Simple rules guide dragonfly migration

Abstract

Every year billions of butterflies, dragonflies, moths and other insects migrate across continents, and considerable progress has been made in understanding population-level migratory phenomena. However, little is known about destinations and strategies of individual insects. We attached miniaturized radio transmitters (ca 300 mg) to the thoraxes of 14 individual dragonflies (common green darners, Anax junius) and followed them during their autumn migration for up to 12 days, using receiver-equipped Cessna airplanes and ground teams. Green darners exhibited distinct stopover and migration days. On average, they migrated every 2.9+/-0.3 days, and their average net advance was 58+/-11 km in 6.1+/-0.9 days (11.9+/-2.8 km d-1) in a generally southward direction (186+/-52 degrees). They migrated exclusively during the daytime, when wind speeds were less than 25 km h-1, regardless of wind direction, but only after two nights of successively lower temperatures (decrease of 2.1+/-0.6 degrees C in minimum temperature). The migratory patterns and apparent decision rules of green darners are strikingly similar to those proposed for songbirds, and may represent a general migration strategy for long-distance migration of organisms with high self-propelled flight speeds.

Figure 1

Attachment of radio transmitters and migration patterns during autumn migration in green darners. (a) Attachment of a 300 mg radio transmitter to the thorax of a green darner. (b) A green darner with transmitter shortly before take-off at Cape May Point (dark blue line at bottom of figure (c)) This individual shows the typically minor wing wear seen during our study, indicative of relatively recently emerged individuals. (c) Trajectories of migrating green darners along the northeastern seaboard of the USA (New York (NY) to Maryland (MD)). Different colours indicate different individuals, numbers depict days since tagging, numbers in brackets show maximum number of days individuals were tracked. Dotted lines indicate that it was unclear on which day individuals conducted their migratory flight. The dashed blue line indicates the most likely route of crossing Delaware bay. Shore lines are depicted by a thick black line, ocean by undulating lines. Pictures by Christian Ziegler.

Figure 2

Migratory pattern in green darners during autumn migration. (a) Overall migratory advances of green darners during our study (black lines show individual migratory vectors). The dashed arrow depicts the average advance and direction of our study population. (b) Frequency histogram showing average migratory distances of 13 of 14 tagged green darners separated into stopover days (less than 4 km directional movement, white bars, one individual was not observed migrating) and migration days (grey bars). (c) Frequency histogram showing average flight direction of individual green darners in relation to wind direction (determined as surface wind at the midpoint of their migration flight).