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Supplementary Information

Information transfer and behavioural inertia in starling flocks

Alessandro Attanasi1,2, Andrea Cavagna1,2,3∗, Lorenzo Del Castello1,2, Irene Giardina1,2,3, Tomas S. Grigera4,

Asja Jelic1,2∗, Stefania Melillo1,2, Leonardo Parisi1,5, Oliver Pohl1,2, Edward Shen1,2, Massimiliano Viale1,21 Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy

2 Dipartimento di Fisica, Universita Sapienza, 00185 Rome, Italy3 Initiative for the Theoretical Sciences, The Graduate Center,

City University of New York, 10016 New York, USA4 Instituto de Investigaciones Fisicoquımicas Teoricas y Aplicadas (INIFTA) and Departamento de Fısica,Facultad de Ciencias Exactas, Universidad Nacional de La Plata, c.c. 16, suc. 4, 1900 La Plata, Argentina

CONICET La Plata, Consejo Nacional de Investigaciones Cientıficas y Tecnicas, Argentina5 Dipartimento di Informatica, Universita Sapienza, 00198 Rome, Italy and

∗ e-mail: [email protected], [email protected]

Appendix A: Border effects

The propagation curve, x(t), namely the distance trav-eled by the information in a given time, has a linear formonly for early and intermediate times, whereas for longertimes there is a clear saturation of x(t), Fig. 2. The causeof this saturation is the border of the flocks. To get thepropagation curve we have assumed that the number ofbirds reached by the information, when this has trav-eled a distance x(t), is proportional to the volume of thesphere with radius x. But this number of birds is exactlythe rank r(t) of the last bird reached by the informationin a time t, and therefore we have r(t) ∼ x(t)3. Thisargument is true in three dimensions and in the bulk,where no border effects are present.

However, when the information to turn reaches theborder of the flock in some direction, things change,as the typical number of birds reached by the informa-tion scales with an exponent smaller than 3. To under-stand this point, let us make the example of flock witha disc-like shape. Once the information has reached theboundary, by traveling a distance equal to the shortestaxis, the problem becomes effectively two-dimensional,so that r(t) ∼ x(t)2. Similarly, in a tube-shaped flock,one would get r(t) ∼ x(t) for long enough times. Hence,in non-spherical systems (as real starling flocks are [1]),we expect in general that for later times, i.e. oncethe information to turn has reached the boundary, therank grows like, r(t) ∼ xlate(t)

α, with α < 3. Giventhat we use a bulk definition of the traveled distance,x(t) ≡ r(t)1/3 ∼ xlate(t)

α/3, we expect a saturation ofx(t) for later times, due to the fact that α/3 < 1. Thisis indeed what we observe, Fig. 2 and SI-Fig. S3.

Appendix B: The spin

Some theories of collective motion have highlightedthe similarity between flocks and magnets [2, 3]. In par-ticular, the Hamiltonian in (2) is the same as that of a

ferromagnet, where the birds velocities vi play the roleof magnetic spins [3]. However, in previous descriptionsspins were only virtual, as they did not obey proper Pois-son rules. Within the present description, things change.

The momentum sz conjugated to the phase ϕ is de-fined as the local generator of the rotations parametrizedby ϕ, around the z axis. Hence, (sz, ϕ) are general-ized action-angle canonical variables. The fact that szgenerates the symmetry parametrized by the phase ϕ isexpressed by Poisson relation, {v, sz} = ∂v/∂ϕ = iv,which in components reads, {vx, sz} = ∂vx/∂ϕ = −vyand {vy, sz} = ∂vy/∂ϕ = vx. If we interpret vx and vyas the x, y components of the spin, these equations showthat sz is a true spin, namely the generator of the rota-tion in the space of the order parameter v. This is themost general and fundamental definition of spin [4]. Ac-cordingly, jz = ρs∇ϕ is the spin current and ρs = a2Jis the spin stiffness [5].

