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The University of Reading
Large-scale processes in a
migratory shorebird
The challenge for behavioural models
Ken Norris
Jenny Gill

Behavioural models
• Predict distribution,
demography and
abundance
• Behavioural dynamics
of resource
competition
• Depletion/interference

Limosa l. islandica
L. l. limosa

Behavioural models:
• Do models work at small-scales?
• If so, how do we move up to larger
scales?

Deben
Alde
Orwell
Stour
Colne
Blackwater

East coast prey
Proportion in diet
1
0.5
0
Oct Nov Dec Jan
Feb Mar
Bivalve density
Bivalves
Polychaetes
Unknown
3000
2000
1000
Deben
Orwell
Stour
0
Oct Nov Dec Jan Feb Mar

Resource depletion models
M
P = T h ∑ ( j − d c
) f j
+1/ a ′ ∑ f j
log( j / d c
)
d c
M
d c
Sutherland & Anderson 1993
• Resource abundance
• Functional response
Intake rate
Prey abundance

Resource depletion models
M
P = T h ∑ ( j − d c
) f j
+1/ a ′ ∑ f j
log( j / d c
)
d c
M
d c
Sutherland & Anderson 1993
• Resource abundance
• Functional response
Intake rate
3
2
1
0
0 2000 4000
Bivalve density

400
Patch
900
Mudflat
225
400
Observed godwit-days ha -1
100
25
0
0 25 100 225 400
196
100
Estuary
100
0
0 100 400 900
36
4
4 36 100 196
Predicted godwit-days ha -1
Gill et al. (2001) Proc. Roy. Soc. B

Intake rates and prey density predict:
• Current godwit distribution at smallscales
• Can we use the same framework to
describe distribution at larger spatial
scales?

100
Population index
80
60
40
20
0
70 74 78 82 86 90 94 98
Year
Source: Wetland Bird Survey

100
101-1000
> 1000
Black-tailed Godwit
Distribution

The buffer effect:
Disproportionate use of poor quality sites with
changing population size
No. in site
Good
Poor
Rate of increase
Population size
Initial population size

Buffer effects may:
1. Explain why spatial distribution changes
with population size
2. Be a key mechanism in population
regulation

Black-tailed godwit buffer effect pattern
Rate of population increase
7
5
3
1
-1
East coast
South coast
-3
0 1 2 3 4
Mean population index (1970-74)

Scaling-up:
• Are the same processes operating at
small and large-scales?
• If so, expect south coast sites to have
higher quality food resources

South coast prey
1
Hydrobia
Polychaetes
Bivalves
Crustaceans
Gastropods
Unknown
Proportion in diet
0.5
0
Oct Nov Dec Jan Feb Mar

South coast switch to
freshwater meadows
Density
1500
1000
500
Annelida
Coleoptera
Diptera
Chironomidae
0
Oct Nov Dec Jan Feb Mar

Seasonal intake rates
Intake Rate (g/sec)
0.0006
0.0004
0.0002
South estuaries
East estuaries
South freshwater
0
Oct Nov Dec Jan Feb Mar

Is intake rate related to rate of
population increase ?
Rate of population increase
6
4
2
0
-2
South coast
East coast
0 .2 .4 .6
Mean spring intake rate (mg/sec)
Gill et al. (2001) Nature

Buffer effects may:
1. Explain why spatial distribution changes
with population size
2. Be a key mechanism in population
regulation

Since 1993, >800 individually
marked
(c. 1-2% of population)
70-90% resighted
Annual survival rate
87%
94%

Does winter ecology
influence breeding
season processes?

April Isotherms 1961-1990
*
*
= Arrival sites
* * *
* * *
*

6
4
2
0
Arrival times in Iceland
Wintering Location
South coast
East coast
Frequency
21.4.00
23.4.00
25.4.00
27.4.00
29.4.00
1.5.00
3.5.00
5.5.00
7.5.00
9.5.00
11.5.00

Is arrival date related to rate of
population increase ?
10
South coast
East coast
Arrival date
8
6
4
2
0
-2 0 2 4 6
Rate of population increase

0.8
Proportion of pairs successful
0.6
0.4
0.2
0.0
-22 -20 -18 -16 -14 -12
Fresh
East
coast
South
coast
δ13C
Portugal
Saline

Buffer effects may:
1. Explain why spatial distribution changes
with population size
2. Be a key mechanism in population
regulation

The Challenge –
Behavioural models at large-scales?
1. What are the behavioural
dynamics operating at largescales?
2. Need to understand ‘carry over’
effects