Abstract
The non-breeding period of pelagic seabirds, and particularly the moulting stage, is an important, but understudied part of their annual cycle as they are hardly accessible outside of the breeding period. Knowledge about the moulting ecology of seabirds is important to understand the challenges they face outside and within the breeding season. Here, we combined stable carbon (δ13C) and oxygen (δ18O) signatures of rectrices grown during the non-breeding period of two pairs of storm-petrel species breeding in the northern (European storm-petrel, Hydrobates pelagicus, ESP; Leach's storm-petrel, Hydrobates leucorhous, LSP) and southern (black-bellied storm-petrel, Fregetta tropica, BBSP; Wilson's storm-petrel, Oceanites oceanicus, WSP) hemispheres to determine differences in moulting ranges within and between species. To understand clustering patterns in δ13C and δ18O moulting signatures, we examined various variables: species, sexes, years, morphologies (feather growth rate, body mass, tarsus length, wing length) and δ15N. We found that different factors could explain the differences within and between the four species. We additionally employed a geographical distribution prediction model based on oceanic δ13C and δ18O isoscapes, combined with chlorophyll-a concentrations and observational data to predict potential moulting areas of the sampled feather type. The northern species were predicted to moult in temperate and tropical Atlantic zones. BBSP was predicted to moult on the southern hemisphere north of the Southern Ocean, while WSP was predicted to moult further North, including in the Arctic and northern Pacific. While moulting distribution can only be estimated on large geographical scales using δ13C and δ18O, validating predictive outcomes with food availability proxies and observational data may provide valuable insights into important moulting grounds. Establishing those, in turn, is important for conservation management of elusive pelagic seabirds.
Highlights
The non-breeding period is an important part of the avian annual cycle, and it often spans the majority of the year in pelagic seabirds
The conditional inference tree (CIT) for both hemispheres showed that carbon and oxygen isotopic signatures differed significantly between species (Node 1; p < 0.001; Fig 1, Table 1), but not between sexes
Within ESP, LSP and WSP individuals could be grouped into different moulting niches as the δ13C and δ18O signatures of their tail feathers differed between groups. These divisions were linked to interannual differences in all three species, and to morphological and δ15N differences in ESP and WSP. These morphological differences were likely caused by differences in foraging ecology and prey availability, rather than sexual segregation
Summary
The non-breeding period is an important part of the avian annual cycle, and it often spans the majority of the year in pelagic seabirds. Knowledge about seabird non-breeding ecology is crucial to understand the entire annual avian cycle as events on the non-breeding grounds may carryover into the breeding period. The non-breeding period is understudied in many pelagic seabird species due to the inaccessibility of the birds beyond the breeding period. An important stage in the non-breeding period of many pelagic seabird species is the moulting stage. Spread the impact of moult by reducing the number of feathers moulting at once [13], increasing the length of the moulting period. The extended moulting period covers a large part of the non-breeding period, and may even overlap with the end of the breeding period [14]
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