productivity of the threatened Marbled Murrelet (Brachyramphus marmoratus). We describe a method for collecting productivity data which allows researchers to objectively evaluate age determinations and their effect on juvenile percentages. This has the potential to reduce the effect of observer variability and the number of misidentified birds, especially late in the breeding season when adults in late pre-basic molt are very similar to juveniles. We analyzed the timing of this molt in adults, and found some variation between regions and years. Therefore, critical dates for identification of adults in late molt versus juveniles may need to be reassessed annually. During our study, productivity estimates ranged from 1% to 7% juveniles. We used two methods for assessing the effect of missed juveniles. Even after adjusting for misclassified murrelets, the resulting juvenile percentages (1% to 17%) would suggest an unstable population in our study area.

Citation

Long, L.L.; Ralph, C.J.; Miller, S. 2001. A new method for ageing Marbled Murrelets and the effect on productivity estimates. Pacific Seabirds 28(2):82-91