MEPS prepress abstract  -  DOI:

Sex differences in migration and demography of a wide-ranging seabird, the northern gannet

Zoe Deakin, Keith Hamer, Richard B Sherley, Stuart Bearhop, Thomas W Bodey, Bethany Clark,W James Grecian, Matt Gummery, Jude Lane, Greg Morgan, Lisa Morgan, Richard A Phillips, Ewan D Wakefield, Stephen C Votier


ABSTRACT: Marine vertebrates show a diversity of migration strategies, including sex differences. This may lead to differential demography, but the consequences of such between-sex variation are little understood. Here we studied the migration of known-sex northern gannets Morus bassanus – a partial migrant with females ~8% heavier than males. We used geolocators to determine the winter areas of 49 breeding adults (19 females and 30 males) from two colonies in the northeast Atlantic (Bass Rock and Grassholm, UK). We also tested for sex-specific survival probabilities using capture-mark-recapture methods (n=72 individuals Bass Rock; n=229 individuals Grassholm; 2010-2018) and applied sex-specific population projection matrices (PPMs) to quantify population-level effects. Tracked gannets wintered in a range of Large Marine Ecosystems (LMEs): Canary Current LME (69%; CCLME), Celtic-Biscay Shelf LME (16%), Iberian Coastal LME (8%), North Sea LME (4%) or Mediterranean LME (2%). Migratory destination differed between the sexes: 90% of females vs. 57% of males wintered in the CCLME. Survival was similar between the sexes at Bass Rock (0.951 ± 0.053 and 0.956 ± 0.047 for females and males, respectively). At Grassholm there was evidence of slight sex differences in breeder survival: females had lower annual survival (mean ± SD = 0.882 ± 0.040) than males (0.946 ± 0.026). At Bass Rock, PMMs with no sex-effect best fitted the observed population increase (1994-2014). Sex-specific PPMs fitted the population estimates for Grassholm (1995-2015). Our results reveal that female gannets are more likely to travel further than males to winter in the CCLME. This difference is unlikely to be due to morphological differences, unlike in other bird species. However, the reason for slightly higher over-winter female mortality at Grassholm is unclear.