2022 Bird Study: Notes

Blackburnian Warbler
Photo by William H. Majoros, on Wikipedia

Forest degradation drives widespread avian habitat and population declines
by Matthew G. Betts et all, 2022 in Nature Ecology & Evolution. The full text is publicly available.


In many regions of the world, forest management has reduced old forest and simplified forest structure and composition. We hypothesized that such forest degradation has resulted in long-term habitat loss for forest-associated bird species of eastern Canada (130,017 km2) which, in turn, has caused bird-population declines. Despite little change in overall forest cover, we found substantial reductions in old forest as a result of frequent clear-cutting and a broad-scale transformation to intensified forestry. Back-cast species distribution models revealed that breeding habitat loss occurred for 66% of the 54 most common species from 1985 to 2020 and was strongly associated with reduction in old age classes. Using a long-term, independent data-set, we found that habitat amount predicted population size for 94% of species, and habitat loss was associated with popula- tion declines for old-forest species. Forest degradation may therefore be a primary cause of biodiversity decline in managed forest landscapes.


These ‘notes’ are intended to provide a summary of the highlights from the paper for non-technical readers. They are comprised of direct extracts of text from the scientific paper; text that is not directly from the paper is given in square parentheses [ ].  Highlighting is inserted to facilitate a quick read on a webpage. Also see Press Reports which include an extended interview with the lead author.

[Key Background]
– most conservation policies have focused on reducing deforestation (that is, permanent conversion to another land-cover type
– Forest degradation is also expected to be a key driver of biodiversity decline and is a com- ponent of broad-scale biodiversity agreements…However, forest degradation has been much more challenging to measure
– Forest degradation is defined as the reduction or loss of biological complexity in forests
due to harvesting, managed forests tend to be younger than those under a natural disturbance regime with potential implications for species associated with mature or old-growth forests.
…managers increasingly convert native forests to plantation…plantations tend to be comprised of only one or two tree species, and thinning is used to shift composition towards merchantable species

[Key Hypothesis]  breeding habitat loss and resultant population declines are driven by forest degradation

[From Methods]

[Area of Study: The Maritime provinces – New Brunswick, Nova Scotia, Prince Edward Island]
[Use of ‘Citizen Science Info] We used MBBA bird point counts (collected 2006–2011) to build species distribution (habitat) models, and we used long-term BBS routes (N = 90) to test whether changes in habitat in landscapes surrounding these routes successfully predicted longterm population trends in 54 species of forest birds. [MBBA is the Maritime Breeding Bird Atlas. “A total of 1,389 citizen scientists dedicated their time and effort to the (2nd) atlas project.”]

The hypothesis that breeding habitat loss and resultant population declines are driven by forest degradation remains largely untested…for methodological reasons.
…we navigated these previous obstacles….We used Landsat Thematic Mapper (TM) reflectance bands as predictor variables in species distribution models (SDMs)… [they examined] correlations between habitat amount and bird abundance over the 1985–2019 period as quantified in an independent dataset, the North American Breeding Bird Survey19

[Key points in the discussion (including summary of results)]

Overall, our results indicate that forest degradation has led to habitat declines for the majority of forest bird species with negative consequences for bird populations, particularly species associated with older forest. Forest changes include conversion from mixed-species forests to single-species conifer-dominated plantations or thin- nings (Fig. 1c,d,e) and clear-cutting old forests without equivalent regrowth into old age classes

For several species, rates of population decline seemed to outpace rates of habitat decline (compare x and y axes in Fig. 4a). For instance, Blackburnian warbler populations have experienced an ~70% decline over 35 years (4.5% per year; Fig. 6b), but only 33% of habitat has been lost.

Our results do not preclude the effects of wintering ground habitat loss, climate change, mortality on migration or contaminants. Population declines in species associated with regenerating forest are particularly cryptic because habitat amount for these species tends to be increasing. One hypothesis is that populations of some species that prefer early seral stages, despite having potentially more habitat, are declining due to climate change over the past three-and-a-half decades (~1 °C increase over 30 years)

It is well established that large-scale intensive forest-management practices in this region have resulted in substantial increases in single-species tree plantations. In areas that have not been planted, ingrowth of shade-intolerant hardwoods and balsam fir (Abies balsamea) predominate; these replace original shade-tolerant deciduous and coniferous species (Fig. 1a) and are unlikely to be succeeded by shade-tolerant species given cur- rent short harvest rotations.

If maintaining non-declining populations of forest birds is the goal, conservation measures that halt the alteration of habitat, particularly in diverse, older forests, will be necessary.

Of course, this may come at the expense of wood production but potentially less so with forest-landscape zoning that maintains reserves, ecological forestry and spatially limited intensive management.

Species cited by Betts (2022) in immature (I), Regenerating (R) and Mature (M) forest,