We modelled how nestling growth rates of Cassin’s Auklet (Ptychoramphus aleuticus (Pallas, 1811)) varied with timing of peak copepod prey availability at two breeding colonies in British Columbia: on Triangle Island, in the California Current Ecosystem, and Frederick Island, in the Gulf of Alaska Ecosystem. We used time series of nestling growth rates and estimated the seasonal timing of peak biomass of the copepod Neocalanus cristatus (Krøyer, 1848) using a temperature-dependent phenology equation. We developed a single model to examine intercolony differences in the effect of the timing of regional peak prey biomass on seabird nestling growth rates. This model indicated nestling growth rates on Triangle Island varied widely and were positively associated with timing of peak zooplankton biomass, such that higher growth rates were observed when the peak biomass occurred later in the breeding season. In contrast, nestling growth rates were consistently high at Frederick Island, where peak copepod biomass always occurred relatively late. If ocean climate warming results in a poleward shift of Neocalanus abundance and induces earlier and more narrow timing of availability, then episodes of poor nestling growth will increase in frequency on Triangle Island and could eventually affect auklets on more northerly colonies.
Latitudinal temperature-dependent variation in timing of prey availability can impact Pacific seabird populations in Canada