It is essential to understand that according to this def-inition sz is the intrinsic spin, not the orbital angularmomentum of the bird. Let us define external space thespace of the birds coordinates and internal space (or tar-get space) the space of the order parameter, namely theplanar velocity. Indeed, v(x, t) is a map between the ex-ternal space R3 × R and the internal space SO(2) (thecircle). The phase ϕ parametrizes rotations in the in-ternal space of the velocity and it must not be confusedwith the orbital angle θ of 2d polar coordinates, whichparametrizes rotations in the external space of positions.The easiest way to understand this is the following: abird flying straight (with respect to an arbitrary fixedreference frame - that of our cameras, for example) has

ϕ = 0, but θ �= 0.

As emphasized in the main text, rotationsparametrized by these two angles, ϕ and θ, corre-spond to two very different types of collective turns.A rotation parametrized by ϕ corresponds to an equalradius turn, i.e. a turn in which all birds have the sameradius of curvature and where trajectories cross. On theother hand, a rotation parametrized by θ corresponds to


Supplementary Information


Alessandro Attanasi1,2, Andrea Cavagna1,2,3∗, Lorenzo Del Castello1,2, Irene Giardina1,2,3, Tomas S. Grigera4,

Asja Jelic1,2∗, Stefania Melillo1,2, Leonardo Parisi1,5, Oliver Pohl1,2, Edward Shen1,2, Massimiliano Viale1,21 Istituto Sistemi Complessi, Consiglio Nazionale delle Ricerche, UOS Sapienza, 00185 Rome, Italy

2 Dipartimento di Fisica, Universita Sapienza, 00185 Rome, Italy3 Initiative for the Theoretical Sciences, The Graduate Center,

City University of New York, 10016 New York, USA4 Instituto de Investigaciones Fisicoquımicas Teoricas y Aplicadas (INIFTA) and Departamento de Fısica,Facultad de Ciencias Exactas, Universidad Nacional de La Plata, c.c. 16, suc. 4, 1900 La Plata, Argentina

CONICET La Plata, Consejo Nacional de Investigaciones Cientıficas y Tecnicas, Argentina5 Dipartimento di Informatica, Universita Sapienza, 00198 Rome, Italy and

∗ e-mail: [email protected], [email protected]

Appendix A: Border effects

The propagation curve, x(t), namely the distance trav-eled by the information in a given time, has a linear formonly for early and intermediate times, whereas for longertimes there is a clear saturation of x(t), Fig. 2. The causeof this saturation is the border of the flocks. To get thepropagation curve we have assumed that the number ofbirds reached by the information, when this has trav-eled a distance x(t), is proportional to the volume of thesphere with radius x. But this number of birds is exactlythe rank r(t) of the last bird reached by the informationin a time t, and therefore we have r(t) ∼ x(t)3. Thisargument is true in three dimensions and in the bulk,where no border effects are present.

However, when the information to turn reaches theborder of the flock in some direction, things change,as the typical number of birds reached by the informa-tion scales with an exponent smaller than 3. To under-stand this point, let us make the example of flock witha disc-like shape. Once the information has reached theboundary, by traveling a distance equal to the shortestaxis, the problem becomes effectively two-dimensional,so that r(t) ∼ x(t)2. Similarly, in a tube-shaped flock,one would get r(t) ∼ x(t) for long enough times. Hence,in non-spherical systems (as real starling flocks are [1]),we expect in general that for later times, i.e. oncethe information to turn has reached the boundary, therank grows like, r(t) ∼ xlate(t)

α, with α < 3. Giventhat we use a bulk definition of the traveled distance,x(t) ≡ r(t)1/3 ∼ xlate(t)

α/3, we expect a saturation ofx(t) for later times, due to the fact that α/3 < 1. Thisis indeed what we observe, Fig. 2 and SI-Fig. S3.

Appendix B: The spin

Some theories of collective motion have highlightedthe similarity between flocks and magnets [2, 3]. In par-ticular, the Hamiltonian in (2) is the same as that of a

ferromagnet, where the birds velocities vi play the roleof magnetic spins [3]. However, in previous descriptionsspins were only virtual, as they did not obey proper Pois-son rules. Within the present description, things change.

