Migrating through a storm: how do songbirds respond to environmental pressures across the annual cycle?

This blog post is provided by Devin de Zwaan and tells the #StoryBehindThePaper for the paper “The relative influence of cross-seasonal and local weather effects on the breeding success of a migratory songbird“, which was recently published in the Journal of Animal Ecology.
Devin de Zwaan is a postdoctoral research fellow wrapping up contracts with the University of British Columbia (UBC) and Environment and Climate Change Canada (ECCC). His research focuses on the life-history strategies of birds living and breeding in extreme, fluctuating environments to understand the capacity for individuals, populations, and communities to respond to increasingly variable conditions under climate change. Much of Devin’s research centres around high mountain systems, which are a particular favourite for both research and exploring.
Horned Lark (Eremophila alpestris) nestlings (Photo credit: Devin de Zwaan)

One early spring day the sun is shining and the snow that once covered the ground has finally melted. The next day a snowstorm plunges everything back into a winter-like state. The few birds that nested early are now faced with temperature challenges and resource limitations, as well as a choice: abandon the nest to invest in their own survival, or weather the storm. This is a typical scenario for birds breeding in alpine habitats, or any strongly seasonal environment. Nesting early is often associated with greater reproductive success, potentially linked to reduced predation risk or greater resource availability during peak nestling growth. However, nesting early is also a gamble, where fluctuating, early-season conditions can lead to nest failure. The ability to breed early depends on the severity of the prevailing environment, as well as conditions in the non-breeding season that carry-over to impact the body condition, energetic state, and behaviour of parental birds. For migrating species, carry-over effects may arise at any point along the migration route. We generally do not know when and where environmental conditions are most influential across the annual cycle, but this information is important if we are to understand the future of many populations.  

An unexpected spring snowstorm in alpine habitat (Photo credit: Devin de Zwaan)

In a recent paper in the Journal of Animal Ecology, we sought to identify the time and place in which weather conditions most impacted reproductive timing and success. We combined 12 years of breeding data with 3 years of migration data collected from light-level geolocators for an alpine-breeding population of horned lark (Eremophila alpestris). Geolocators are small tracking devices carried like a backpack, allowing us to estimate locations throughout the year based on day length and the timing of sunrise. Knowing roughly where and when individuals were during migration, we extracted temperature and precipitation variables from local weather stations along their route. We then tested the influence of weather variables at winter, stopover, and breeding sites on clutch initiation date (onset of breeding) and age at fledging (time within the nest). Finally, we addressed how delays in clutch initiation date and age at fledging may combine to influence nest success, given that predation risk increases with time within the nest.

An adult male horned lark with a light-level geolocator attached using a leg-loop harness (Photo credit: Devin de Zwaan)

We found that conditions on the wintering grounds generally did not influence clutch initiation or offspring development, indicating that even the effects of harsh winters do not cascade across the annual cycle. However, colder temperatures at spring stopover sites and the breeding site both delayed clutch initiation. The most influential periods were just before birds departed the stopover site and just prior to clutch initiation, and thus birds are responding to prevailing conditions that occur closer to breeding in time and distance. Interestingly, even though horned larks are ground-nesting birds, snow depth did not influence clutch initiation, which may be because females begin nesting as soon as small patches become exposed on wind-swept ridges. Additionally, age at fledging was delayed by extreme cold (days below 0°C) at the stopover site only, independent of clutch initiation date. In other words, birds that experienced severe conditions at stopover sites took longer to raise offspring and this was not a result of delayed onset of breeding, suggesting an underlying mechanism linked to energy availability and investment in parental care. Finally, later initiated nests and those with slower growing offspring were more likely to be preyed upon. This means that conditions prior to breeding that delayed clutch initiation and prolonged time within the nest combined to severely reduce the probability of nest success.

The location of the first four nests in a season, built in relatively small exposed patches on wind-swept ridges (Photo credit: Devin de Zwaan)

We demonstrate that environmental conditions across the annual cycle can have nuanced effects on migratory behaviour and subsequent breeding parameters, which can combine synergistically to influence reproductive success. Ultimately, parents that overcome time constraints or energy limitations and breed early experience pronounced fitness benefits. While our results suggest birds breeding in seasonal environments may have some capacity for flexibility in their breeding phenology, there is a limit, and the ability to respond to prevailing conditions depends on reliable environmental cues. Climate change is already associated with greater environmental variability and more severe weather events. For migratory birds that spend time in separate habitats over the course of an annual cycle, decoupled conditions and therefore environmental cues between non-breeding and breeding sites may make it increasingly challenging to track early season conditions and reproduce successfully. Understanding the capacity for birds to respond to variable environmental conditions will better allow us to predict the future for climate-sensitive populations.

A horned lark nest surrounded by snow after a severe cold snap (Photo credit: Devin de Zwaan)
Read the paper

Read the full paper here: de Zwaan, D. R., Drake, A., Camfield, A. F., MacDonald, E. C. & Martin, K. (2022). The relative influence of cross-seasonal and local weather effects on the breeding success of a migratory songbird. Journal of Animal Ecology, 00, 1– 13. https://doi.org/10.1111/1365-2656.13705

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