Size and accumulation of fuel reserves at stopover predict nocturnal restlessness in a migratory bird

Abstract

Early arrival at the breeding site positively affects the breeding success of migratory birds. During migration, birds spend most of their time at stopovers. Therefore, determining which factors shape stopover duration is essential to our understanding of avian migration. Because the main purpose of stopover is to accumulate fat as fuel for the next flight bout, fuel reserves at arrival and the accumulation of fuel are both expected to affect stopover departure decisions. Here, we determined whether fuel reserves and fuel accumulation predict a bird's motivation to depart, as quantified by nocturnal migratory restlessness (Zugunruhe), using northern wheatears (Oenanthe oenanthe) that were captured and temporarily contained at spring stopover. We found that fuel reserves at capture were positively correlated with Zugunruhe, and negatively correlated with fuel accumulation. This indicates that fat birds were motivated to depart, whereas lean birds were set on staying and accumulating fuel. Moreover, the change in fuel reserves was positively correlated with the concurrent change in Zugunruhe, providing the first empirical evidence for a direct link between fuel accumulation and Zugunruhe during stopover. Our study indicates that, together with innate rhythms and weather, the size and accumulation of fuel reserves shape stopover duration, and hence overall migration time.

Migratory restlessness in captive individuals predicts actual departure in the wild

“…Migratory restlessness is a key measure in avian migration research, and many studies have used inter-individual variation in migratory restlessness to explain how environmental and intrinsic factors temporally affect migration [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. Our findings therefore justify the conclusions of a large body of research on avian migration.…”

“…As migratory restlessness has also been defined as the activity recorded in a given time period, usually (part of) the night [14,16,[20][21][22]25], we repeated the analyses with migratory restlessness defined as the total number of activity counts recorded between 1 and 5. Levels of migratory restlessness were lower than those observed previously in northern wheatears on Helgoland during spring migration [23]. The most likely explanation for this seasonal difference is that in spring migrants generally travel faster, and make shorter stopovers, than in autumn [31].…”

“…Upon capture, C.E. measured wing length (maximum chord) to the nearest 0.5 mm, and body mass to the nearest 0.1 g. Wing length was used to calculate lean body mass, employing a linear regression based on 220 'lean' northern wheatears caught on Helgoland in previous years: lean body mass (g) ¼ 0.29 g mm 21 Â wing length (mm) -6.85 g (linear regression: n ¼ 220, F 1,218 ¼ 95.07, adj-R 2 ¼ 0.30, p , 0.0001, after [23]). Fat reserves were then calculated as: (body mass (g) -lean body mass (g))/lean body mass (g).…”

Ghrelin affects stopover decisions and food intake in a long-distance migrant

“…That is, which physiological signals tell the brain that the fuel reserves are sufficient to resume migration? This link that connects physiological condition and brain is still unknown.Previous work has shown that fat stores are the best predictor for migratory disposition and stopover duration: Birds with larger subcutaneous (s.c.) fat stores show more migratory restlessness in captivity (8,10,13,14), stay shorter times at stopover sites (9, 11), and migrate faster (12). Because birds fuel their migratory flights primarily through fatty acid metabolism (15-17), we expect that factors reflecting fuel load or the energetic condition must be involved in the behavioral switch between resting at stopover sites and resuming migration.…”

Experimental temperature manipulations alter songbird autumnal nocturnal migratory restlessness

“…During the migration season energy stores and feeding/refueling rates are a critical determinant modulating migratory behaviour (Fusani et al 2009, Eikenaar and Schläfke 2013, Lupi et al, 2016. In addition, external factors also affect the costs and benefits of migration and can modulate migration behaviours.…”