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Haverford College
Departments of Physics and Astronomy
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Haverflock  Summer 2009

This summer we are studying collective animal behavior using the stereo video techniques pioneered by the STARFLAG group.[1]-[8]  Our topic of interest is the phenomenon called mobbing, whereby smaller prey animals (in our case, passerines such as crows or songbirds) harass and attack a larger predator (in our case, the red-tailed hawk). [9]  We have used particle-tracking methods and statistical analyses to understand and model the dynamical rules governing this behavior.

From left to right:  American crow (Corvus brachyrhynchos); red-tailed hawk (buteo jamaicensis); four crows mobbing a hawk.  (wikipedia and flickr)

Introduction • Sample images & video • Methods •Results• Participants• References

Introduction

Why study collective animal behavior? In addition to its intrinsic interest, flocking has been studied to enable us to understand and identify universal features of collective behavior in general (such as financial markets, traffic and human crowds during evacuations), to model the motion of animals in CGI, and to avoid collisions between bird flocks and airplanes. Collective animal behavior provides a laboratory for understanding the way cooperation factors into evolution;  models have shown that flocking can reduce the risk to individuals from predators on average, or that cooperating with other fish reduces the risk during inspection of a predator, for example. 

We are interested in mobbing in particular because it presents an intriguing case of nonintuitive behavior whose evolutionary origins are uncertain.  What drives smaller prey birds to seemingly enhance their risk of predation by confronting a predator at close range?  Do their attacks take advantage of strategies that deter the predator while avoiding the most likely scenarios of attack?  How do the trajectories of mobbing birds coordinate with their characteristic mobbing cries? Mobbing attacks take place in groups ranging from individuals to small flocks of a dozen or more birds, with the average number found to be around six.  How do mobbing attack strategies depend upon the number of mobbers?

Methods:  We are making our observations around Haverford's 216-acre campus and Arboretum, so you may have seen us filming.  Our goal is to record video of aerial mobbing incidents between hawks and various passerines (principally tree swallows, barn swallows and American crows) as well as mobbing of a fixed predator target.   Earlier studies have shown that mobbing can be induced by using a taxidermy predator as a target near a passerine roosting site [10]-[11] and by playing back predator and mobbing cries. Our field data consists of stero images recorded on two high-def 30 fps progressive scan Canon Vixia HF-200 digital video cameras precisely positioned and aimed so as to provide stereo images of our experimental subjects.  These will be analyzed to provide 3D video of mobbing attacks.   These will be analyzed using the image analysis package SigmaScan, with particle tracking software adapted from that developed by John Crocker and Eric Weeks.  [12]

Results

7/8/09  So far we have designed and fabricated our stereo video apparatus and tested it out in the field.   We have surveyed Haverford's campus for the best sites for our studies.  We have recorded numerous incidents of mobbing involving individual birds, as well as entire mixed-species flocks,  as well as reconstructing the 3D coordinates of test objects.  Back in the lab, our video analysis software now allows us to automatically track birds in motion by subtracting off even complex backgrounds that include trees, sky and other objects, and to reconstruct the 3D trajectories of each bird in detail.  We are now studying videos of mobbing incidents taken here at Haverford, as well as successful hawk attacks that have been posted on the web, to provide a first analysis of the dynamics of both predation and mobbing attacks.

Participants:

Suzanne Amador Kane Associate Professor, Haverford Physics Department

M. Elias Tousley (Haverford Physics '11) is working on this study as a summer research project in 2009.

Owen Glaze (a '09 graduate of Lower Merion High School) is performing summer research before attending Penn State's main campus in the fall.  

References

  1. STARFLAG website
  2. Physics Today article on bird flocking & the STARFLAG group
  3. Interaction Ruling Animal Collective Behaviour Depends on Topological rather than Metric Distance: Evidence from a Field Study PNAS, 105, 1232-1237 (2008) (free download).
  4. An empirical study of large, naturally occurring starling flocks: a benchmark in collective animal behaviour
    Animal Behaviour 76, 201-215 (2008). Here is the Preprint version
  5. The STARFLAG handbook on collective animal behaviour: Part I, empirical methods Animal Behaviour 76, 217-236 (2008). Here is the Preprint version
  6. The STARFLAG handbook on collective animal behaviour: Part II, three-dimensional analysis Animal Behaviour 76, 237–248 (2008). Here is the Preprint version
  7. New statistical tools for analyzing the structure of animal groups, Mathematical Biosciences 214, 32-37 (2008).
  8. Collective behavior in animal groups: theoretical models and empirical studies, HFSP Journal 2, 205-219 (2008).
  9. Roni Ostreiher, "Is mobbing altruistic or selfish behaviour?", Animal Behaviour, Volume 66, Issue 1, July 2003, Pages 145-149, ISSN 0003-3472, DOI: 10.1006/anbe.2003.2165. (http://www.sciencedirect.com/science/article/B6W9W-48FSV4T-7/2/cc5318bf3e76a0a15b47f0a2cf1f50c5)
  10. Hendrichsen, DK; Christiansen, P; Nielsen, EK, et al., Exposure affects the risk of an owl being mobbed - experimental evidence, Hendrichsen, DK; Christiansen, P; Nielsen, EK, et al. Journal Of Avian Biology,  Volume 37, Issue 1,  Pages 13-18, Jan. 2006
  11. Mark T. Nolen, Jeffrey R. Lucas, Asymmetries in mobbing behaviour and correlated intensity during predator mobbing by nuthatches, chickadees and titmice, Animal Behaviour, Volume 77, Issue 5, May 2009, Pages 1137-1146, ISSN 0003-3472, DOI: 10.1016/j.anbehav.2009.01.023. (http://www.sciencedirect.com/science/article/B6W9W-4VRWNNM-3/2/61330586040906cd3a12ca2d64d84f47)
  12. "Methods of Digital Video Microscopy for Colloidal Studies", John C. Crocker and David G. Grier, J. Colloid Interface Sci. 179, 298 (1996). Crocker & Weeks Particle tracking website