The momentum sz conjugated to the phase ϕ is de-fined as the local generator of the rotations parametrizedby ϕ, around the z axis. Hence, (sz, ϕ) are general-ized action-angle canonical variables. The fact that szgenerates the symmetry parametrized by the phase ϕ isexpressed by Poisson relation, {v, sz} = ∂v/∂ϕ = iv,which in components reads, {vx, sz} = ∂vx/∂ϕ = −vyand {vy, sz} = ∂vy/∂ϕ = vx. If we interpret vx and vyas the x, y components of the spin, these equations showthat sz is a true spin, namely the generator of the rota-tion in the space of the order parameter v. This is themost general and fundamental definition of spin [4]. Ac-cordingly, jz = ρs∇ϕ is the spin current and ρs = a2Jis the spin stiffness [5].

It is essential to understand that according to this def-inition sz is the intrinsic spin, not the orbital angularmomentum of the bird. Let us define external space thespace of the birds coordinates and internal space (or tar-get space) the space of the order parameter, namely theplanar velocity. Indeed, v(x, t) is a map between the ex-ternal space R3 × R and the internal space SO(2) (thecircle). The phase ϕ parametrizes rotations in the in-ternal space of the velocity and it must not be confusedwith the orbital angle θ of 2d polar coordinates, whichparametrizes rotations in the external space of positions.The easiest way to understand this is the following: abird flying straight (with respect to an arbitrary fixedreference frame - that of our cameras, for example) has

ϕ = 0, but θ �= 0.

As emphasized in the main text, rotationsparametrized by these two angles, ϕ and θ, corre-spond to two very different types of collective turns.A rotation parametrized by ϕ corresponds to an equalradius turn, i.e. a turn in which all birds have the sameradius of curvature and where trajectories cross. On theother hand, a rotation parametrized by θ corresponds to

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a parallel path turn, typical of rigid bodies. In this kindof turn, paths do not cross and this implies differentradii of curvature for different points. The generatorof the external θ-rotations is the standard angularmomentum, lz, whereas the generator of the internalϕ-rotations is the spin, sz, which is conserved by thecontinuity equation.In order to have some intuition about the physical

nature of sz we must connect external to internal spaces.This connection is established by the kinematic equation,

x = v0 eiϕ , (B1)

expressing the simple fact that birds are not anchoredto a lattice, but they follow their velocity vectors. If weconsider the speed v approximately constant, equation(B1) implies,

ϕ = v0/R , (B2)

where R is the instantaneous radius of curvature. Using(6), we also get,

ϕ =szχ

, (B3)

so that we finally have,

sz =v0 χ

R∼ κ , (B4)

where κ = 1/R is, by definition, the curvature of thetrajectory. Therefore, once the connection with the ex-ternal space is performed, the spin turns out to be essen-tially the curvature. This is why a bird flying straight(R = ∞, κ ∼ 0) has sz ∼ 0, while it has nonzero stan-dard angular momentum lz. A change (in time) of thespin siz of bird i, due to the social force exerted by theneighbours of i, corresponds to a change (in time) of itsinstantaneous radius of curvature, Ri, and curvature, κi.Hence, what actually propagates across the flock duringthe turn is a fluctuation (in space and time) of the cur-vature field, κ(x, t). Before the turn, the flock is flyingalmost straight, R � 1, κ ∼ 0, sz ∼ 0. Then the turnssparks in some part of the flock, causing an increase ofthe curvature κ, i.e. an increase of sz. This changesweeps through space and time until the whole flock hasturned. Finally, after the turn, the flock relaxes back toR � 1, κ ∼ 0, sz ∼ 0. Mathematically, this propagationof the curvature, i.e. of sz, derives from the canonicalequations(6): by taking the second derivative with re-spect to time one obtains a D’Alembert wave equationfor sz(x, t) identical to that obeyed by ϕ(x, t), eq.(8).Conservation law (7) states that the spin-curvature,

sz(x, t) ∼ κ(x, t), obeys a continuity equation. As wehave seen, this conservation law is crucial to determinesound-like propagation. Continuity means that the tra-jectory curvature in a given volume cell of the systemcannot change unless it is transported into, or out of,that cell by a spin current, jz = ρs∇ϕ. We can refor-mulate this by saying that, if a certain excess of cur-vature, namely a strong misalignment among a certain

group of individuals, forms in a given point of the sys-tem, it cannot be simply dissipated out. Rather, suchexcitation creates a social force that makes the neigh-bours turn, and their neighbours too, and so on, so thatthe excess curvature is transported away, instead of be-ing dissipated.Finally, note that the spin sz is not the z component

of the velocity. Also note that the rotation generated bysz is the very transformation under which Hamiltonian(5) is symmetric.

