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

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 wintering areas of 49 breeding adults (19 females and 30 males during 2010 to 2014) from 2 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 (CCLME; 69%), 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 (mean ± 95% CI = 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 (0.882 ± 0.040) than males (0.946 ± 0.026). At Bass Rock, PPMs with no sex effect best fitted the observed population increase (1994−2014). Sexspecific 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 due to morphological differences, unlike in other bird species. However, the reason for slightly higher over-winter female mortality at Grassholm is unclear.

Sexual segregation of gannet foraging over 11 years: movements vary but isotopic differences remain stable

“…Fieldwork took place on Grassholm Island, Wales, UK (51°43' N, 05°28' W), during the chick-rearing periods (late June to August) of 11 breeding seasons ( −2017( , with GPS tracking in 2006( −2017( , and stable isotope sampling in 2006. The colony held ~30 000 pairs during the study period (Murray et al 2015, Deakin et al 2019. Adult gannets were captured at the nest using a carbon fibre pole with a noose or crook during the changeover between partners so that chicks were not left unattended.…”

Individual Spatial Consistency and Dietary Flexibility in the Migratory Behavior of Northern Gannets Wintering in the Northeast Atlantic

“…Gannets exhibit a southward-oriented chain migration following a flyway running along the coast of western Europe and Africa (Fort et al, 2012). Variation in migratory behavior, the migration path, final non-breeding destination and foraging behavior during these periods, occurs both among and within populations (Kubetzki et al, 2009;Fort et al, 2012;Deakin et al, 2019), and one recent study has shown that individuals in the NW Atlantic exhibit consistent behavioral strategies in successive years (Fifield et al, 2014). Additionally, the nonbreeding distributions of gannets may have changed in recent decades (Kubetzki et al, 2009), suggesting a degree of plasticity in migratory behavior.…”

So far, so good… Similar fitness consequences and overall energetic costs for short and long-distance migrants in a seabird

“…among sexes [60], ages and individuals in migratory behaviour [61]. Recently, GLS loggers were used to study migration strategies of gannets nesting in the Northeast Atlantic [10,23,24,26,60] and Northwest Atlantic [25]. These loggers also made it possible to track long-term migration to assess marine pollution impact (e.g.…”