Travelling on a budget: predictions and ecological evidence for bottlenecks in the annual cycle of long-distance migrants

Abstract

Long-distance migration, and the study of the migrants who undertake these journeys, has fascinated generations of biologists. However, many aspects of the annual cycles of these migrants remain a mystery as do many of the driving forces behind the evolution and maintenance of the migrations themselves. In this article we discuss nutritional, energetic, temporal and disease-risk bottlenecks in the annual cycle of long-distance migrants, taking a sandpiper, the red knot Calidris canutus, as a focal species. Red knots have six recognized subspecies each with different migratory routes, well-known patterns of connectivity and contrasting annual cycles. The diversity of red knot annual cycles allows us to discuss the existence and the effects of bottlenecks in a comparative framework. We examine the evidence for bottlenecks focusing on the quality of breeding plumage and the timing of moult as indicators in the six subspecies. In terms of breeding plumage coloration, quality and timing of prealternate body moult (from non-breeding into breeding plumage), the longest migrating knot subspecies, Calidris canutus rogersi and Calidris canutus rufa, show the greatest impact of bottlenecking. The same is true in terms of prebasic body moult (from breeding into non-breeding plumage) which in case of both C. c. rogersi and C. c. rufa overlaps with southward migration and may even commence in the breeding grounds. To close our discussion of bottlenecks in long-distance migrants, we make predictions about how migrants might be impacted via physiological 'trade-offs' throughout the annual cycle, using investment in immune function as an example. We also predict how bottlenecks may affect the distribution of mortality throughout the annual cycle. We hope that this framework will be applicable to other species and types of migrants, thus expanding the comparative database for the future evaluation of seasonal selection pressures and the evolution of annual cycles in long-distance migrants. Furthermore, we hope that this synthesis of recent advancements in the knowledge of red knot annual cycles will prove useful in the ongoing attempts to model annual cycles in migratory birds.

Migratory common blackbirds have lower innate immune function during autumn migration than resident conspecifics

“…Our study clearly indicates that migrating blackbirds did not boost their innate immune defences. Instead, it lends support to the idea that immune function is compromised during migration as a consequence of physiological or energetic trade-offs [4]. However, only innate immune function (BKA and haptoglobin-like activity) was lower in migrants than in residents; baseline acquired (antibody-mediated) immunity, measured as total immunoglobulins, did not differ between migrants and residents.…”

“…However, the immune system is costly in terms of its production, maintenance and activation [2,3]. For migratory animals, it has therefore been hypothesized that they need to reduce immune function during the physiologically demanding migration seasons [4,5]. A contrasting hypothesis proposes that migrants need to boost immune function because they encounter more and/or different pathogens during migration [6,7].…”

Prolonged stopover and consequences of migratory strategy on local-scale movements within a regional songbird staging area

“…Although it is still unclear why songbirds exhibit this behavior, these extended stops may be an adaptive strategy for accumulating large fat stores at food-rich sites to fuel long migratory flights (Tøttrup et al 2012, Callo et al 2013), especially just before or after a barrier (Delmore et al 2012, Fraser et al 2013, Gómez et al 2017). Regardless of their purpose, effective conservation requires identifying prolonged stopover areas (McKinnon et al 2017, Van Loon et al 2017) and understanding behavioral patterns at these sites, because localized loss of stopover resources can pose an ecological bottleneck (Myers 1983, Buehler and Piersma 2008, Gómez et al 2017.…”

Flyway evolution is too fast to be explained by the modern synthesis: proposals for an ‘extended’ evolutionary research agenda

“…2). The subspecies rufa and islandica are both relatively largebodied (Tomkovich 1992), but despite its name, rufa is the palest subspecies, whereas islandica is among the darkest (Buehler and Piersma 2008). While rufa has the longest seasonal migration, commuting between the sub-Antarctic during the nonbreeding season and the Canadian High Arctic during the breeding season (see Fig.…”

“…The discontinuous circumpolar breeding range of Red Knots incorporates breeding areas of at least six populations that are morphologically sufficiently distinct to count as subspecies (Tomkovich 1992(Tomkovich , 2001), but which appear to have diversified recently from a small founder population that survived the Last Glacial Maximum at approximately 20,000 years ago. The subspecies are certainly distinct when it comes to their migratory trajectories and the seasonal timing of their movements Buehler and Piersma 2008;Piersma 2007). For the classic evolutionary mechanisms of random point mutations and gene selection mechanisms to explain the divergence of qualitatively and quantitatively distinct and non-overlapping traits, populations and time since divergence have, simply, been too small (Haldane 1957;Nunney 2003).…”