Appendix C: The generalized moment of inertia

As we have seen, the spin sz is not the standard angu-lar momentum lz. Accordingly, the generalized momentof inertia χ, is not the standard moment of inertia, whichin the case of circular motion is, I = mR2, where m isthe mass. So, what is the physical and biological mean-ing of χ? From the canonical point, the answer is clear:χ is the inertia to changing ϕ. Indeed, equation (8) canbe rewritten as,

χ =aFs

ϕ, (C1)

where Fs = aJ∇2ϕ is the social force exerted by theneighbours. Hence, the generalized moment of inertia χis defined as the ratio between the social force (the cause)and the change of angular velocity (the effect). This isthe standard definition of inertia: the ratio between force(cause) and acceleration (effect). However, in this caseFs is a generalized (social) force, and aFs is a generalizedtorque, hence χ is not the standard moment of inertia.To better grasp the biological meaning of χ we must,

once again, bridge the gap between internal and externalspace. By using equations (B1), (B2) and (C1) we obtainseveral cause-effect relations clarifying the physical andbiological meaning of χ. The first relation connects thesocial force to the change of radius R, or equivalently tothe change of curvature κ,

χ = −(R2

v

)aFs

R=

(1

v

)aFs

κ. (C2)

Hence, χ is the resistance of a bird to change its instan-taneous radius of curvature R (the effect), when a givensocial force Fs (the cause) is exerted.

Another interesting relation can be obtained in termsof the banking angle γ. A banked turn [6, 7] is the typicalway birds (and planes) change their heading. It consistsin a gentle roll, so to form an angle γ between the axisof the wings and the horizontal plane. In this way, partof the total lift goes into a centripetal force, Fc = mgγ,making the bird turn (m is the mass, g the gravitationalacceleration and γ � 1). From (C2), it is straightfor-ward to prove that,

χ =

(v

g

)aFs

γ. (C3)

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According to (C3), the generalized moment of inertia χis the resistance of a bird to change its banking angle γ.Here, γ is the effect of the social force, Fs, and χ setsthe ratio between cause and effect. Notice that equations(C2) and (C3) are clearly non-canonical definitions of theinertia χ, because at the denominator they both have afirst order derivative in time, rather than a second orderone, as in the canonical equation (C1).

Let us emphasize once again that χ is not the stan-dard, mechanical moment of inertia, I = mR2, butrather a social, or sensorimotor, resistance of the birdto change R or γ. It is not possible to write an ab inicioexpression for χ in terms of primary mechanical quanti-ties, like mass, radius, etc. To understand this fact, letus imagine that at some point the neighbours of bird iall sharpen their banking angle γ, thus creating a strongsocial force, Fs = aJ∇2ϕ, acting on i. What we call asocial ‘force’ is in fact a shortcut to describe a very com-plex sensorimotor process: a nonzero ∇2ϕ means thatsome of the neighbours of i are now about to crash intoi. This is most likely perceived by i, which decides tochange its own γ and make it equal to that of the neigh-bours, thus avoiding the crash. However, the degree bywhich i will react to the imminent crash, or converselythe resistance to this reaction (which is χ), is the resultof a very complex trade-off. Let us analyze this trade-offby pretending to be i.

On one side, there is the price of the crash. Howimminent is it? This will depend on both the nearestneighbour distance and on the mutual velocity. Howbad would that be? Perhaps, I can ignore my neigh-bours, and just change them, without any real crash.How confident I am into my capability to change γ? If Iam very agile, I can wait a bit longer before changing myγ. On the other side, there is the price to changing γ.How much will it cost me energetically? By increasing γI will increase the drag against air, otherwise I fall down.But to do this I must increase the power, which is costly.Can I manage to do that?

The generalized moment of inertia χ is the very finalproduct of this very complicated neural and sensorimotorprocess. Clearly, we cannot know a priori its value. Butwe can define it, and measure it. This is exactly whatwe have done by measuring the speed of propagation ofthe turn across the flock. In this sense, the situation isthe same as in real magnetic systems: the parameter χ isthe magnetic susceptibility to an external field coupledto sz [5], which cannot be simply expressed as a functionof the microscopic parameters of the theory, but it canbe experimentally measured.

