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History of deer colonization - Response of vegetation to deer - Response of animals to deer and squirrel - Deer biology
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Invertebrates - Songbirds - Nest predation and reproductive success

Response of songbirds to deer browsing

Aim & scope - Where & when - How - Main results - Conclusion - Scientific Publications

Aim and scope
Vegetation changes associated to deer browsing history are likely to affect habitat quality for songbirds by:
  • Reducing the availability of fruits and nectar (food resources)
  • Limiting the availaibility of nest sites
  • Indirectly reducing the availaibility of insects (food resources) (Response of invertebrates to deer browsing)

We hypothesised that these changes should affect the abundance and diversity of songbirds.
 
We used two methods to measure songbird abundance and diversity :
  • Point count sampling
  • Spot mapping
 sapsucker

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Where and when
In 1999, Sylvain Allombert and co-workers conducted point counts and mapping sessions on 7 islands with different deer-browsing histories:
  • 3 were deer-free: Low, Lost and South Low (no browsing impact on the vegetation)
  • 2 had been colonised by deer for less than 20 years: West Skedans and South Skedans
  • 2 had been colonised by deer for more than 50 years: West Limestone and Haswell (heavy browsing impact on the vegetation)
Laskeek Bay

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How

Point counts

Point counts were done in 2 forest interior plots on each island.

Point counts covered a 50 meter radius around the centre of the census plots. Birds seen and heard within this circle were recorded during exactly 20 minutes. Each point count station was visited 4 times, at 4 different dates.


Spot mapping

Spot mapping was conducted on the entire area of the 7 islands.
Spot mapping consisted in mapping individual territories.
Each survey of an island lasted an average of 4 hours, and each bird seen or heard was located on a map; its behaviour (singing, foraging...) was also recorded. Each island was visited 4 times.
counting birds

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Main Results

Variation in songbird abundance (number of individuals)
The following graph presents the mean number of individuals recorded per point count on 3 types of islands (no deer, deer, deer present for less than 20 years, deer present for more than 50 years).
abundance

Songbird abundance decreased with increasing length of deer-browsing history.

Variation in songbird diversity (number of species)

The following graph presents the mean number of species recorded per point count on the same islands.
Diversity

Songbird diversity decreases as well with an increasing deer-browsing impact.

Homogeneization and simplification of bird assemblages

These charts represent the proportion of the total number of pairs for several species, ranked from the most abundant to the less abundant.
 
assemblages

The abundance distributions is more "even" on islands with no or with a short browsing history than on islands with a long browsing history.

Effect of browsing history on bird habitat selection

Data from spot mapping and point counts show that the effect of browsing history on songbirds varies in relation to a species habitat selection.

scheme songbirds diversity
Drawings : J.A. Crosby

The species that decrease most in abundance when length of browsing history increases are those which are the most dependant on the ground and shrub vegetation layers for feeding and nesting (red arrows).
 
This is interpreted as the consequence of a reduction in habitat quality through decreased food resources and nest site availability and quality. A decrease in nest site quality can also mean a higher exposure to predators (Nest predation and reproductive success)

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Conclusion
Deer browsing seems to cause a cascade of direct and indirect effects that, via changes in the vegetation and the invertebrate fauna, will cause changes in song bird abundance and diversity. The species most negatively affected are those that depend most on the understory vegetation.

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Scientific Publications

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