Bescherming tegen roofdieren: Zelforganisatie van visscholen en vogelzwermen Charlotte K. Hemelrijk...

Bescherming tegen roofdieren:

Zelforganisatie van visscholen en vogelzwermen

Charlotte K. HemelrijkBehavioural Ecology and Self-organisation

Centre for Ecological and Evolutionary StudiesUniversity 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 Squid Pike Perch

School sizes1 6 20 1 6 20 1 6 201 6

predators

Ca

ptu

re/c

on

tact

ra

tio

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

LevelFourth 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


Level Third Outline


Sixth Outline 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

Wid

th

0 50 100 200 300 400 500

→ School Size

2

1.8

1.6

1.4

1.2

1

Leng

th/W

idth

(Kunz & Hemelrijk 2003; Hemelrijk & Kunz 2004; Hemelrijk & Hildenbrandt,2008)

Start ball-shaped school

N = 600

0 10 20 30 40 5012

14

16

18

20

22

24

26

t/s

leng

th w

idth

/ B

LU

length

width

26

24

22

20

18

16

14

12

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

De

nsi

ty

Larger schools shorter Nearest Neighbour Distance -> more frequent avoidance -> more oblong as a side-effect

0 50 100 200 300 400 500

→ School Size

2

1.8

1.6

1.4

1.2

1

Leng

th/W

idth

10 20 30 60 100 200 300 600 1000 20000

0.5

1

1.5

2

2.5

NN

DGroup Size 10 30 100 300 500→ School Size

Empirical data of Mullets

Corresponds to the patterns of the model!

Hemelrijk, Reinders, Hildenbrandt, Stamhuis (2010) Ethology

WIDE

School size (# individuals)

10 20 30 40 50 60

Leng

ht t

o W

idth

(BL

U)

0

2

4

6

8

10

Leng

th /

Wid

th

10 20 30 40 50 60 # individuals

NARROW

School size (# individuals)

0 10 20 30 40 50 60 70

Mea

n N

ND

(B

LU)

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

Mea

n N

ND

10 20 30 40 50 60 # individuals10 20 30 40 50 60 # individuals

Stamhuis


Level Third Outline


Sixth Outline Level





» Fifth level


Level Third Outline


Sixth Outline Level





» Fifth level

Faster Schools Are

10 100 2000School Size

1

0.995

0.99

0.985

FastPolarisation

Slow

Turning Less Are More aligned (real fish: Viscido et al 2004) Less Collision Avoidance Falling Back Less Oblong

10 30 100 300 2000→ School Size

Fast

Length / Width

2.5

1.5

1.0

Slow

Partridge et al, 1980, Saithe

Faster Schools: Denser Core, And Loser Tail,Because Less Fall BackContrary to Breder (1959); Radakov (1973)

0.6

0.5

0.4

0.3

0.2

0.1

0 10 20 30 60 100 200 300 600 1000 2000School Size

Density#Individuals / BLU3 Core

School

Tail

Speed and Density

Fast

Fast

Slow

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

iiii avoidalignattrSocial fffF

Stay over the Sleeping Site

Horizontal attractionVertical attraction

iii RoostSocialSteering fFF

sleeping area

Fixed Wing Aerodynamics

Balanced constant level flight

Lift (L)

Drag (D)Thrust(T)

Weight (W)

Speed (v)

mgv

v=L

v

v=L

20

2

020

2

LC

C=D

L

D

Lift and drag

gTDLF 0 miiFlighti

Flight force

1( ) ( )ii i steering Flight i

i i i

t + Δt = t + F t +F t Δtm

t+ Δt = t + t+ Δt Δt

v v

r r v

Update

Velocity

Location

FF

Parameters From Starlings

Parameter Description Default value

Δu Reaction time 50 ms

v0 Cruise speed 10 m/s = 36 km/h

M Mass 80 g

CL/CD Lift-drag coefficient 3.3

Lo Default lift 0.78 N

D0,T0 Default drag, default thrust 0.24 N

nc Number of interaction partners 6.5

rh Radius of max. separation (“hard sphere”) 0.2 m

RRoost Radius fo Roosting Area 150 m

(Hildenbrandt, Carere, Hemelrijk, 2010, Behavioural ecology)

Resemblance to real flocks?

QualitativeQuantitative

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


Resembles real flocks in (10 events) (Ballerini et al 2008) :

Model StarDisplay

Model

Videos

Qualitative similarity to empirical data of Rome


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

Wid

th

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 = 2000Trajectory 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

Ban

king

(dg

)

Banking

NND

Vol

ume

(m3) Volume

0 10 20 30 40 50 60-> Time

Asp

ect

ratio I3/I1

I2/I1 I3/I2

Flocks of rock doves

Similar to model, volume changes during turning

Nea

rest

Nei

ghbo

ur D

ista

nce

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

Alti

tude

(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 velocityduring movement approx. straightforward

temporary sub flocks

Corr

elati

on le

ngth

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

pola

risati

on

0 500 1500 2000 3000 4000 5000---> N

High # interaction partners (50)N=2000

causes stable shape due to more global interaction, stronger synchronisation

Vol

ume

(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

Pola

risati

on

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/I25

4.5

4

3.5

3

2.5

2

1.5

1

0.5

100

500

1000

2500 individuals

L

W

Oblong in forward directionLength/Width

movement direction

Usually oblong in all kinds of directions, seldom in the movement direction

Shape of flock when turning mildly

Turning of a flockModel 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

Fre

quen

cy

-100 -60 0 60 100Speed 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 = 5t = 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 Split Flash expansion

Predator

Starlings under attackCarere 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

Bescherming tegen roofdieren: Zelforganisatie van visscholen en vogelzwermen Charlotte K. Hemelrijk...

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