The international journal of science / 20 June 2024

spotlight
Agriculturalsciences

SOAR
POINT

Unpopular vote
Election-watchingscientists feel thepolitical heat

2050 vision
How best to update
the Sustainable
Development Goals

Reality check
Large language models
curb hallucinations inlarge language models

Vol. 630, No. 8017
£10.00 nature.com

Air sacs below thewings help soaring
birds to glide

Article

The respiratory system influences flightmechanics in soaring birds
https://doi.org/10.1038/s41586-024-07485-y
Received: 28 October 2023

Emma R. Schachner1 ✉, Andrew J. Moore2, Aracely Martinez3, Raul E. Diaz Jr4,
M. Scott Echols5, Jessie Atterholt6, Roger W. P. Kissane7, Brandon P. Hedrick8 & Karl T. Bates7

Accepted: 29 April 2024
Published online: 12 June 2024
Check for updates

The subpectoral diverticulum (SPD) is an extension of the respiratory system in birds
that is located between the primary muscles responsible for flapping the wing1,2.
Here we survey the pulmonary apparatus in 68 avian species, and show that the SPD
was present in virtually all of the soaring taxa investigated but absent in non-soarers.
We find that this structure evolved independently with soaring flight at least seventimes, which indicates that the diverticulum might have a functional and adaptive
relationship with this flight style. Using the soaring hawks Buteo jamaicensis andButeo swainsoni as models, we show that the SPD is not integral for ventilation,that an inflated SPD can increase the moment arm of cranial parts of the pectoralis,and that pectoralis muscle fascicles are significantly shorter in soaring hawks thanin non-soaring birds. This coupling of an SPD-mediated increase in pectoralis
leverage with force-specialized muscle architecture produces a pneumatic systemthat is adapted for the isometric contractile conditions expected in soaring flight.
The discovery of a mechanical role for the respiratory system in avian locomotion
underscores the functional complexity and heterogeneity of this organ system, andsuggests that pulmonary diverticula are likely to have other undiscovered secondaryfunctions. These data provide a mechanistic explanation for the repeated appearanceof the SPD in soaring lineages and show that the respiratory system can be co-opted toprovide biomechanical solutions to the challenges of flight and thereby influence theevolution of avian volancy.

The avian respiratory system is considered to be a key innovation thathas facilitated the diversification of birds into numerous ecologicalniches and has enhanced their ability to perform powered flight3.
Emerging from the gas-exchanging lung and air sacs is an array of
secondary non-ventilatory diverticula; these weave around adjacentorgans, pass between muscles and joints, variably pneumatize thepostcranial skeletal tissues and in some taxa even extend subcutaneously to the wing tips1,2. Although it is the ventilatory air sacs that drive
inspired air through the gas-exchanging portions of the respiratory
system, both the sacs and the diverticula have been co-opted for a variety of functions, including vocalization4,5, visual displays6 and myriad
other hypothesized roles, including thermoregulation7 and protection
from trauma8,9. The subpectoral diverticulum (SPD) is a variably sized
and variably present structure that emerges from the interclavicular
air sac and leaves the body cavity2,10,11, diving ventrally between the
major flight muscles, the supracoracoideus muscle (responsible forthe elevation of the wing in flapping) and the pectoralis (the wing
depressor in flight)12. The SPD has been qualitatively described as particularly large in procellariiforms, suliforms and accipitriforms1,2,13,14,
but its phylogenetic distribution has not been systematicallyevaluated.

