Bescherming tegen roofdieren: Zelforganisatie van visscholen en vogelzwermen Charlotte K. Hemelrijk Behavioural Ecology and Self-organisation Centre for Ecological and Evolutionary Studies University of Groningen The Netherlands

Fish in schools Under attack of a shark, the school forms a vacuole around the predator

Starlings Starling display above the roost in Utrecht: shape is highly variable (Brodie 1976, Carere et al 2009)

Safety In Numbers  Neil and Cullen (1974):  Larger schools-> lower succes of attack by predator Cuttlefish SquidPikePerch School sizes predators Capture/contact ratio

Advantages of grouping Protection against predation: – Dilution effect – Confusion effect – Early warning Energetics Finding food Finding mates How do individuals travel in groups?

‚Self-organisation‘ models Cognitively, simple behavioural rules of individuals Complex patterns at a group level Self-organisation ‚Understanding by building‘ (Pfeifer & Scheier, 1999)

Click to edit the outline text format Second Outline Level  Third Outline Level Fourth Outline Level  Fifth Outline Level  Sixth Outline Level  Seventh Outline Level  Eighth Outline Level  Ninth Outline LevelClick to edit Master text styles  Second level  Third level  Fourth level » Fifth level Click to edit the outline text format Second Outline Level  Third Outline Level Fourth Outline Level  Fifth Outline Level  Sixth Outline Level  Seventh Outline Level  Eighth Outline Level  Ninth Outline LevelClick to edit Master text styles  Second level  Third level  Fourth level » Fifth level Models Of Fish Schools Attraction/ Individuals move at a certain speed and react to others depending on their distance: Robust Group-level phenomenon: Coordinated schooling Huth & Wissel 1992, 1994; Reuter & Breckling, 1994; Couzin et al, 2002; Hemelrijk & Hildenbrandt,2008 Avoid Align Move to t=1, t=1, t=2 Avoidance Alignment Attraction Blind Angle

Model of fish school Moving individuals follow three rules: Avoidance Alignment Attraction Hemelrijk & Hildenbrandt, 2008, Ethology

School shape: Adaptive? Lower detectability, because predators attack at front (Bumann, Krause, Rubenstein1997) How organised ? Intention or self-organisation? (Kunz & Hemelrijk, 2003; 2005; Hemelrijk & Hildenbrandt 2008) Oblong

Oblong Shape as a side - effect Two and three dimensional models, several group sizes, group compositions, two cruise speeds High Low Movement Length Width → School Size Length/Width (Kunz & Hemelrijk 2003; Hemelrijk & Kunz 2004; Hemelrijk & Hildenbrandt,2008)

Start ball-shaped school N = Length Length, Width time (/s) Width Development of Shape Collision Avoidance → usually Slow Down & Move Inwards → Lengthening of Swarm

Supporting evidence Fast Hemelrijk & Hildenbrandt,2008; Kunz & Hemelrijk 2003 Density Larger schools shorter Nearest Neighbour Distance -> more frequent avoidance -> more oblong as a side-effect → School Size Length/Width → School Size

Empirical data of Mullets Corresponds to the patterns of the model! Hemelrijk, Reinders, Hildenbrandt, Stamhuis (2010) Ethology Length / Width # individuals Mean NND # individuals # individuals Stamhuis

Click to edit the outline text format Second Outline Level  Third Outline Level Fourth Outline Level  Fifth Outline Level  Sixth Outline Level  Seventh Outline Level  Eighth Outline Level  Ninth Outline LevelClick to edit Master text styles  Second level  Third level  Fourth level » Fifth level Click to edit the outline text format Second Outline Level  Third Outline Level Fourth Outline Level  Fifth Outline Level  Sixth Outline Level  Seventh Outline Level  Eighth Outline Level  Ninth Outline LevelClick to edit Master text styles  Second level  Third level  Fourth level » Fifth level Faster Schools Are School Size Fast Polarisation Slow Turning Less  Are More aligned (real fish: Viscido et al 2004)  Less Collision Avoidance  Falling Back  Less Oblong → School Size Fast Length / Width Slow Partridge et al, 1980, Saithe

Faster Schools: Denser Core, And Loser Tail, Because Less Fall Back Contrary to Breder (1959); Radakov (1973) School Size Density #Individuals / BLU3 Core School Tail Speed and Density Fast Slow Densest 10% 25% backwards core

Oblong form Arises as a side-effect of coordination!  Due to falling back to avoid collision Question Why are bird flocks seldom oblong and why are they variable in shape?

Variable flock shape in all contexts Explanation: ‘telepathy’ (Selous, 1930) This talk: by self-organisation? Starlings above roost Starlings avoiding a predator Dunlins travelling

Our model of starling flocks, StarDisplay Flocking model with: 1. local coordination (attraction, alignment, avoidance) 2. few interaction partners (6-7, Ballerini et al 2008) 3. flying following simplified aerodynamics and banking while turning (Norberg, 1990) 4. attraction to the sleeping site (roost) (Carere et al 2009) (Hildenbrandt, Carere, Hemelrijk, 2010) Behavioural Ecology traits of birds, starlings also in fish model Hildenbrandt

Number of interaction partners (Ballerini et al 2008ab) Fixed (topological)  6 à 7 in Rome Interact by:  attraction  alignment  avoidance Part of flock Ri

Stay over the Sleeping Site Horizontal attraction Vertical attraction sleeping area

