MEPS 687:163-172 (2022)  -  DOI: https://doi.org/10.3354/meps14015

ABSTRACT: Many bird species use features of the physical environment to cue breeding activity. We show that for 2 species of Pacific auks (zooplanktivorous Cassin’s auklet Ptychoramphus aleuticus and generalist rhinoceros auklet Cerorhinca monocerata), spatio-temporal variation in marine production indicators surrounding a major breeding colony in the northeast Pacific Ocean can cue laying date. By utilizing a multi-year phenological time series and a spatio-temporal sliding-window analysis spanning November up until median lay dates in spring, chlorophyll a (chl a) concentrations in the surface ocean around the colony strongly predicted lay date. However, the response to this cue differed between species by over 2 mo, as each species was exposed to different, sequential water-masses. We show that for Cassin’s auklets, chl a levels at Triangle Island, British Columbia (Canada) in February, nearly 2 mo prior to actual laying, strongly correlated with lay date in April (average r = 0.83). At this time, the ocean environment around Triangle Island is dominated by water-masses from the south-southeast. However, for sympatric rhinoceros auklets, chl a along broad shelf-break areas in April prior to lay dates in May strongly correlated with lay date (average r = 0.76). Occurring after the spring transition, these water-masses flow primarily from the northwest. Consistent with other bird species, both auks appear to respond to information about food production, but at different spatio-temporal scales. We suggest that these different responses to environmental cues reflect species-specific differences in female migration behaviour, and an attempt by Cassin’s auklets to bet-hedge against phenological mismatches.

KEY WORDS: Lay date · Chlorophyll a · Breeding phenology · Egg-laying date · Seabirds · Time-series · Pytchoramphus aleuticus · Cerorhinca monocerata