A range-wide domino effect and resetting of the annual cycle in a migratory songbird

Abstract

Latitudinal differences in timing of breeding are well documented but how such differences carry over to influence timing of events in the annual cycle of migratory birds is not well understood. We examined geographical variation in timing of events throughout the year using light-level geolocator tracking data from 133 migratory tree swallows ( Tachycineta bicolor) originating from 12 North American breeding populations. A swallow's breeding latitude influenced timing of breeding, which then carried over to affect breeding ground departure. This resulted in subsequent effects on the arrival and departure schedules at autumn stopover locations and timing of arrival at non-breeding locations. This 'domino effect' between timing events was no longer apparent by the time individuals departed for spring migration. Our range-wide analysis demonstrates the lasting impact breeding latitude can have on migration schedules but also highlights how such timing relationships can reset when individuals reside at non-breeding sites for extended periods of time.

Keywords: Tachycineta bicolor; carry-over effects; life history; migration; timing of breeding; tree swallow.

Figure 1.

(a) Locations of 12 tree swallow (Tachycineta bicolor) breeding sites where geolocators were deployed and recovered (circles; n = 133 individuals), and non-breeding locations (triangles) are superimposed over the breeding (yellow) and non-breeding (pink) range (BirdLife International and NatureServe 2015). The breeding site coordinates and samples sizes within each latitude category (gradients of blue) are in the electronic supplementary material. (b) Timing of events within the annual cycle of tree swallows using light-logging geolocators originating from 12 breeding sites. The numbered circles represent the median timing of events by breeding latitude category (shown as the gradient of blue; y-axes). The orange dotted lines link events across different latitudes. The light blue arrows across each latitude show the direction of the annual cycle (x-axes). Figures designed by KAP Design.

Figure 2.

Timing of major events throughout the annual cycle of 133 tree swallows (Tachycineta bicolor) originating from 12 breeding sites. The shape and length of the violins are based on kernel density estimations of the distribution of the dates of each event within the annual cycle. Circles represent individual tree swallows with colour coding based on the breeding latitude category. (Online version in colour.)

Figure 3.

Relationships between timing of various events throughout the annual cycle in tree swallows (Tachycineta bicolor). (a) Departure from the non-breeding site interacts with non-breeding latitude to predict breeding site arrival date. (b) Breeding site arrival and breeding latitude positively predict the first egg date of tree swallows. (c) The fledge date and breeding latitude interact to predict breeding site departure date. (d) Breeding site departure date interacts with breeding latitude to positively affect first stopover site arrival date. (e) Arrival at the first stopover site and stopover latitude interact to influence stopover departure date. (f) First stopover departure date and the distance between the first stopover site and the non-breeding site interact to affect non-breeding arrival date. (b–f) The black dashed lines indicate the 1 : 1 line of the variables on the x-axis and y-axis. (a,c–f). The coloured lines represent the predicted quartiles (minimum, lower, median, upper and maximum) of the interaction and their associated range in values. (a–f) The colour-coded points represent individual tree swallows based on the predicted quartile categories. Dates are expressed as number of days past 1 January.