Taking the Bite Out of Winter: Common Murres (Uria aalge) Push Their Dive Limits to Surmount Energy Constraints

Abstract

Diving seabirds that overwinter at high latitudes experience persistent cold exposure, short days and associated declines in ocean productivity that can challenge their ability to balance daily energy budgets. We used dive-immersion geo-locators to test the hypothesis that pursuit-diving Common murres (Uria aalge) will respond to the challenges of winter in the North Atlantic through increased daily energy expenditures (DEE) that will be met by increased foraging effort and adjustments in dive tactics. Largely flightless in winter (<5% of daylight hours flying), murres spent most of their time on the water (>85% resting and swimming). Accordingly, when sea surface temperatures (SST) were consistently near freezing in late winter (1.9 ± 0.8 • C), mean DEE (2463.2 ± 10.9 kJ day −1 ) exceeded the theoretical limit to sustainable energy expenditure in vertebrates (i.e., 7 X Basal Metabolic Rate or 2450 kJ day −1 for murres). Consistently deep (70% >50 m) and long dives in late winter, 38% of which exceeded their calculated aerobic dive limit indicate that targeted prey was distributed in deep (dark) waters. Consequently, foraging was largely diurnal; likely because capture efficiency of deep-water prey is poor under low light. Murres responded to these late winter time and energy constraints with a nearly two-fold increase in daily time spent diving (95.2 ± 5.6 and 178.3 ± 6.3 min day −1 during early and late winter, respectively), an increase in dive bout frequency and duration, and correspondingly less time resting between bouts. Uniquely adapted for deep-diving, pursuit-diving can push their dive limits to maximize daily energy intake when energy demands are high and prey are distributed in deep water. Our study highlights late winter as an extremely challenging phase in the annual cycle of North Atlantic murres and provides critical insights into the behavioral mechanisms underlying their winter survival.

A year in the life of a North Atlantic seabird: behavioural and energetic adjustments during the annual cycle

“…Despite our values of DEE being based on a number of assumptions 21 , we are confident in the patterns of guillemot behaviour and energetic expenditure that we have identified for this annual cycle. Although the pattern of DEE is different to those that have been recorded in guillemot populations breeding in Newfoundland, Canada 21 and Svalbard, Norway 20 , our estimates are within the ranges reported within these studies. For each of these high latitude guillemot populations, the winter (January -February) was an energetically challenging period.…”

“…20 th July 2005-2 nd April 2006) were created using the adehabitatHR package 56 , using a least squared cross validation method and a 50 km grid size. Core use areas during the non-breeding period were represented by 50% kernel density contours 21 . (2020) 10:5993 | https://doi.org/10.1038/s41598-020-62842-x www.nature.com/scientificreports www.nature.com/scientificreports/ Daily air temperature was extracted from either Lerwick (60° 14′N, 01°18′W), Leuchars (56° 38′N, 02°86′W), Bridlington (54° 1′N, 00°21′W) or Sandettie (51° 10′N, 01°80′W) weather stations 57 , depending on which was in closest proximity to the centroid of an individual's fortnightly GLS fix cluster.…”

“…Days which did not have temperature and depth data for the entire 24 hour period were removed (n = 9) and the remaining data (n bird days = 179) were utilised to derive daily time-activity budgets. Time-activity budgets were based on the identification of five key activities: a. diving (T d ), b. flying (T f ), c. at the colony (T c ), d. active on water (T a ) and e. inactive on water (T i ) 21,60 . Active on water (T a ) included intervals on the surface between dives and longer periods between dive bouts when activities such as swimming and preening were undertaken.…”

“…S2A). Bouts of flight (T f ) were separated from bouts of inactivity (T i ) on the assumption that guillemots do not fly at night 61,62 and logger temperature being less than daily air temperature + 4 o C, as opposed to the higher temperatures expected during leg-tucking events (T i 21,60 ). We validated this classification of T f and T i by modelling the impact of behavioural state on the relationship between the duration of time spent in the activity and the maximum temperature recorded ( Supplementary Fig.…”

“…For example, those that wintered in the Norwegian, Barents and White Seas increased their foraging effort ahead of several weeks of polar night, potentially maximising prey intake prior to this period of intense environmental constraint 20 . Furthermore, guillemots that over-wintered on the Newfoundland Shelf increased their diurnal foraging effort in response to seasonally varying vertical distributions of prey resources 21 . Thus guillemots, as with other seasonally breeding diving birds, make good models to investigate behavioural and energetic responses to seasonally varying ecological drivers.…”