1. 1
Bicknell's and Swainson's thrushes in
regenerating New Brunswick clearcuts:
niche separation by nest site and diet
Emily A. McKinnon, Kevin C. Fraser,
Antony W. Diamond
University of New Brunswick, Fredericton

2. 2
Outline
• Intro
– BITH and SWTH
– NB context
– niches
• Part 1: Nest site
– nest patch vegetation
• Part 2: Diet
– trophic niche width
– SIA
• Summary

3. 3
Intro: study species
Catharus bicknelli = BITH
Catharus ustulatus = SWTH

4. 4
Bicknell’s Thrush
• ‘Specialist’
• Stunted, dense balsam fir and spruce
• High elevation
• Endemic to NE NA

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Swainson’s Thrush
• ‘generalist’
• Found in
many forest
types
• abundant

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Study Site

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NB highlands
• Heavily managed for timber/pulpwood
• Lots of balsam fir
• BITH in regenerating clearcuts -
recently colonized?

8. 8
Niche separation
Sabo 1980: New Hampshire
montane bird community
SWTHBITH

9. 9
Niche separation
Do BITH and SWTH nest in
similar locations?
Do BITH and SWTH show
overlapping dietary niche?
Is BITH really a ‘specialist’?

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Part 1: Nest site

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Nest patch characteristics
= non-nest site
= nest site
• 5-m radius
patch centered
on the nest
• non-nest
patches
selected 100m
away from nest
patch

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Nest patch analyses
• Nest patch data  biologically meaningful
variables
– Snags
– % balsam fir
– Small trees
– Hardwoods
– Total density

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Nest patch composition

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BITH vs. SWTH

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BITH and SWTH nests
Non-nest patch

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BITH and SWTH
• BITH are more selective than SWTH
• Differences in vegetation association on a
small scale

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Part 2: Diet
• Do they consume different foods?

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Dietary studies
• Morphology (Dilger 1956)
– BITH more ground forager, SWTH arboreal
• Foraging technique/location (Sabo 1980)
– BITH ground forager, SWTH generalist
• Stomach contents analysis (Beal 1918, Wallace 1939)
– BITH: Coleoptera, Hymenoptera, SWTH:especially Diptera

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‘New’ method to quantify diet
• Stable Isotope Analysis (SIA)
• Isotopic niche = trophic niche
15
N/14
N
15
N/14
N
15
N/14
N
15
N/14
N
14
N
14
N
14
N

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Isotope timelines
bone muscle
whole
blood/RBC
plasma
liver
feather
Long time Short time
feces

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Isotopic niche
BITH?SWTH
Bearhop et al. 2004

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Collecting isotope samples

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When does blood reflect local
diet?
∀δD = H2
/H1
• Expected
value: -83‰ +
fractionation

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δD throws a wrench in the plan
BITH SWTH
20 June

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SWTH: Specialist or generalist?

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BITH: Specialist or generalist?

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Summary
Nest patch
Diet
Very
specialized
Very
generalized
Very
specialized
Very
generalized
SWTH
BITH

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Summary
• BITH choose nest patches on a small
scale
• Suggests ability to nest in small patches?
• SWTH nest patches are less specific
• less affected by management?
• BITH - Type 2 generalists
• Individual specialization?
• SWTH - Type 1 generalists
• reduced competition with BITH?

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Future work
• Nest patch
characteristics- How
flexible are they?
• Diet throughout the
season
• Could blood tell us
where BITH or SWTH
came from?

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Acknowledgements
• Committee: Dr. D. MacLean, Dr. D. Keppie
• T. Vanbuskirk, B. Whittam, G. Campbell, C. Rimmer, K.
McFarland, A. Albert, R. Aracil, H. Askanas, K. Friars, M. Godin,
C. Wegenschimmel, Operations 1 Staff, Mark McLelland at the
Governor’s Lodge

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Questions?
SWTH bad hair day.