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

Outline• Intro

– BITH and SWTH– NB context– niches

• Part 1: Nest site– nest patch vegetation

• Part 2: Diet– trophic niche width– SIA

• Summary

3

Intro: study species

Catharus bicknelli = BITHCatharus ustulatus = SWTH

4

Bicknell’s Thrush

• ‘Specialist’

• Stunted, dense balsam fir and spruce

• High elevation

• Endemic to NE NA

5

Swainson’s Thrush

• ‘generalist’• Found in many forest types• abundant

6

Study Site

7

NB highlands

• Heavily managed for timber/pulpwood

• Lots of balsam fir

• BITH in regenerating clearcuts - recently colonized?

8

Niche separationSabo 1980: New Hampshire

montane bird community

SWTHBITH

9

Niche separation

Do BITH and SWTH nest in similar locations?

Do BITH and SWTH show overlapping dietary niche?

Is BITH really a ‘specialist’?

10

Part 1: Nest site

11

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

12

Nest patch analyses• Nest patch data biologically meaningful

variables

– Snags

– % balsam fir

– Small trees

– Hardwoods

– Total density

13

Nest patch composition

14

BITH vs. SWTH

15

BITH and SWTH nests

Non-nest patch

16

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

small scale

17

Part 2: Diet• Do they consume different foods?

18

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

19

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

15N/14N15N/14N

15N/14N

15N/14N

14N

14N

14N

20

Isotope timelines

bone muscle

whole blood/RBC

plasma

liver

feather

Long time Short time

feces

21

Isotopic nicheBITH?SWTH

Bearhop et al. 2004

22

Collecting isotope samples

23

When does blood reflect local diet?

D = H2/H1

• Expected value: -83‰ + fractionation

24

D throws a wrench in the plan

BITH SWTH

20 June

25

SWTH: Specialist or generalist?

26

BITH: Specialist or generalist?

27

Summary

Nest patch

Diet

Veryspecialized

Verygeneralized

Veryspecialized

Verygeneralized

SWTHBITH

28

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?

29

Future work

• Nest patch characteristics- How flexible are they?

• Diet throughout the season

• Could blood tell us where BITH or SWTH came from?

30

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

31

Questions?

SWTH bad hair day.