Action Synopsis: Bird Conservation About Actions

Manually control or remove midstorey and ground-level vegetation (including mowing, chaining, cutting etc) in forests

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Of fifteen studies captured, one, a replicated controlled study from the USA, found higher bird species richness in areas with midstorey thinning, compared to control areas. One study from the USA found similar bird species richness in areas with mid- and understorey control, compared to other management types. A study from Canada found fewer species in treated sites than controls.
  • Seven studies from Europe and the USA found that total bird densities or those of some species or guilds were higher in areas with mid- or understorey management, compared to before management or to areas without management. Four of these studies used understorey removal as part of a wider management regime.
  • Five studies from the USA and Canada found that densities of some species were lower in areas with midi or understorey control, or that overall bird densities did not different between managed and unmanaged areas. Two of these studies investigated several interventions at once.
  • A replicated controlled study from the USA found similar survival for black-chinned hummingbirds in areas with understorey management, compared to areas with other interventions. Two replicated, controlled studies from Canada found higher nest survival in forests with removal of deciduous trees, compared to controls. A controlled study found that northern bobwhite chicks had greater foraging success in areas with cleared understorey vegetation compared to burned areas, but lower than under other managements.
  • A replicated, controlled study from the USA found that midstorey control did not appear to affect competition between species for nesting sites.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A before-and-after study at Minsmere reserve (151 ha), Suffolk, UK, in 1978-1988 (Burgess et al. 1990), investigated how European nightjars Caprimulgus europaeus responded to a series of management interventions, including the clearing of understory vegetation (1 m2 of heather Calluna vulgaris at the base of 1-3 m tall birch Betula spp. trees). This study is discussed in detail in ‘Clear or open patches in forests’.

    Study and other actions tested
  2. A series of before-and-after trials in four open pine forests in Texas, USA (Conner et al. 1995), found that red-cockaded woodpecker Picoides borealis populations increased at all four sites in the early 1990s after management, including mid- and under-storey thinning, was intensified in 1989. Vegetation was removed from 1988-1993 from 310-1450 ha of cluster areas each year and 30 of 39 (77%) new breeding clusters (i.e. breeding groups of woodpeckers) established over the study period were in areas with extensive mid- and under-storey thinning. This study is discussed in detail in ‘Provide artificial nesting sites’, ‘Translocate individuals’ and ‘Use prescribed burning’.

    Study and other actions tested
  3. A replicated study in 1991 in Ocala National Forest, an area of sand pine Pinus clausa scrub in Florida, USA (Greenberg et al. 1995), found similar densities and species richness of birds in areas that were clearcut and had the understorey mown, compared to areas that were clearcut and ‘brake-seeded’. This study is discussed in detail in ‘Clearcut and re-seed forests’.

    Study and other actions tested
  4. A replicated controlled study in 1992-3 in 33 pine-grassland stands in Ouachita National Forest, Arkansas, USA (Wilson et al. 1995), found that overall bird species richness and abundances were similar in stands with midstorey thinning, compared to control stands. This study is discussed in more detail in ‘Use prescribed burning’.

    Study and other actions tested
  5. A controlled, replicated study in 1990-1 in mixed loblolly pine Pinus taeda and shortleaf pine P. echinata forests in eastern Texas, USA (Conner et al. 1995), found no differences in occupancy rates of nest cavities by red-cockaded woodpeckers and southern flying squirrels Glaucomys volans between stands with thinned midstorey vegetation and control stands. This study is discussed in detail in ‘Reduce inter-specific competition for nest sites by modifying habitats to exclude competitor species’.

    Study and other actions tested
  6. A before-and-after study in mixed pine Pinus spp. forests in 1985-1996 in South Carolina, USA (Franzreb 1997) found that a population of red-cockaded woodpeckers increased following the clearance of midstorey vegetation amongst other interventions. The authors emphasise that hardwood midstorey control using cutting and herbicides and prescribed burning mimicked the natural fire regime and was essential to the success of the project. The results of this study are discussed in more detail in ‘Use prescribed burning’.