Appendix D: Relationship between Φ and J.

The relationship between alignment strength, J , andpolarization, Φ, is a consequence of the Gaussian natureof the theory in the spin-wave limit, equation (5). The

polarization is the modulus of the magnetization vector,

Φ = ‖ 1

N

∑i

�vi‖ , (D1)

so that it has values between 0 and 1 (for simplicity weassume here ‖�v‖ = 1). Using,

�v = eiϕ , (D2)

and expanding the velocity for small values of the phase,we obtain,

Φ = 1− 1

2

∑i

ϕ2i = 1− 1

2〈ϕ2〉 . (D3)

As expected, the polarization is larger the smaller thephase fluctuations. The small-ϕ (namely large-Φ) ex-pansion is called spin-wave approximation. We want toremark that spin wave approximation has been found tobe accurately verified in real flocks of starlings in pre-vious works [3]. To compute 〈ϕ2〉 we use the fact thatin the spin-wave limit the probability distribution of thephase is Gaussian (equation (5)),

P (ϕ) ∼ exp

(−1

2β

∫d3x

a3ρs(�∇ϕ)2

)(D4)

= exp

(−1

2β

∫d3k

a3ρs k

2 ϕkϕ−k

), (D5)

where β is the inverse temperature and the stiffness ρs =a2J is (up to the lattice spacing constant) the strengthof the alignment interaction. We therefore have,

Φ(J, β) = 1− 1

2〈ϕ2〉 = 1−

∫d3k

a

βJk2∼ 1− 1

βJ. (D6)

This equation has a rather simple meaning: a large po-larization (Φ ∼ 1) can be obtained either by reducingthe temperature (large β) or by increasing the alignmentcoupling constant, J . By inverting equation (D6) we get,

J =1/β

1− Φ(J, β). (D7)

The fact that J seems to diverge for Φ → 1 may be con-fusing. To understand this point we must keep in mindthat Φ is a function of both J and β (eq.(D6)). Hence,the polarization can become very close to 1 even with nodivergence of J : this happens if β increases (low tem-perature) at fixed J , in which case both the numeratorand denominator of (D7) go to zero, so that J remainsfinite. In this case, though, no increase of the secondsound speed cs would be observed, because cs only de-pends on J , c2s = a2J . On the other hand, if J increasesat constant β, then the polarization Φ approaches 1 andthe second sound speed cs also increases.





4

Appendix E: Dissipation

According to Noether’s theorem, the spin conserva-tion law is present as long as only even derivatives ofthe phase with respect to time appear in the equationof motion, as in the D’Alembert equation. Hence, whenphase-dissipation occurs, i.e. when a term ϕ appearsin the equation of motion, strict conservation no longerholds. However, we shall show here that as long as dis-sipation is low, the qualitative results are the same asin the case of strict conservation. This analysis will alsobe useful to understand how the diffusive equation ofmotion (4) derived from the standard theory can be ob-tained as the overdamped limit of the new theory.We introduce a dissipative term proportional to ϕ in

the equation of motion and obtain,

χ∂2ϕ

∂t2+ η

∂ϕ

∂t− ρs ∇2ϕ = 0 , (E1)

with ρs = a2J and where η is a generalized friction co-efficient. From this we get the dispersion law,

χω2 − iηω − ρsk2 = 0 . (E2)

In the limit η � χ we simply get the diffusive result,ω = i(ρs/η)k

2. In general, however, we obtain,

ω = iη

2χ± csk

√1− k20/k

2 , (E3)

where, as usual, the propagation speed is cs =√

ρs/χand,

k0 ≡ η

2√ρsχ

. (E4)

If we define the dissipation time scale, τ ≡ 2χ/η, andthe zero-dissipation frequency, ω0 ≡ csk, we can rewritethe dispersion law as,

ω = i/τ ± ω0

√1− k20/k

2 . (E5)

