Abstract
Shifts in the timing of animal migration are widespread and well-documented; however, the mechanism underlying these changes is largely unknown. In this study, we test the hypothesis that systematic changes in stopover duration—the time that individuals spend resting and refueling at a site—are driving shifts in songbird migration timing. Specifically, we predicted that increases in stopover duration at our study site could generate increases in passage duration—the number of days that a study site is occupied by a particular species—by changing the temporal breadth of observations and vise versa. We analyzed an uninterrupted 46-year bird banding dataset from Massachusetts, USA using quantile regression, which allowed us to detect changes in early-and late-arriving birds, as well as changes in passage duration. We found that median spring migration had advanced by 1.04 days per decade; that these advances had strengthened over the last 13 years; and that early-and late-arriving birds were advancing in parallel, leading to negligible changes in the duration of spring passage at our site (+0.07 days per decade). In contrast, changes in fall migration were less consistent. Across species, we found that median fall migration had delayed by 0.80 days per decade, and that changes were stronger in late-arriving birds, leading to an average increase in passage duration of 0.45 days per decade. Trends in stopover duration, however, were weak and negative and, as a result, could not explain any changes in passage duration. We discuss, and provide some evidence, that changes in population age-structure, cryptic geographic variation, or shifts in resource availability are consistent with increases in fall passage duration. Moreover, we demonstrate the importance of evaluating changes across the entire phenological distribution, rather than just the mean, and stress this as an important consideration for future studies.
Highlights
The timing of animal migration—the seasonal movements between breeding and non-breeding sites—depends on reliable biotic and abiotic cues, such as food availability and temperature (Dingle, 1996)
Average stopover duration was significantly shorter in spring than in fall (1.98 days vs. 3.34 days), but did not vary across migration distances (Table 2; ANOVA, season F(1, 15) = 15.24, p = 0.001; distance F(2, 15) = 0.94, p = 0.42; distance × season F(2, 15) = 1.74, p = 0.21)
We did not find widespread systematic changes in stopover duration, we found strong variation in stopover duration among years for some species but not others, as evidenced by the variance terms in our hierarchical model
Summary
The timing of animal migration—the seasonal movements between breeding and non-breeding sites—depends on reliable biotic and abiotic cues, such as food availability and temperature (Dingle, 1996). Beluga whales postpone their autumn migration in response to delayed sea ice formation (Hauser et al, 2017), and potato leafhoppers have advanced their northbound migration due to earlier springtime (Baker, Venugopal & Lamp, 2015). These shifts can be inconsequential when animals track their resources (Bartomeus et al, 2011), they can lead to decreased fitness when animals become mismatched in space and time with their resources (Reed, Jenouvrier & Visser, 2012; Mayor et al, 2017; Kharouba et al, 2018). Information on phenological patterns and the mechanisms driving them is needed to understand the relationship between migration and environmental change and to develop effective conservation guidelines for imperiled species (Bowlin et al, 2010; Allen & Singh, 2016; Guérin et al, 2017)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
