Bescherming tegen roofdieren: Zelforganisatie van visscholen en vogelzwermen

1. 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

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

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

4. Safety In Numbers Cuttlefish Squid Pike Perch predators Capture/contact ratio 1 6 20 1 6 20 1 6 1 6 20 School sizes • Neil and Cullen (1974): • Larger schools-> lower succes of attack by predator

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

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

7. Models Of Fish Schools Avoid Blind Angle Avoidance Align Alignment Attraction Move to t=1, t=1,t=2 Huth & Wissel 1992, 1994; Reuter & Breckling, 1994; Couzin et al, 2002; Hemelrijk & Hildenbrandt,2008 Individuals move at a certain speed and react to others depending on their distance: Attraction/ Robust Group-level phenomenon: Coordinated schooling

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

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

10. Oblong Shape 2 1.8 1.6 1.4 1.2 1 High Low Length/Width Movement 0 50 100 200 300 400 500 → School Size (Kunz & Hemelrijk 2003; Hemelrijk & Kunz 2004; Hemelrijk & Hildenbrandt,2008) Width Length Two and three dimensional models, several group sizes, group compositions, two cruise speeds as a side - effect

11. Length, Width 26 24 22 20 18 16 14 12 Length Width time (/s) Development of Shape Start ball-shaped school N = 600 Collision Avoidance → usually Slow Down & Move Inwards → Lengthening of Swarm

12. 2 1.8 1.6 1.4 1.2 1 Length/Width Density 0 50 100 200 300 400 500 → School Size Supporting evidence Hemelrijk & Hildenbrandt,2008; Kunz & Hemelrijk 2003 Fast 10 30 100 300 500 → School Size Larger schools shorter Nearest Neighbour Distance -> more frequent avoidance -> more oblong as a side-effect

13. Empirical data of Mullets Length / Width Mean NND 10 20 30 40 50 60 # individuals 10 20 30 40 50 60 # individuals 10 20 30 40 50 60 # individuals Hemelrijk, Reinders, Hildenbrandt, Stamhuis (2010) Ethology Stamhuis Corresponds to the patterns of the model!

14. Faster Schools Are Length / Width Polarisation Fast 2.5 1.5 1.0 1 0.995 0.99 0.985 Slow Slow Fast 10 100 2000 School Size 10 30 100 300 2000 → School Size Turning Less Are More aligned (real fish: Viscido et al 2004) • Less Collision Avoidance Falling Back  Less Oblong Partridge et al, 1980, Saithe

15. Speed and Density Density #Individuals / BLU3 0.6 0.5 0.4 0.3 0.2 0.1 0 Core School Tail 10 20 30 60 100 200 300 600 1000 2000 School Size Densest 10% Fast core Slow 25% backwards Slow Fast Faster Schools: Denser Core, And Loser Tail, Because Less Fall Back Contrary to Breder (1959); Radakov (1973)

16. 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?

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

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

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

20. Stay over the Sleeping Site Vertical attraction sleeping area Horizontal attraction

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

22. Parameters From Starlings (Hildenbrandt, Carere, Hemelrijk, 2010, Behavioural ecology)

23. Resemblance to real flocks? Qualitative Quantitative

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

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

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

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

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

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

30. Shape When Turning Sharply sleeping area Banking Banking (dg) I3/I1 Aspect ratio I2/I1 I3/I2 Volume Volume (m3) NND 0 10 20 30 40 50 60 -> Time N = 2000 changes due to changes in volume due to different behaviour inside and outside roost

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

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

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

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

35. Deviations of global velocityduring movement approx. straightforward temporary sub flocks

36. Larger flocks less synchronised Deviation of Velocity Correlation length ξ (m) Flock size L (m) 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

37. Larger flocks: weaker global polarisation local polarisation polarisation global 0 500 1500 2000 3000 4000 5000 ---> N Larger sub flocks differ in direction more flock shape is more variable

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

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

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

41. Shape of flock when turning mildly 2500 individuals 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 1000 Oblong I3/I2 Oblong in forward direction Length/Width 100 L 500 W movement direction Usually oblong in all kinds of directions, seldom in the movement direction

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

43. N = 2000 birds, 10 m/s Frequency -100 -60 0 60 100 Speed deviation (% cruise speed) Variability of speed is low

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

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

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

47. Conclusion: • Variableshape 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

48. Predation Ultrasound attack by fish on herring

49. Predation? Model

50. Models of Fish Schools under attack Split Flash expansion Herd • Predator approaches • Prey has two tendencies • Stay with the school • Move away from predator Predator