With zero dissipation, we get k0 = 0, τ = ∞ and ω = ω0,which is the case we studied in the main text. For η �= 0,on the other hand, we have two regimes, according to thevalue of the friction coefficient and of the wave numberk. For k ≥ k0 we have attenuated propagating waves, asthe frequency has both a real and an imaginary part. Fork < k0 we have overdamped (or evanescent) waves: thefrequency is purely imaginary, there is no propagation,but pure exponential decay.The smallest value of k in the system is kmin ∼ 1/L,

where L is the linear size of the flock. Hence, smalldissipation is defined by the relation,

η <

√ρsχ

L: small dissipation . (E6)

With small dissipation there is linear propagation of theinformation and the time scale of the exponential decayis set by τ = 2χ/η. From (E6) we get,

τ >√χ/ρs L = L/cs . (E7)

Therefore, small dissipation implies that the dampingtime constant is larger than the time the information em-ploys to travel across the flock. In other words, the signalis effectively very weakly damped across the length scaleof interest. We conclude that even when a small dissi-pation is present, propagation of information is qualita-tively the same as that described by the zero dissipationtheory.We finally note that the existence of a threshold

momentum k0 implies that a continuum theory devel-oped to described only the asymptotically correct long-wavelength hydrodynamics of a flock, i.e. a theory work-ing in the k → 0 limit, would miss linear propagation ofthe phase due to spin conservation, even in very weakdamping. This is the case of the hydrodynamic theoryof Toner-Tu [8], and its later developments [9].

Appendix F: Dimensional analysis

In the novel theory we have an Hamiltonian that isthe sum of two parts. Hence, we have to be carefulwith physical dimensions. The phase is of course a purenumber, [ϕ] = [1], whereas the alignment coupling con-stant has the dimensions of an energy, [J ] = [e]. Inthis way the social force has the dimensions of a trueforce, [Fs] = [aJ∇2ϕ] = [e · x−1] and the spin has thedimensions of an angular momentum, [sz] = [e · t], i.e.of an action. Accordingly, χ has the dimensions of atrue moment of inertia, [χ] = [e · t2]. Notice that theterm appearing in equation (6) is a2J∇2ϕ = aFs, whichis dimensionally a torque. Hence, the derivative of anangular momentum is a torque, as it should.By definition, the polarization is a pure number, [Φ] =

[1]. This is why in the relation linking alignment cou-pling constant to polarization, J = 1/[β(1−Φ)], we needa dimensional constant with the dimensions of an energy,[1/β] = [e]. As we have said, 1/β sets the scale of thenoise. Finally, with the above physical dimensions, thespeed of propagation of information across the flock, cs,is measured in meters per second, as it should.

Appendix G: General off-plane case

Our initial assumption that the birds’ velocities lieon a plane during the turn, namely that the turn hasvery small torsion, although experimentally satisfied (seeFig.1b,c), is not at all a necessary condition for our math-ematical description. The most general formulation ofour result holds even with a truly 3d order parameter vi

[5].





5

If we assume that the mean direction of motion of theflock points in the x direction, then there will be full3d fluctuations of the individual velocities vi around theoverall direction of motion of the flock, generating smallcomponents of vi along the two orthogonal axes, z andy. Therefore, we must define two phases, ϕz and ϕy andthe Hamiltonian can be spin-wave expanded in terms ofthese two fields. The phase ϕz parametrizes rotationsof vi around the z axis (as in the planar - zero torsioncase), whereas ϕy parametrizes rotations of vi aroundthe y axis. In this fully 3d case the Hamiltonian is givenby [5],

H =

∫d3x

a31

2ρs

[(∇ϕz)

2+ (∇ϕy)

2]+

1

2χ

[s2z + s2y

],

(G1)where ρs = a2J is, as usual, the stiffness. The equationsof motion are,

∂ϕα

∂t=

δH

δsα=

sαχ

, (G2)

∂sα∂t

= − δH

δϕα= a2J ∇2ϕα = ∇ · jα , (G3)

with α = y, z, giving rise to two D’Alembert equations,

∂2ϕα

∂t2− c2s ∇2ϕα = 0 , c2s = ρs/χ . (G4)

In the full 3d case we therefore obtain two, rather thanone, propagating dissipationless modes, along the trans-verse directions y and z. This is just a manifestation ofGoldstone’s theorem [10].These equations are exactly the same as for model

