1. This document discusses research into how starling flocks form complex flocking behaviors through self-organization and local interactions between individuals. 2. A computer model called StarDisplay was able to replicate the shapes, internal structures, and variations in shape of empirical starling flocks by including behaviors like banking while turning and differences in speed. 3. The model also showed that asynchronous behaviors during turning, large flock sizes, and fewer interaction partners between individuals could lead to more complex and variable flock shapes, matching observations of birds.

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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
Volume(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