Timing of nocturnal autumn migratory departures in juvenile European robins (Erithacus rubecula) and endogenous and external factors

Abstract

Many passerine medium distance nocturnal migrants take off from stopover sites not only at the beginning of the night, but also in the middle and at the end of the night. In this paper, we tested two explanations for this phenomenon: (1) that departure time is governed by fuel stores, and (2) that departure time is influenced by the weather. The relationship of temporal distribution of migratory nocturnal departures with body condition and weather factors was studied in juvenile European robins (Erithacus rubecula) during autumn migration. The study was done on the Courish Spit on the Baltic Sea in 1997-2003 by retrapping 74 ringed birds in high mist nets during nocturnal migratory departure. Departure time was not related to fuel stores at arrival and departure, stopover duration, fuel deposition rate or progress of the season. Nor did the local weather at departure influence departure time. A possible reason was a large variation in the behaviour of the birds. European robins which made 1-day stopovers arrived and departed during better weather conditions than birds that stopped over for longer periods. In the former cohort, a significant model with four predictors explained 55% of variation in departure time. It is assumed that weather at the night of departure and during the previous night influenced the time of take-offs in these birds. In robins which made long stopovers, departure time is probably governed by their individual endogenous circadian rhythms of activity, which are related to the environment in a complex way.

Effect of fuel load, date, rain and wind on departure decisions of a migratory passerine

“…Moreover, rain and tailwind were included in the time-dependent models. Stopover duration is also a relevant variable determining departure decisions, since birds which would have stopped over for longer may be more motivated to depart (Bulyuk and Tsvey 2006;Bolshakov et al 2007;Tsvey et al 2007). However, our dataset was too small to also consider this variable.…”

“…In line with this reasoning, Arizaga et al (2008) observed that stopover duration was negatively correlated with fuel load (measured at first capture event) in migrating Blackcaps (Sylvia atricapilla). However, other authors found no clear correlations between arrival fuel load and stopover duration (Bulyuk and Tsvey 2006;Tsvey et al 2007). To what extent these differences could be due to a highly adaptable individual behaviour or to a hierarchical consideration of those variables governing the departure decisions is still a question that is scarcely understood.…”

“…Contrasting with wind or rain, which could be considered as environmental or exogenous variables, those associated with the circannual time programme may be considered as endogenous ones (Bulyuk and Tsvey 2006). A possibility for analysing the influence of the endogenous circannual time programme would be to test for the effect of timing (date) on departure decisions.…”

“…Data used to date to analyse the effects of weather on departure decisions come from three main sources: (1) direct observations (counting) of departing specimens (Hebrard 1971;Chan 1995;Bolshakov and Rezvyi 1998;Bolshakov and Bulyuk 1999), (2) timing of departure of individually marked specimens, either ringed (Fransson 1998;Dänhardt and Lindström 2001;Bulyuk and Tsvey 2006) or equipped with a radioemitter (Å kesson and Hedenström 2000;Å kesson et al 2001Bolshakov et al 2007;Tsvey et al 2007), and (3) estimations of the emigration likelihood (equivalent to the departure one) using Cormack-Jolly-Seber (CJS) models (Schaub et al 2004), which are based on mark-recapture data. A constraint with the use of direct observations is that number of departing specimens is related to that of stopping-over ones.…”

Variation in songbird migratory behavior offers clues about adaptability to environmental change

“…Yet the timing of migration is the product not only of decisions made at the point of departure (wintering or breeding grounds) but also of differences in behavior during travel (e.g., Bauer et al 2008b). At stopover sites, some individuals may seek brief sanctuary while awaiting favorable conditions to re-initiate migration or to relocate elsewhere within the landscape Taylor et al 2011), while others remain longer for rest and refuelling prior to another migratory flight (e.g., Bulyuk and Tsvey 2006;Bayly 2007). Because flexibility in en-route decisions permits individuals to respond to environmental conditions encountered over the course of a migratory journey (Drent et al 2003;Eichhorn et al 2009;Muraoka et al 2009), and because these conditions are likely to vary from year to year, average population behavior during stopover may track environmental change.…”