G in the Halperin-Hohenberg classification of dynami-cal universality classes [5, 11]. Model G does not de-scribe superfluid liquid helium, but an isotropic Heisen-berg antiferromagnet with staggered magnetization as anon-conserved order parameter, and total magnetizationas a constant of motion. An experimental realization ofa 3d isotropic antiferromagnet is RbMnF3, a compoundwhose dynamics is characterized by the transverse spin-wave modes (G4). Notice that also in this system thereis superfluid transport. As discussed in the main text,superfluidity is not restricted to liquid helium II, but it israther built into the mathematical details of the theory.In particular, it is the product of symmetry and conser-vation laws. In the full 3d case described here (model G)these ingredients give rise to superfluid transport exactlyas in the planar (He-II) case.To write (G1) and (G4) we have assumed that the two

excitations ϕz and ϕy are equally likely, so that the onlysymmetry breaking direction is that of motion. In fact,recent studies on individual diffusion in starling flocksshow that gravity is another symmetry breaking direc-tion, heavily suppressing fluctuations along the verticalplane [12]. If we identify z with the axis of gravity, thissuppression would imply that rotations of the velocityaround the y axis are suppressed, and therefore that ϕy

is less relevant a degree of freedom than ϕz. This sup-pression induced by gravity is likely the cause of theplanar-like turns we observe in flocks and it thus justi-fies the adoption of the simpler planar description of themain text.

Appendix H: Mutual delay vs reaction time

One may think that the mutual delay between twobirds, τij , is the same as the reaction time, τR, namelythe time between the stimulus provided by j and theconsequential action of i. However, this is not the case.Let us assume j is the first bird to turn, and that i is

second. By definition of reaction time, i begins its turnτR seconds after j. However, we do not define τij as thedifference between the starting instants of the two turns:there is no practical and robust way to do that, becauseeach birds turns smoothly, so that it is impossible to de-fine the ‘start’ of the turn. To compute τij we use theentire trajectory of both birds, by finding the time shiftthat maximally overlaps the accelerations of i and j (seeFig.1). If the function ai(t) were exactly the same as thefunction aj(t− τij), then we would have τij = τR. This,however, is never the case. First of all there is noise,so that the two curves, ai(t) and aj(t), are only approx-imately shifted with respect to each other. But moreimportantly, the second bird, i, can try to ‘catch up’during the turn, so that the delay at the end of the turnis shorter than the delay at the beginning of the turn,which is the reaction time. In this case, the delay τijwould be a value intermediate between those two times,hence giving a value smaller than τR. The opposite canhappen too: bird i could in fact lose ground during theturn, so that the delay at the end of the turn is longerthan the reaction time, and τij is larger than τR.





6

Event label N Φ cs (ms−1) ∆cs (ms−1)

20110208 ACQ3 176 0.806 20.20 0.25

20111124 ACQ1 125 0.959 42.64 0.97

20111125 ACQ1 50 0.866 32.38 1.68

20111215 ACQ1 384 0.801 22.74 0.71

20111125 ACQ2 502 0.841 23.86 2.45

20110217 ACQ2 404 0.854 37.70 1.63

20111214 ACQ4 F1 154 0.940 38.46 1.47

20111214 ACQ4 F2 T1 139 0.890 37.32 3.42

20111214 ACQ4 F2 T2 139 0.808 35.40 0.48

20111220 ACQ2 197 0.907 27.54 1.01

20111201 ACQ3 F1 133 0.793 18.82 1.55

20110211 ACQ1 595 0.757 21.96 2.71

TABLE S1. Polarization and speed of propagation. Nis the number of birds in the flock. The polarization is de-fined as, Φ = ||(1/N)

∑i vi/vi||. The values of Φ reported

here are on average smaller than those reported in previousinvestigations [1, 13]. The reason for this is that previousdata were obtained with cameras sampling at 10Hz, whereasthe present data are obtained at 170Hz. At this sampling fre-quency experimental noise and wing flapping reduce the po-larization. This reduction, however, affects equally all flocksby uniformly rescaling (1− Φ), hence it does not change thecorrelation in Fig.3. The speed of propagation of the infor-mation, cs, is found by fitting the linear regime of the propa-gation curve, x(t). The error ∆cs on the speed of propagationcs is obtained from its variability under changing the linearfitting time interval of x(t).

