Phenological cues to breeding and the differential response of Pacific auks to variation in marine productivity

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.