AUTHOR BLOG: How Canada Warblers Keep Up with the Joneses

Anjolene Hunt

Linked paper: Forestry and conspecifics influence Canada Warbler (Cardellina canadensis) habitat use and reproductive activity in boreal Alberta, Canada by A.R. Hunt, E.M. Bayne, and S. Haché, The Condor: Ornithological Applications 119:4, November 2017.

YMC_2616

Photo credit: Torin Heavyside

Could a behavioral phenomenon, like the tendency of birds to live near neighbors, change the way we think about the effects of habitat disturbance? Results from new paper in The Condor: Ornithological Applications suggest that for Canada Warblers, this may be the case.

These iconic, brightly colored boreal songbirds are declining in number. Habitat loss and degradation are likely the main culprits. While it is true that forestry changes the landscape across much of the Canada Warbler breeding range, it remains contentious whether this results in habitat loss or whether they will use and thrive in regenerating postharvest areas. We suspected this discrepancy in our understanding of forestry effects could be explained by social factors influencing how Canada Warblers choose where to live.

Just as you might choose a neighborhood based on criteria like access to green space, birds also assess their physical environment when choosing where to live. These species-habitat relationships help biologists understand which areas to protect.

But the physical environment is far from the only cue birds use to choose a place to live, nor is it the only cue we use to understand why they live where they do. Just as you might rely on friends’ opinions of a neighborhood to snatch up a good place before it’s off the market, birds may choose to live near members of their own species. If another bird picks an area, chances are it has high quality food, nesting areas, and mates. However, if everybody makes the same decision, problems can arise as they pack into the same location. When habitat is disturbed, choosing to live near neighbors can result in overcrowding in the remaining undisturbed areas and potentially force some birds into the outskirts of high-quality neighborhoods.

After returning from a long spring migration, Canada Warblers have limited time to find a breeding site (they have the shortest breeding season of wood warblers in Alberta!), and strangely, researchers always seem to find Canada Warblers clustered together in some areas while absent from others. Based on this information, we suspected that where Canada Warblers choose to breed might depend on where their neighbors chose to settle. If this is the case, it could explain why they will live in postharvest areas, and in turn, how we perceive the effects of forestry.

Taking quads down muddy trails and bushwhacking through dense shrubbery to our destination, we surveyed for Canada Warblers in boreal Alberta, Canada. We used a recorded song to mimic an intruding male and lure territory owners into our nets. Once captured, we attached color bands to their legs to distinguish between individuals and followed them throughout the breeding season. We documented their space use in and around postharvest areas, how close they were to neighbors, and whether they found a female mate and raised young. See what a day in the life of Canada Warbler biologist looks like here.

Our results showed that numbers of male Canada Warblers were much lower in postharvest areas compared to unharvested forest. The few males that did live partially in postharvest were typically at the edge of nearby unharvested forest. Males were also more likely to live nearer to neighbors rather than spacing out. Hence, Canada Warblers in the boreal forest may not prefer to live in postharvest areas, but may live there as a side effect of trying to be near neighbors in unharvested forest.

But are Canada Warblers reaping the benefits of living near neighbors, or are they feeling the pressure to keep up with the Joneses? We found males living in areas with more neighbors were more likely to be “single” (without a female mate) than males living in areas with fewer neighbors. Overcrowding may lead to increased male-male competition for limited females. These rivalries also mean that males have to spend more time and energy defending their territory, leaving less time and energy to court females.

Our results suggest that protecting large stretches of unharvested forest near sites occupied by Canada Warblers will be important to provide enough habitat and prevent crowding effects. It also goes to show that appearances can be deceiving when it comes to the use of disturbed habitat and that the influence of social behavior should not be underestimated.

Follow Anjolene on Twitter: twitter.com/AnjoleneHunt

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