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Spreeuwen, hoe zwermen ze?

Prof. Dr. Charlotte K. Hemelrijk Behavioral Ecology and Self-organization

Centre for Ecological and Evolutionary Studies University of Groningen

The Netherlands

Beautiful movements

Youtube Hoograven

‘Dancing’ above the sleeping site

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Advantages of complex movements

• Protection against predation:

– Confusion effect

How do individuals move collectively?

Agitation wave

Complex flocking behaviour

• Telepathy (Selous, 1931)

• Selforganisation

• Interactions of starlings are local,

7 neighbours (Ballerini et al 2008)

• Models of self-organised moving groups • schools of fish (Couzin et al, 2002;Hensor et al 2005;

Hemelrijk et al 2011)

Starling flocks by self-organisation?

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‚Understanding by building‘

Rules of local interaction

Complex patterns of the group

Self-organization

Pfeifer and Scheier 1999

Hypotheses for empirical studies

Model of fish school

Shape is oblong,

but bird flocks have all kinds of shapes

Individuals move and

interact with neighbours:

• attraction

• alignment

• avoidance:

through slowing down

Hemelrijk & Hildenbrandt, 2008, Ethology

(Katz et al 2011;

Herbert-Read et al 2011)

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Starling flocks

slaapplaats

- coordination (attraction, alignment, avoidance)

- with 7 closest neighbours

- staying above sleeping site

Hildenbrandt

Model with

Hildenbrandt, Carere, Hemelrijk, 2010, Behav Eco

Wrong, not like starlings

What is missing?

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Flying behaviour!

Spreeuwen

Lift (before rolling)

Lift (after rolling)

Gravity

Centripetal

force Centrifugal

force

Flying:10 m/s = 36 km/h

Banking while turning

Hildenbrandt, Carere, Hemelrijk, 2010, Behav Eco

Model StarDisplay with banking

Resembles empirical data in shape, orientation, internal structure

Hildenbrandt, Carere, Hemelrijk, 2010, Behav Eco

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Variation of shape

• Turning behavior

– Fixed speed

– Asynchrony

• Large flock size

• Low number of interaction partners

Hemelrijk, Hildenbrandt, 2011, PLOS ONE

Model StarDisplay

wide

oblong

Rock doves

oblong

wide

Turning causes changes of shape

Flying: Low variation of speed Hemelrijk, Hildenbrandt, 2011, PLOS ONE

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Asynchrony during turning

Causes shape changes due to temporary compression Resembles Rock doves (Pomeroy and Heppner,1992)

N = 2000

NND V

olu

me

(m3 )

Volume

0 10 20 30 40 50 60 -> Time turn turn

Large flock size, 20.000 individuals

Complex flock shape

Hemelrijk, Hildenbrandt, 2011, 2012, PLOS ONE, InterfaceFocus

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Low number of interaction partners

-> flock members are less aligned and further apart

-> shape is complex and variable

High number of neighbours (50) -> more synchronous

Low (6 or 7) -> asynchronous

Variation of flock shapes in birds

Due to

– Biophysics of flying behaviour:

• Low variability of speed (-> change of shape)

• Banking in turn (-> loss of altitude)

– Asynchrony of behaviour within flock:

• Turning,

• Low number of interaction partners,

• High number of flock members

Hemelrijk, Hildenbrandt, 2011, PLOS ONE

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Agitation wave in starling flock

Waves reduce capture succes

Procaccini at el 2011, Animal Behaviour

What behaviour underlies it? Flocks are too far away to observe this. Density wave or orientation wave ?

Change in visible wing surface

Maximal

Minimal projected area Zig-manoeuvre

Orientation wave?

Hemelrijk, van Zuidam, Hildenbrandt, under review

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Agitation waves

• ‘Wave of orientation’

– In dunlins (Potts 1984), white belly, brown back

– In anchovies (Radakov 1973; Gerlotto et al. 2006a) silvery belly, dark back

• ‘Density wave’ in herring (Axelsen et al. 2001)

What escape manoeuvre is used by starlings?

Study in model

Extensions to Stardisplay:

• Location of predator attack: escape manoeuvre

• Escape manoeuvre (Rüdebeck 1950, 1951)

– Repeated by close neighbours

• Two manoevres – Speeding-up-forward into the flock (-> density wave)

– A ‘Zigzag’ like movement (-> orientation wave)

Hemelrijk, van Zuidam, Hildenbrandt, under review

Zigzag escape

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Repeated speeding up manoeuvre

No visible agitation wave

Insufficient contrast in density or density-changes too slow?

Repeated Zig (Rolling)

Observable agitation wave Wave speed as in empirical data

Agitation wave is an orientation wave rather than density wave

Hemelrijk, van Zuidam, Hildenbrandt, under review

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‚Understanding by building‘

Rules of local interaction

Complex patterns of the group

Self-organization

Pfeifer and Scheier 1999

Hypotheses for empirical studies

Thanks

• Collaborators

– Hanno Hildenbrandt (scientific programmer)

– Claudio Carere (empirical data)

– Lars van Zuidam (master student)

• Grants

– EU grant StarFlag

– Pilot grant NWO