Fixed Wing Aerodynamics Balanced constant level flight Lift (L) Drag (D) Thrust (T) Weight (W) Speed (v) Lift and drag Flight force Update Velocity Location FF

Parameters From Starlings ParameterDescriptionDefault value Δu Reaction time50 ms v0 Cruise speed10 m/s = 36 km/h M Mass80 g CL/CD Lift-drag coefficient3.3 Lo Default lift0.78 N D0,T0 Default drag, default thrust0.24 N nc Number of interaction partners6.5 rh Radius of max. separation (“hard sphere”)0.2 m RRoost Radius fo Roosting Area150 m (Hildenbrandt, Carere, Hemelrijk, 2010, Behavioural ecology)

Resemblance to real flocks? Qualitative Quantitative

Model StarDisplay: Flocking manouevres by self-organisation shapes distance and angle to nearest neighbours, orientation of shape and density distribution but the flock-volume is smaller than real (Hildenbrandt, Carere, Hemelrijk, 2010, Behavioural ecology) Resembles real flocks in (10 events) (Ballerini et al 2008) : Model StarDisplay

Model Videos Qualitative similarity to empirical data of Rome (Hildenbrandt, Carere, Hemelrijk, 2010, Behavioural ecology)

Similarity to empirical data of Rome: Two Flocks Merge Video Model Very similar also in duration (1s between pictures)

Question Why are bird flocks seldom oblong and why are they variable in shape?

Measure shape of flocks Movement Length Width parallel to movement direction (L/W) Like in fish schools I1 I2 I3 aspect ratios, I3/I2 based on bounding box parallel to longest dimension Flocks and schools are flat (I1= thickness) Movement

Results Default situation sleeping area N = 2000 Trajectory of center of gravity of flock

Shape When Turning Sharply changes due to changes in volume due to different behaviour inside and outside roost N = 2000 sleeping area Banking (dg) Banking NND Volume (m3) Volume > Time Aspect ratio I3/I1 I2/I1 I3/I2

Flocks of rock doves Similar to model, volume changes during turning Nearest Neighbour Distance Pomeroy & Heppner 1992 Turning Time

Due to rolling while turning Starlings Lift (before rolling) Lift (after rolling) Gravity Centripetal force Centrifugal force Model without rolling: Rolling induces loss of altitude Altitude (m) Time

Loss of altitude during turns in Rock Doves and Steppe Eagles Altitude Turning Pomeroy & Heppner 1992; Gillies et al 2008 Rock doves

Flock size Small flocks have relatively smaller changes in volume due to more similar condition (above roost, or outside) more global interaction in flock Large (2000) Small (200) Time ->

Deviations of global velocity during movement approx. straightforward temporary sub flocks

Correlation length ξ (m) Deviation of Velocity Larger flocks less synchronised Larger groups have greater sub-flocks of similar velocity deviation like in real starlings (Cavagna et al 2010) Gradient in model, 0.44, in flocks of real starlings 0.35 Flock size L (m)

Larger flocks: weaker global polarisation Larger sub flocks differ in direction more flock shape is more variable global local polarisation polarisation > N

High # interaction partners (50) N=2000 causes stable shape due to more global interaction, stronger synchronisation Volume (m3) Time 6.5 int p 50 int p more static volume stronger polarisation

More interaction partners (50 vs 6.5) More polarised more ‘synchronised’ -> less variable shape Local 6.5 nb Global 6.5 nb local50 nb Global 50 nb Polarisation

Causes of changes of volume Different headings in flock, asynchrony (above roost, or outside it) no attraction to the roost no banking high # interaction partners time no attraction to the roost no banking high # interact.- partners default, 2000 inds N=200, small flock size (Hemelrijk, Hildenbrandt, PlosONE, 2011)

Oblong I3/I individuals L W Oblong in forward direction Length/Width movement direction Usually oblong in all kinds of directions, seldom in the movement direction Shape of flock when turning mildly

Turning of a flock Model StarDisplay Rock doves (Pomeroy & Heppner, 1992) Change of orientation of longest dimension of flock to the movement direction, repositioning of individuals in the flock: due to low variability of speed

Variability of speed Frequency Speed deviation (% cruise speed) N = 2000 birds, 10 m/s is low

Low variability of speed in fish model During turn, shape changes relative to movement direction, not oblong in movement direction

Turns and oblong shape in fish-model due to high variability of speed: individuals remain at ‘fixed’ positions t = 1 movement direction t = 5 t = 10 Speed deviation % cruise speed Slow Fast

Advantage of lower variability of speed Saving energy Confusing predators Avoiding collisions -> traffic

Conclusion: Variable shape of travelling flocks is a consequence of – larger flock size – fewer interacting neighbours – a heterogeneous environment – rolling-movement during turning – but not due to higher variability of speed Lower variability (adjustability) of speed induces Change of shape relative to movement direction Repositioning during turns Testable hypotheses for empirical studies

Predation Ultrasound attack by fish on herring

Predation? Model

Models of Fish Schools under attack Predator approaches Prey has two tendencies – Stay with the school – Move away from predator Herd SplitFlash expansion Predator

Starlings under attack Carere et al 2009

Model with bird-flocks under attack

Financial support European framework, StarFlag NWO pilot grant Startup grant from Rosalind Franklin Fellowship at University of Groningen

Starlings Flock shapes  Many flock types also in absence of predators  Singletons and small flocks more often where predation is low  Higher catch rate where predation is low Carere et al 2009