Seasonal variations in migration strategy of a longdistance Arctic-breeding seabird

Abstract

Long-distance migratory seabirds need to adjust their migration strategy according to internal (breeding, molting) and external factors (seasonality, resource availability). Time-minimizing strategies are common during spring migration to arrive at the optimal time to breed. We studied the annual movements and migration strategy of the long-tailed jaeger Stercorarius longicaudus, a small arctic-nesting seabird. First, we documented year-round movements (routes, wintering sites) of male and female jaegers breeding in the Canadian Arctic. We then compared their migration strategies between seasons (phenology, stopover use, travel distance, speed) to determine whether they adopt a time-minimizing strategy in spring. Over 6 yr, we collected 43 tracks from geolocators deployed on Bylot and Igloolik Islands. Jaegers departed the breeding site over a 5 wk period and traveled on average 32375 km (round trip) before returning to breed, one of the longest documented migrations on Earth. Birds used a major stopover area east of the Grand Banks of Newfoundland in spring and fall, and wintered in high marine productivity areas of the South Atlantic. Unexpectedly, the spring migration was 40% longer and 32% slower than in fall, and birds increased their time spent on water (foraging and/or resting) by 61%. A time-minimizing strategy in fall may help to reach the wintering site rapidly and start molting early. In spring, a fly-and-forage strategy seems to be adopted to increase foraging effort, probably for the accumulation of body reserves before breeding and in anticipation of unfavorable conditions that may prevail at arrival on their arctic breeding site.

Sympatrically breeding congeneric seabirds (Stercorarius spp.) from Arctic Canada migrate to four oceans

“…These modifications may have led to longer tracking durations than were previously attained (maximum 86 days for LTJA and approximately 275 days for POJA), although our small sample size limits general inferences. Rather than a backpack‐style harness that loops over the wings (used previously with LTJA [Seyer et al, 2021; Sittler et al, 2011]), we used a leg‐loop attachment (Mallory & Gilbert, 2008) as had been trialed for POJA (Troy, 2007). For acrobatic birds like jaegers, we felt a leg‐loop harness would have the lowest risk of detrimental impact to the bird but may have a higher risk of being shed (sliding off the tail and legs of the bird possibly due to weight changes or interactions with other birds).…”

“…For acrobatic birds like jaegers, we felt a leg‐loop harness would have the lowest risk of detrimental impact to the bird but may have a higher risk of being shed (sliding off the tail and legs of the bird possibly due to weight changes or interactions with other birds). For LTJA and POJA, we also used smaller tags than in previous studies: LTJA, 5 g instead of 9.5–10 g (Seyer et al, 2021; Sittler et al, 2011) and POJA, 9.5 g instead of 18 g (Troy, 2007). For LTJA and POJA, our 1% tag and harness to bird mass ratios were conservative in the context of conventional rules for seabird tracking studies (i.e., <3% of the body weight of the bird [Phillips et al, 2003]).…”

“…Given the omission of movements during the staging period, this is most likely conservative. We identified the arrival and departure times to staging and wintering areas based on a combination of net‐squared‐displacement values (birds are resident to an area when the rate of change of net‐squared‐displacement plateaus; Seyer et al, 2021) and visual evaluation of maps.…”

“…Previous results from light-level geolocator tracking of jaegers in North America showed that LTJA from the eastern Canadian Arctic over-wintered off west and southern Africa (Seyer et al, 2021) and PAJA from eastern Greenland migrated to the Caribbean region (Bemmelen, 2019). A presumed-complete southward migration of a POJA from Alaska, USA, was recorded by satellite and this bird spent the postbreeding period off the coast of southeastern Australia (Troy, 2007).…”

“…(2) describe the ocean habitats they used; and (3) provide the first direct information on full-annual-cycle movements and nomadism of a pomarine jaeger, an irruptive species whose movements are still largely a mystery (Wiley & Lee, 2020a). Like LTJA tracked from the eastern Canadian Arctic (Seyer et al, 2021), and LTJA and PAJA tracked from Greenland (Bemmelen, 2019), we expected that all three species would spend the nonbreeding period in the Atlantic Ocean. We also expected that PAJA would use shallow, coastal habitats and that LTJA and POJA would use deeper habitats in areas of consistent upwelling.…”