Adaptations to migration in birds: behavioural strategies, morphology and scaling effects

Abstract

The annual life cycle of many birds includes breeding, moult and migration. All these processes are time and energy consuming and the extent of investment in any one may compromise the others. The output from breeding is of course the ultimate goal for all birds, while the investment in moult and migration should be selected so that lifetime fitness is maximized. In particular, long-distance migrants breeding at high latitudes face severe time pressures, which is a probable reason why natural selection has evolved efficient behaviours, physiological and morphological adaptations allowing the maximum possible migration speed. Optimal migration theory commonly assumes time minimization as an overall strategy, but the minimization of energy cost and predation risk may also be involved. Based on these assumptions, it is possible to derive adaptive behaviours such as when and at which fuel load a stopover site should be abandoned. I review some core components of optimal migration theory together with some key predictions. A review of accumulated empirical tests of the departure rule indicates that time minimization is an important component of the overall migration strategy, and hence gives support to the assumption about time-selected migration. I also briefly discuss how the optimal policy may be implemented by the bird by applying a set of simple rules. The time constraints on migrants increase with increasing body size. Some consequences of this are discussed.

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

“…Rapidly accruing and carrying large fuel stores can increase energetic costs (Alerstam andLindström 1990, Hedenström andAlerstam 1997) and predation risks (Hedenström andAlerstam 1992, Kullberg et al 1996) but may be an optimal choice for later migrants facing declining food stores, temperatures, and day lengths (Schaub andJenni 2000, La Sorte et al 2015). A seasonal increase in fat stores can result from seasonal intensification of fuel deposition rates (Schaub andJenni 2000, Dänhardt andLindström 2001) and tailwinds (Koch et al 2006, Pena-Ortiz et al 2013.…”

“…Optimal migration theory predicts that birds balance the costs and benefits of different migratory decisions to minimize total time spent on migration, energy expenditure, predation risk, or some combination of these factors (Alerstam andLindström 1990,Åkesson andHedenström 2000). Many songbirds minimize migration time by seeking out optimal stopover habitats and accumulating large departure fuel stores rapidly in order to fuel long-distance flights (Hedenström 2008), while others minimize risk or energy expenditure by carrying smaller fuel loads, stopping more frequently, and making shorter flights (Bolshakov et al 2003,Åkesson et al 2012, Tøttrup et al 2012. The shorebird literature dubs these tactics ''skip'' and ''hop'' strategies, respectively, and outlines an additional strategy in which individuals accumulate extreme fat stores during long staging events to fuel subsequent ''jump'' flights .1,000 km (Piersma 1987, Warnock 2010.…”

A phantom road experiment reveals traffic noise is an invisible source of habitat degradation

“…We focus on birds during migratory stopover, because energy budgets are streamlined; foraging, vigilance, and rest dominate activity (14). To meet the amplified physiological needs of sustained nocturnal migratory flights, birds must increase foraging during periods of stopover while maintaining appropriate vigilance levels (14,15).…”

“…We focus on birds during migratory stopover, because energy budgets are streamlined; foraging, vigilance, and rest dominate activity (14). To meet the amplified physiological needs of sustained nocturnal migratory flights, birds must increase foraging during periods of stopover while maintaining appropriate vigilance levels (14,15). Any interference with foraging will decrease stopover efficiency and thus reduce migration speed, a likely surrogate for fitness (14), thereby increasing exposure to significant mortality risks during what can be the most perilous stage of a migratory bird's life cycle (16).…”

“…To meet the amplified physiological needs of sustained nocturnal migratory flights, birds must increase foraging during periods of stopover while maintaining appropriate vigilance levels (14,15). Any interference with foraging will decrease stopover efficiency and thus reduce migration speed, a likely surrogate for fitness (14), thereby increasing exposure to significant mortality risks during what can be the most perilous stage of a migratory bird's life cycle (16). Anthropogenic noise might disrupt the foraging-vigilance tradeoff by acting as a form of perceived predation risk (17,18) or by reducing sensory awareness via distraction or acoustic masking (3,19).…”