Research Spotlight: Variable Retention and Migratory Birds

Isabelle Lebeuf-Taylor

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Research Spotlight: Variable Retention and Migratory Birds

Summary of how implementing variable retention in harvest areas can increase bird diversity.

Paul Reeves

This section is a summary of results presented in Lebeuf-Taylor et al. 2025^[1].

Retention patches within regenerating harvest areas introduce structural and compositional heterogeneity that is beneficial for wildlife. This study explored how variable retention in regenerating harvests may speed up forest bird use of harvested areas. Results show that any amount of retention is valuable for some songbirds, but benefits take a minimum of 10 years to come into effect.

Introduction

Natural disturbances in the boreal forest—such as wildfires, insect outbreaks, and wind events—create a patchy mosaic of forests with different ages and structural diversity, including variations in canopy height, tree species, and deadwood.

This natural variation supports biodiversity by providing a range of habitats for different species.
In contrast, harvested stands often create younger, more uniform forests without the same habitat diversity as naturally disturbed forests of similar age, reducing habitat availability for bird species that depend on older or more complex forest conditions.
To mitigate this loss of forest heterogeneity and to promote wildlife use of harvested areas, foresters implement variable retention in harvested stands. This entails retaining standing live trees singly or in patches of various sizes within the harvest areas. Over time, these retained trees remain standing, become snags, or fall, adding mature forest characteristics, like structural complexity and variety of tree age, in otherwise even-aged stands.
Harvested areas with retention have been shown to attract a greater diversity of forest birds than stands without retention^[2]. However, it is unknown how the shape and size of retention patches benefit biodiversity, including birds.
In addition, retention patches in Alberta are often smaller than those tested in previous research and it remains unclear whether they provide the same value to birds.

            This section highlights research examining how six forest songbird species respond to the size and shape of small retention patches within regenerating harvest areas in Alberta’s boreal and foothills forests.
        

Forest songbirds, like the Yellow-rumped Warbler, have different habitat requirements for nesting and foraging.

Features like canopy cover, snag density, and forest patch size influence how suitable retention patches are as habitat for different forest birds.

The presence and diversity of bird species help indicate how effectively forest retention patches provide habitat within managed landscapes.

Al-Pac

More About Retention

Provincial rules require foresters to retain up to 5% of harvest areas as live trees at the Forest Management Unit level^[3]. This means:

Individual harvested stands can have a range of retention—from no trees, to a few scattered trees, to patches several hectares in size—with no minimum size requirement.
Retained trees can be grouped in patches or dispersed, and must represent the composition of the original forest.
The patches must be isolated and not connected to riparian buffers or large unharvested forests.

As a result, the rules are flexibly applied, leading to a variety of patch shapes and sizes. Understanding how birds respond to these retained patches helps evaluate whether current retention practices effectively support wildlife habitat in managed forests.

Methods

The purpose of this research was to determine whether small-scale retention attracts songbirds to regenerating harvest areas.

Over three years (late May to early July), birds were surveyed at 392 harvest sites across Alberta’s boreal and foothills forests to compare bird abundance in harvested areas with and without retained trees.
Bird numbers were recorded at 246 retention patches and 146 harvested stands without retention, ranging in age from 1 to 22 years.
Retention patches, which varied widely in size—from a single tree up to 1.2 hectares (0.1% to 5% of the harvest block)—occurred in a range of shapes and were located 300 m to 1 km away from undisturbed forest.

**Map of Study Sites.** Map of study sites in forested regions of Alberta where birds were sampled in regenerating harvest areas with retention patches.

More About the species

Six forest songbirds were used as indicators in this study.

These six species have different forest habitat preferences, nesting and foraging behaviours, and vary in their tolerances to forest harvesting. These species are not strictly interior forest or old forest specialists, and have been documented using regenerating harvests with varying amounts of tree retention. They include:

Martha Marks

Olive-sided Flycatcher

Nest in mature coniferous forests.
Perch at forest edges where they catch insects on the fly.

jelofson

Red-eyed Vireo

Associated with mid-aged deciduous or mixedwood forests in Alberta.
Nest in the midstory.

Parkland Pictures

Yellow-rumped Warbler

Nest in mature coniferous trees but can live in a variety of forest types.
Can change their foraging behaviour in harvested stands to feed in open areas^[4].

jelofson

Tennessee Warbler

Breed in deciduous, mixedwood, and White Spruce forests, and are associated with shrubby open forests.
Nest along forest edges near open areas.

jelofson

White-throated Sparrow

Found in young forested stands or at forest edges.
Forage in dense understory and nest close to the ground.

Chris Hill

Ruby-crowned Kinglet

Associated with older White Spruce stands.
Nest high in the forest canopy.

Key Results

Bird Responses to Retention Patch Characteristics.

This research found that species respond differently to retention patches, and that these responses shift over time. Overall, birds are observed at areas with retained live trees more quickly than those without. However, patch shape and size influenced how the retained areas were used.

Click on a tab to explore research highlights.

Retention Patch Presence

Any amount of retention anywhere in the harvest area is beneficial for some of the indicator species, but not all.

