Change in breeding phenology is often a response to environmental forcing, but less is known of the mechanism underlying such changes and their fitness consequences. Here we report on changes in the breeding phenology from a 27-year longitudinal study (1991-2017) of individually marked, known-aged grey seals (Halichoerus grypus) on Sable Island, Nova Scotia, Canada.

We used Generalized Linear Mixed Models to test hypotheses about the influence of fixed factors (maternal age, parity, previous reproductive success, pup sex, colony density, AMO, NAO, and SST) and a random factor (female identity) on parturition dates. We also examined the consequences of the shift in birthdates on maternal energy investment in offspring as measured by pup weaning mass. Birthdates were known for 2768 pups of 660 known-age females. For 494 females with ≥ 2 parturition dates, repeatability as measured by the intraclass correlation was high (mean=0.66). 87% of the variation in birthdates was explained by a mixed effects model that included intrinsic and extrinsic fixed effects. Most of the explained variation was associated with the random effect of female identity.

Parity was the most important intrinsic fixed effect, with inexperienced mothers giving birth later in the season than multiparous females. Over almost 3 decades, mean birthdates advanced by 15 days. The mixed model with intrinsic effects and population size, the detrended AMO from the previous year and mean NAO in the previous 3 years explained 80 % of the variation with 21 % of variation from the fixed effects. Both primiparous and multiparous individuals responded to the climate forcing, and there was strong evidence for heterogeneity in the response. The shift in birthdates did not impact pup weaning mass. Females exhibited considerable phenotypic plasticity indicating that climate driven changes in breeding phenology may not always have measurable consequences.

BDmodeldat.csv: Data for the intrinsic model of birthdate (Year can be used to link to environmental covariates reported in MeanBDdat.csv).

• MomID: unique identifier for female grey seal
• Year: breeding season (December of year-1 to January of year) pup was born (also PupCohort)
• BirthDay: day of birth in breeding season (days since Dec 1)
• MomAge: female age in year pup was born
• MomParity: number of pups or pregnancies observed
• PupSex: 1 is male, 2 is female, 0 is unknown
• MomPrevSuccess: 1 is previously successful, 0 is previously unsuccessful

MeanBDdat.csv: Environmental covariates and mean birthdate estimated from random effects of the intrinisc model. 

• Year: breeding Season (December of year-1 to January of year)
• MeanBD: estimated mean birthdate in the breeding season (days since Dec 1) from random effects in the intrinsic birthdate model
• NumSeals: estimated total population supported by breeding colony on Sable Island in previous year
• AMOdt: AMO in previous year
• AMOdt3yr: Mean AMO in previous 3 years
• NAO: NAO in previous year
• NAO3yr: Mean NAO in previous 3 years
• SprSST: sea surface temperature from the Fisheries and Oceans Research Vessel Trawl survey in the previous year

WMmodeldat.csv: Data required to model pup weaning mass as a function of maternal and pup covariates including birthdate.

• Year: breeding season (December of year-1 to January of year) pup was born (also PupCohort)
• MomID: unique identifier for female grey seal
• PupWM: pup weaning mass in kg
• MomAge: female age in year pup was born
• MomParity: number of pups or pregnancies observed
• PupSex: 1 is male, 2 is female, 0 is unknown
• MomPrevSuccess: 1 is previously successful, 0 is previously unsuccessful
• BirthDay: day of birth in breeding season (days since Dec 1)
• MeanBD: estimated mean birthdate from random effects in the intrinsic birthdate model
• BDAnomaly: day of birth in breeding season (days since Dec 1) less the MeanBD for that year