    Study and other actions tested
  7. A replicated, controlled study from May-July in 1992-1995 in three replicate plots of mixed forest in British Columbia, Canada (Easton & Martin 1998), found that bird abundance did not vary between sites with manual thinning of the mid- and understorey vegetation and controls, although thinned areas held fewer species than controls. Abundance of birds increased annually (no significant differences between the control and treated areas) due primarily to the significant increase in numbers of common species. Nesting success was higher in manually thinned areas (46%) than in controls (28%). Manual thinning reduced the volume of deciduous trees by 90-96 % by removing deciduous trees within 1 m of conifer seedlings, or that were 1 m taller than nearby conifers.

    Study and other actions tested
  8. A replicated, controlled study in 1993-1994 in oak Quercus-hickory Carya forests in the Ozark Mountains, Arkansas, USA (Rodewald & Smith 1998), found that one of the 14 species analysed was more abundant on plots from which understorey vegetation was removed, compared to those with both understorey and overstorey control. Three species were more abundant in plots with both over- and understorey control, whilst three tree-nesting species and ground- and shrub-nesting species were more abundant in control stands.

    Study and other actions tested
  9. A replicated study in 1995-1996 in pine savanna in South Carolina, USA (Krementz & Christie 1999), found that there were fewer scrub-successional species in stands managed for red-cockaded woodpeckers (including midstorey thinning) than in stands which were clearcut to remove non-native pines and replanted with longleaf pines Pinus palustris. This study is discussed in ‘Clearcut and re-seed forests’.

    Study and other actions tested
  10. A replicated study in four oak- hazel Corylus avellana woodlands (average size 5.3 ha) in 1996-1999 in Uppland and Åland, Sweden (Hansson 2001), found that sites that were subject to brush cutting and tree thinning (see ‘Thin trees within forests’) had similar numbers of migrant and breeding birds as grazed sites, and more than some abandoned sites. Sites under traditional management (cleared in spring, mown in mid-late summer and grazed in autumn) had higher abundances of migrant birds. This study is discussed in detail in ‘Employ grazing in natural and semi-natural habitats’.

    Study and other actions tested
  11. A replicated, randomised, controlled study from 1992-1995 in nine 11-22 year old regenerating coniferous plantations (22-47 ha) in British Columbia, Canada (Easton & Martin 2002), found that bird nesting density was lower, but success higher, in areas where deciduous trees and saplings were cut, compared to controls (40 nests and 28% success in treatment areas vs. 79 nests and 18% success in controls). Overall, density and success increased with increasing area of deciduous vegetation remnants. Three years after treatment removed 90-96% of deciduous vegetation, experimental areas had similar numbers of deciduous trees to controls. The effect also applying herbicide to the deciduous stumps is discussed in ‘Apply herbicide to mid- and understorey vegetation’.

    Study and other actions tested
  12. A controlled study within a loblolly pine plantation in Louisiana, USA, in 2003-2005 (Burke et al. 2008) found that northern bobwhite Colinus virginianus chicks were significantly more likely to successfully capture arthropods in areas of forest that were mown, compared to areas that were burned. However, success was significantly lower than in areas that were both burned and treated with imazapyr herbicide. There was only a very small difference between mown and control areas.

    Study and other actions tested
  13. A replicated, controlled study in riparian forest along the Middle Rio Grande, New Mexico, USA, in 2002-2004 (Smith et al. 2009), found an 18% increase in the number of black-chinned hummingbird Archilochus alexandri nests (from 114 to 134) across four sites where exotic shrubs and woody debris were removed and chipped before herbicide was applied to the root crowns of exotic species. However, an increase was only seen at one site, with the other three showing a 27% decline from 73 to 53 nests. This compared with an 8% increase at three sites with planting of native shrubs (see ‘Plant native shrubs following fuel reduction’) after fuel reduction and a 42% decrease at two sites where debris was burned (‘Use prescribed burning’). Across all fuel reduction treatments, nest survival was around 67% before fuel reduction and 43% after; in three control plots it remained similar (54 vs. 57%).

    Study and other actions tested
Please cite as:

Williams, D.R., Child, M.F., Dicks, L.V., Ockendon, N., Pople, R.G., Showler, D.A., Walsh, J.C., zu Ermgassen, E.K.H.J. & Sutherland, W.J. (2020) Bird Conservation. Pages 137-281 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.


Where has this evidence come from?

List of journals searched by synopsis

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Bird Conservation

This Action forms part of the Action Synopsis:

Bird Conservation
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What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

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