Combination of At-Sea Activity, Geolocation and Feather Stable Isotopes Documents Where and When Seabirds Molt

Abstract

Key facets of the foraging ecology of seabirds during the inter-breeding period still remain poorly understood because of the difficulty of studying them at sea, including during the energy-demanding molting stage. Here, the extent to which three sympatric petrels (Antarctic and thin-billed prions, and blue petrel) from the subantarctic Kerguelen Islands modify their foraging ecology during molt was investigated using a combination of complementary tools, namely miniaturized saltwater immersion geolocators (GLS) and the isotopic method. Firstly, molting behavior was first characterized in the blue petrel, a reference species that is known to renew its plumage in autumn. GLS and feather stable isotopes ( 13 δ C as a proxy of the birds' foraging habitat) indicated that the post-breeding molt of blue petrel occurred in Antarctic waters. Importantly, activity recorders showed that molt was marked by a strong peak in time spent daily sitting on water, which thereafter declined to lower values during the remaining winter months. Secondly, the peak in time spent sitting on water was used as a proxy to characterize the contrasted molt strategies of the two prion species. As blue petrels demonstrated, thin-billed prions molted during the post-breeding period in cold Antarctic waters where they fed primarily on low trophic level prey, most likely Antarctic krill ( 15 δ N as a proxy of the birds' diet). By contrast, Antarctic prions presented an unexpected pre-breeding molt of longer duration that took place further north, in warm subtropical waters. Interestingly, the two Antarctic molting species, the blue petrel and thin-billed prion, renewed their plumage at the same time and within the same oceanic zone that is likely to be a previously undescribed hot spot of seabird diversity during the Austral autumn. The study contributes to a growing body of evidence that closely-related species exhibit various foraging strategies allowing ecological segregation and sheds new light on the poorly known critical molting stage of seabirds.

Consistent variation in individual migration strategies of brown skuas

“…Values of 0 (entirely dry), 1 to 199, and 200 (entirely wet) in each 10 min period were categorised as either 'flight' (as skuas remain at sea during the non-breeding period), 'foraging' or 'sitting on water', respectively, assigned to daylight or darkness periods based on nautical twilight hours and summarised accordingly (see Mattern et al 2015). Although some intermediate values (from 1 to 199) will reflect non-foraging behaviour, in general this categorisation is assumed to provide a reasonable indication of foraging activity among seabirds (McKnight et al 2011, Cherel et al 2016.…”

Escaping the oligotrophic gyre? The year-round movements, foraging behaviour and habitat preferences of Murphy’s petrels

“…As Murphy's petrels decrease their flight activity substantially from November to February (Fig. 6), it is likely this period of relative inactivity coincides with moult (Cherel et al 2016), supported by observations of fresh plumage on birds at sea in April and May (Howell 2012). Whilst many gadfly petrels are known to be predominantly nocturnal (Brooke & Prince 1991, Ramirez et al 2013, the night-time activity of Murphy's petrels appears to be related to commuting rather than feeding behaviour, as during stages when birds travel shorter distances (pre-laying and non-breeding), they spent more time on the water at night.…”

“…Energy requirements are probably lower, yet during this period they also moult flight feathers which is an energetically demanding process. Indeed, recent studies have documented a quasi-flightless period associated with moult in many procellariids (Gutowsky et al 2014, Cherel et al 2016. Although very little is known about feather moult in Murphy's petrels, gadfly petrels perform a simple descendent moult, replacing primary feathers sequentially over a period of 3 to 4 mo (Bridge 2006).…”

Contrasting migratory responses of two closely related seabirds to long-term climate change

“…Prions undergo a complete moult of primary feathers during the non-breeding period, and primary coverts are usually moulted sequentially with the corresponding primary feather (Marchant & Higgins 1990, Brooke 2004, Cherel et al 2016. To limit damage, one primary covert from each museum skin was sampled.…”