[1] Ballerini, M., Cabibbo, N., Candelier, R., Cavagna, A.,Cisbani, E., Giardina, I., Orlandi, A., Parisi, G., Procac-cini, A., Viale, M., & Zdravkovic, V. Empirical investiga-tion of starling flocks: A benchmark study in collectiveanimal behaviour. Anim. Behav. 76, 201–215 (2008).

[2] Vicsek, T., Czirok, A., Ben-Jacob, E., Cohen, I., andShochet, O. Novel type of phase transition in a systemof self-driven particles. Phys. Rev. Lett. 75, 1226–1229(1995).

[3] Bialek, W., Cavagna, A., Giardina, I., Mora, T., Sil-vestri, E., Viale, M. & Walczak, A. M. Statistical me-chanics for natural flocks of birds. Proc. Natl. Acad. Sci.USA 109, 4786–4791 (2012).

[4] Pauling, L. &Wilson, E. B. Introduction to quantum me-chanics: with applications to chemistry (McGraw-Hill,New York and London, 1935).

[5] Halperin, B.I. & Hohenberg, P.C. Hydrodynamic Theoryof Spin Waves. Phys. Rev. 188, 898–918 (1969).

[6] Norberg, U. M. Vertebrate Flight: mechanics, physiol-ogy, morphology, ecology and evolution (Springer Verlag,New York, 1990).

[7] Videler, J. J., Birkhead, T. ed. Avian flight (Oxford Uni-

versity Press, Oxford, 2005).[8] Toner, J. & Tu, Y. Flocks, herds, and schools: A quan-

titative theory of flocking. Phys. Rev. E 58, 4828–4858(1998).

[9] Simha, R. A. & Ramaswamy S. Hydrodynamics Fluctu-ations and Instabilities in Ordered Suspensions of Self-Propelled Particles. Phys. Rev. Lett. 89, 058101 (2002).

[10] Goldstone, J. Field Theories with Superconductor Solu-tions. Nuovo Cimento 19, 154–164 (1961).

[11] Hohenberg, P.C. & Halperin, B. I. Theory of dynamiccritical phenomena. Rev. Mod. Phys. 49, 435–479 (1977).

[12] Cavagna, A., Queiros Duarte, S. M., Giardina, I., Ste-fanini, F. & Viale, M. Diffusion of individual birds instarling flocks. Proc. R. Soc. B 280, 20122484 (2013).

[13] Ballerini, M., Cabibbo, N., Candelier, R., Cavagna, A.,Cisbani, E., Giardina, I., Lecomte, V., Orlandi, A.,Parisi, G., Procaccini, A., et al. Interaction ruling an-imal collective behavior depends on topological ratherthan metric distance: Evidence from a field study Proc.Natl. Acad. Sci. USA 105, 1232–1237 (2008).





7

FIG. S1. Acceleration correlation function. The correlation function Gij(τ) measures the overlap between the accelerationof bird i, ai(t) and the time-shifted acceleration of bird j, aj(t − τ), as a function of τ (see Methods). The point where thisoverlap is maximum corresponds to the delay τij between the two birds. In the figure we report the correlation function for atriplet of birds within one of the studied flock.

FIG. S2. Check of the time ordering relation. We report the time ordering relation test for several of our flocks and forone non-turning flock (lowest-right panel). Temporal consistency requires that τij ∼ τik + τkj , so to have the data scatteredalong the identity line with clear correlation. In the case of the non-turning flock, on the other hand, the delays are justrandom numbers, so no temporal consistency is found.





8

FIG. S3. Ranking and propagation. The ranking curve, r(t) (panels (a) and (c)) and the propagation curve, x(t) =

[r(t)/ρ]1/3 (panels (b) and (d)) are reported for several turning flocks in our pool of data. Together with the data for 3 turningflocks reported in Fig. 2, these represent all the 12 turning events we analyze. The speed of propagation, cs, is obtained asthe slope of the linear regime of x(t) for early and intermediate times (panels (b) and (d)). Moreover, by fitting the rankingcurve (panels (a) and (c)) to a power law, r(t) = tα, for early and intermediate times, we find on average α = 3.2. Therefore,

x ∼ r1/3 ∼ t1.07, thus supporting the statement that propagation is linear.





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