Crossing the Pillars of Hercules: Understanding transoceanic migrations of seabirds throughout their breeding range

Abstract

Variability in long‐distance migration strategies is still poorly understood due to the fact that individuals are often tracked from a single colony/population. Transoceanic migrations of Scopoli's shearwaters ( Calonectris diomedea ) across the Strait of Gibraltar (SoG) have been tracked from several breeding colonies isolatedly, and factors related to the variability in phenological schedules among different populations remain, therefore, not well‐understood. Using light‐level geolocator data, I examined the autumn (postbreeding) and spring (prebreeding) migratory passage dates through SoG of four populations of Scopoli's shearwater spread along the longitudinal breeding range of the species. Additionally, I also estimated the at‐sea activity patterns (from immersion data) during both migratory passages, as well as the body size (from morphometric data) of the individuals of these populations. On average, Scopoli's shearwaters leave the Mediterranean (cross SoG) on 31 October ± 1.8 days on their autumn migrations and return on 03 March ± 1.6 days on their spring migrations. At the population level, there was a clear gradient in the timing of crossing SoG: birds from the westernmost populations (Murcia, SE Spain) were the first ones in leaving the Mediterranean while easternmost breeders (Paximada, Crete) were the last ones. In spring, only birds from the largest breeding population (Zembra, Tunisia) seemed to advance their return and crossed SoG significantly earlier than birds tracked at the remaining populations. In both passages, shearwaters from central and eastern populations spent more time flying than their conspecifics from the western Mediterranean. Scopoli's shearwater populations display a differential phenology and behavior in their migratory passages through SoG. The longitudinal gradient in body size already reported for the species could be an evolutionary response to an obvious trade‐off between sharing common wintering grounds in the Atlantic Ocean and the temporal constraints of restoring physiological condition in those grounds. OPEN RESEARCH BADGES This article has earned an Open Data Badge for making publicly available the digitally‐shareable data necessary to reproduce the reported results. The data is available at https://hdl.handle.net/2445/128784 .

Non‐breeding distribution and at‐sea activity patterns of the smallest European seabird, the European Storm Petrel (Hydrobates pelagicus)

“…The Strait of Gibraltar is an important landmark for migratory animals not only because it defines the limits between the Atlantic Ocean and the Mediterranean Sea, but also because it is frequently used to study the migratory passage and phenology of seabirds (Ramos 2019), including European Storm Petrels (Hashmi & Fliege 1994). Some of the tracked European Storm Petrels passed through the Strait of Gibraltar more than once at the beginning and/or at the end of the non‐breeding period.…”

Methods to detect spatial biases in tracking studies caused by differential representativeness of individuals, populations and time

“…In our example with these three phylogenetically close species that share nonbreeding areas, we found that the Cape Verde shearwater, the species with the most restricted breeding and nonbreeding distributions, had consistently higher inclusiveness values, The relationship with longitude, although not significant, might suggest a west-to-east decline in inclusiveness values. Birds breeding in the easternmost colonies of the Mediterranean used the northernmost nonbreeding areas preferentially (Ramos, 2019), which made their use of the southernmost nonbreeding locations less likely.…”

“…The relationship with longitude, although not significant, might suggest a west‐to‐east decline in inclusiveness values. Birds breeding in the easternmost colonies of the Mediterranean used the northernmost nonbreeding areas preferentially (Ramos, 2019), which made their use of the southernmost nonbreeding locations less likely. Thus, they were not good in representing the entire nonbreeding distribution of the species, although necessary for the complete knowledge of the wintering grounds of the species.…”

At-sea behavioural ecology of the endangered MacGillivray’s prion from Saint Paul Island: combining tracking and stable isotopes

“…In a still unexplained way, the birds overwintering over the Southwest Indian Ridge did not exhibit such synchronicity in their cycle of at-sea activity. Among seabirds, the influence of the moon phase on at-sea activity appears poorly marked for some species (Scopoli's shearwaters Calonectris diomedea (Ramos 2019); Bulwer's petrel Bulweria bulwerii (Dias et al 2016)), while clearly evidenced in others (e.g. Yamamoto et al 2008, Mackley et al 2011, Pinet et al 2011.…”