In the first 1 to 10 years after harvesting, bird abundance was low for all species at sites, whether or not retention patches were present.
Three of the six bird species—Tennessee Warbler, White-throated Sparrow, and Red-eyed Vireo—recolonized regenerating harvests sooner than harvest areas with no retention, with effects starting around 10 years post-harvest. By 22 years post-harvest, these three species had 2- or 3-fold increases in abundance in harvest areas with retention compared to those without. Without retention, numbers of Tennessee Warbler and Red-eyed Vireo had barely increased after 22 years.
Olive-sided Flycatcher and Ruby-crowned Kinglet were initially present in low numbers at both types of harvest sites and continued to decrease in abundance over time.
Yellow-rumped Warbler abundance increased over time, regardless of retention presence.
For all species, retention patches further from forested edges were used less than those closer to undisturbed forests. However, White-throated Sparrow, Red-eyed Vireo, and Tennessee Warbler were still more abundant in sites with retention regardless of distance, compared to harvest areas with no retention.

In harvest retention sites...

Image Credit: jelofson

Tennessee Warbler was 2-3 times more abundant.

Olive-sided Flycatcher abundance decreased over time.

Image Credit: Al-Pac

Patches closer to intact forest were used more often.

Retention Patch Size

Patch size alone had limited effects on bird abundance.

White-throated Sparrow, an edge-tolerant species, was the only species to respond strongly to patch size, preferring smaller patches.
For all other species, patch size did not affect whether birds used the patch or not, indicating that size matters less than other factors, like the shape of the patch or how close it was to unharvested forest.

Image Credit: Ashley Hillman

White-throated Sparrow prefer smaller retention patches.

Retention Patch Shape

The shape of retention patches was more important than patch area for most species, with species' responses linked to their habitat preferences and behaviour.

Edge-associated species—Olive-sided Flycatcher and Yellow-rumped Warbler—preferred retention patches with a lot of edge habitat in relation to patch area, i.e., high edge-to-area ratios.
Ruby-crowned Kinglet, a species more often associated with older forests, preferred compact patches with the smallest amount of edge, typical of larger, rounder patches. However, they did not increase in abundance over time in regenerating harvest areas, regardless of patch size, indicating patch sizes studied were not large enough to provide suitable habitat conditions for this mature forest species.
Red-eyed Vireo abundance—a species that prefers to nest away from forest edges—was also higher when retention patches were larger, with less edge (small edge to area ratio).
Tennessee Warbler and White-throated Sparrow showed no preference for retention patch shape.

Image Credit: jelofson

Yellow-rumped Warbler prefer patches with more edge.

Image Credit: Shutterstock

Retention patches did not provide suitable habitat for Ruby-crowned Kinglet.

Image Credit: Lorelei Galardi

Red-eyed Vireo prefer larger patches with less edge.

Management Implications

To maximize the benefit of retention patches for a range of species, management considerations include:

Maintaining a mix of retention patch sizes.
Placing retention patches near larger unharvested forest blocks, which increase bird abundance and use.
Distributing multiple smaller patches across harvest areas to improve habitat connectivity and reduce travel distances for birds.
Retaining some large, older forest remnants to provide habitat for mature-forest specialists.

Until longer-term outcomes are known, a balanced approach of mixing small patches for connectivity with larger remnant forests offers the best strategy for maintaining bird diversity in managed landscapes.

References

1.

Lebeuf-Taylor, I., E. Knight, E. Bayne. 2025. Improving Bird Abundance Estimates in Harvested Forests with Retention by Limiting Detection Radius through Sound Truncation. Ornithological Applications 127: 1–13. Availabe at: https://academic.oup.com/condor/article/doi/10.1093/ornithapp/duae055/7824177.

2.

Fedrowitz, K. et al. 2014. Can Forestry Help Conserve Biodiversity? A Meta‐analysis. Journal of Applied Ecology 51: 1669–79. Available at: https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2664.12289.

3.

Government of Alberta. 2024. Alberta Timber Harvest Planning and Operating Ground Rules. Available at: https://open.alberta.ca/publications/timber-harvest-planning-and-ogr-2024.

4.

Franzreb, K.E. 1978. Tree Species Used by Birds in Logged and Unlogged Mixed-Coniferous Forests. Wilson Bulletin 90(2): 221–38. Available at: https://digitalcommons.usf.edu/cgi/viewcontent.cgi?article=8101&context=wilson_bulletin.

Contributor

Isabelle Lebeuf-Taylor is a PhD student at the University of Alberta in Dr. Erin Bayne’s lab.

Her research interests lie in improving methods of linking bird populations to changing landscapes for more effective conservation.

            If you have questions about this research, please get in touch: lebeufta@ualberta.ca
        

This research was supported by Alberta-Pacific Forest Industries Inc. through a Mitacs Accelerate grant (Number IT34177), the Forest Research Improvement Alliance of Alberta (FRIAA), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Northern Scientific Training Program (NSTP), and the University of Alberta Northern Research Awards (UANRA).

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