Life in the fast and slow lanes: contrasting timing of annual cycle events in high- and mid-latitude breeding Whooper Swans (Cygnus cygnus)

Shorter duration of ice-free conditions at successively northern latitudes increasingly constrains large-bodied waterbirds from completing their reproductive cycle before freeze-up, affecting their migration and breeding schedules. We compared migration timing and duration of stay in summering areas between 127 tracked East Asian Whooper Swans Cygnus cygnus breeding at high latitude (HLP, 67°N, Japan wintering) and mid-latitude (MLP, 48°N, China wintering) based on tracking and remote-sensing data. Adult breeding site arrival dates were positively correlated with latitude. HLP adults arrived just before 50% snow melt, and summered for 142 days (the duration of the ice-free period there), just over the c.130 days required to hatch and raise cygnets to fledging. MLP adults arrived 3 weeks after 50% snow melt, and summered for c.209 days (c.80% of the total ice-free period of 267 days), laying eggs c.33 days after arrival. These results suggest that HLP breeders were more likely to invest stored nutrients and energy brought from winter quarters and/or on migration to enable rapid egg-laying post-arrival at the breeding grounds (a more ‘capital’ breeding strategy). By contrast, MLP females appear able to obtain most, if not all, of their energetic and nutritional reproductive needs exogenously post-arrival at breeding areas (a more ‘income’ breeding strategy). HLP individuals left the breeding grounds before MLP, but all departed before freeze-up. Migration duration did not differ between the two groups, so differential summer residency times affected duration on wintering areas. We recommend follow-up investigations of the relative degree of capital/income investment among birds from both groups and their effects on their respective demographics, population dynamics and growth rates.