By Lakesha Smith
Have you ever wondered how birds know when it is time to migrate? They do not have a calendar or a watch which tells them when it is time to go. So how exactly do they know when it is time to leave Winnipeg and migrate down south? This is one of the questions I am currently trying to shed more light onto during my thesis for my master’s of science degree at the University of Manitoba.
Previous research on the topic of migratory timing has found migration to be determined by a combination of internal clock-like mechanisms, called endogenous rhythms, individual variability in timing depending on the individual, and environmental conditions such as day length. Both (2010) proposed an effect where nestlings exposed to longer day lengths in the nest developed earlier migration schedules. In other words, birds that are born earlier will migrate earlier than birds that are born later in the summer. Therefore, the aim of my study is to further investigate if the timing of migration in Purple Martins (Progne subis) is determined by the daylength nestling are exposed to while in the nest.
What are Purple Martins, you ask? Purple Martins are beautiful birds: the adult males have dark bluish-purple feathers from head to toe if caught in the right light. The females and young Purple Martins are a little less eye-catching with the bluish-purple feathers limited to their head and back and their chest colored buffy white. Purple Martins are aerial insectivores, in other words they catch and eat insects midair, and the eastern population nests colonially in human-made nest boxes. In summer their breeding grounds in the east range from Florida in the south to Alberta in the north, including Winnipeg. If you take a trip to FortWhyte Alive or Oak Hammock Marsh between May and August, you may just see them perched on the white houses located at both sites. In the winter, to escape the cold, Purple Martins migrate to warm and sunny South America, mainly Brazil.
The study sites where I conducted my research were FortWhyte Alive, Oak Hammock Marsh, and a private residence in Howden. I hypothesized that if the timing of migration is determined by the day length young birds (aka juveniles) encounter in the nest, then delaying hatch date will result in a delayed timing of migration. Delaying hatch date will cause the eggs to hatch later therefore they will be exposed to slightly shorter day lengths than birds that are not delayed.
Above: Purple Martins (submitted)
Above: Purple Martin Nest (submitted)
We, the Purple Martin team, manipulated the hatch date of 35 nests which were either control (1-day delay) and delayed nests (6-day delay) (using methods by Ouwehand et al., 2017). Once the eggs hatched and the nestlings were around 20 days old, they were given silver and purple bands used to identify the individual from a distance. Then one or two nestlings from each nest were randomly tagged with a radio-telemetry tag (see above photo of a juvenile with a tag). Radio-telemetry tags are very useful to track birds and other animals to see where they go during migration. The tag emits a radio frequency that is picked up by automated radio-telemetry receivers, which store the data. So, by using these tags I will know when the juvenile starts its fall migration, since the receiver will stop picking up the tags signal. Then in spring, when the birds hopefully return to Winnipeg, the receiver will pick up the signal of the tag so I will be able to determine when the arrival date is.
Preliminary data collected in fall show that the young Purple Martins from the control nests on average leave earlier than manipulated nests (see Figure 1). Figure 1 shows a boxplot of the fall departure dates comparing control juveniles against manipulated juveniles. These results are what I predicted would happen with Purple Martins that hatched earlier (control nests) exposed to longer day lengths starting migration before the martins that hatched later (manipulated nests).
Above: A boxplot comparing fall departure dates of control (1 day delayed) and manipulated (6 day delayed) juvenile Purple Martins. The pink circles represent control juveniles and the blue circles represent manipulated juveniles.
This research is important since it shows that the development of Purple Martins’ migration timing may be set initially by the day length they encounter while in the nest. If this is the case then in order for Purple Martins and other long-distance migrants to keep up with their changing environment, they will need to advance their egg laying date so that the juveniles will be born earlier, thus developing an earlier migration schedule. If Purple Martins are able to lay eggs earlier each year, then they might be able to keep up with their changing climate.
For more information on Purple Martins you can go to the Purple Martin Conservation Association website. If you have a large backyard (close to water) and a lot of patience, then I would encourage you to look into becoming a Purple Martin landlord.
I want to say a big thank you to the Connie Holland Bird Studies Fund for supporting this and other wonderful avian research! I also want to thank the wonderful members of the Purple Martin team last summer, Christophe Turcotte-van de Rydt, Katie Smith, Elliot Kinnear, Jennifer Graham and Jordyn Ojah, for all their help. Last but not least I want to thank my advisor Kevin Fraser for the guidance and support throughout the project.
Both, C. 2010. Flexibility of timing of avian migration to climate change masked by environmental constraints en route. Current Biology, 20: 243-248.
Ouwehand, J., C. Burger, & C. Both. 2017. Shifts in hatch dates do not provide pied flycatchers with a rapid ontogenetic route to adjust offspring time schedules to climate change. Functional Ecology, 31(11): 2087-2097.