To better understand the variability and functional role of the SPD,given its positioning between the primary muscles of flight, we use
gross dissection, three-dimensional (3D) segmentation of live anddeceased specimens and 3D multibody dynamics analysis (MDA)
modelling in two exemplar soaring birds (B. jamaicensis and B. swainsoni) (Fig. 1), perform muscle fascicle analysis in four additional taxa
and evaluate the presence or absence of the SPD in a total of 68 avian
taxa representing 25 orders and 42 families using micro-computed
tomography (µCT). We find that the SPD evolved independently atleast seven times in phylogenetically disparate lineages in concert
with the evolution of soaring, and that it is not present in any of thenon-soaring taxa examined (Fig. 2). We show using MDA modellingthat the SPD increases the moment arm of the pectoralis when fully
inflated in both deceased (intubated and inflated) and live (sedated)birds (Fig. 3d–f). Our data reveal that this diverticulum is not integral
to ventilation, because the live sedated birds can breathe when the
SPD is asymmetrically or bilaterally collapsed (Fig. 1g–i). Furthermore,
we identify commensurate modifications to the fascicle architectureof the pectoralis muscle related to the presence of the SPD in soaring
taxa (Fig. 3a–c), which we hypothesize to represent complementarycomponents of a pneumatic system to enhance the function of the1
Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL, USA. 2Department of Anatomical Sciences, Renaissance School of Medicine, Stony
Brook University, Stony Brook, NY, USA. 3Department of Cell Biology and Anatomy, School of Medicine, Louisiana State University Health Sciences Center, New Orleans, LA, USA. 4Departmentof Biological Sciences, California State University Los Angeles, Los Angeles, CA, USA. 5The Medical Center for Birds, Oakley, CA, USA. 6Department of Medical Anatomical Sciences, College of
Osteopathic Medicine of the Pacific, Western University of Health Sciences, Pomona, CA, USA. 7Department of Musculoskeletal and Ageing Science, Institute of Life Course and Medical
Sciences, University of Liverpool, Liverpool, UK. 8Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY, USA. ✉e-mail: eschachner@ufl.edu

Nature | Vol 630 | 20 June 2024 | 671

Article
a

c

b

g
d

e

j

sc

cr

hu
ic
spd(r)hco
spd(I)
fu
h

f

i

pc
fu
spd
st

Fig. 1 | Anatomy of the SPD in the red-tailed hawk (Buteo jamaicensis).
a,b, Segmented surface models of the skeleton and respiratory system in a
B. jamaicensis (Ptolemy), with a complete respiratory system inflated tomaximum inspiratory capacity, in ventral view (a) and in left lateral view (b)with just the left SPD shown and the left distal forelimb made translucent.
c, Lysimachus shown with a deflated SPD in left lateral view and the distal
forelimb made translucent. d, Gross dissection of an intubated and inflated
B. jamaicensis individual (Ptolemy), showing the sternal plate with the pectoralis
reflected cranially, and the SPD fully inflated (indicated by the arrow;scale bar, 1 cm). e,f, Segmented surface models of the pectoral girdle, left

forelimb and SPD of Ptolemy in cranial (e) and ventral (f) views, with thepectoralis sculpted on to demonstrate the anatomical relationships. g–j, Axial(transverse) slices from three different CT scans of Roxana (g–i), a live sedated
B. jamaicensis individual (volume rendered in j from scan h), demonstratingchanges in the inflation levels of the right SPD during sedated ventilation andnatural bilateral asymmetry in a live hawk. Abbreviations: co, coracoid; cr,cervical sac; fu, furcula; hu, humerus; ic, interclavicular sac; pc, pectoralis; sc,scapula; spd, subpectoral diverticulum; spd(l), left subpectoral diverticulum;
spd(r), right subpectoral diverticulum; st, sternum.

pectoralis in soaring flight. Given that the respiratory system independently evolved a similar mechanism in at least seven separate soaring
lineages, we propose that the SPD substantially enhances the mechanics of flight in soaring birds.

morphology (Extended Data Fig. 2) as well as bilaterally asymmetric levels of inflation in the hawks that were imaged live and sedated(Fig. 1g–I and Extended Data Fig. 2e,g,l,n,s,u). This variation is also evident when visualized through axial (transverse) µCT images selected
where the coracoids articulate with the sternal plate (Extended Data
Fig. 3) in live and deceased (both inflated and deflated) individuals of
B. jamaicensis and B. swainsoni (Extended Data Fig. 4 and Extended
Data Table 1). Live sedated individuals (intubated via the glottis) can
push air both in and out of one SPD during ventilation while leaving
the contralateral SPD completely collapsed (Fig. 1g–I and Extended
Data Fig. 4e,h), or can collapse both diverticula entirely (Extended
Data Fig. 4f). In addition, another live bird that was used for the study(Vultur gryphus) had an essentially collapsed SPD, probably as a resultof obesity (diagnosed by M.S.E.; Extended Data Fig. 6e), but was alsoable to breathe. These data indicate that both species of Buteo (andpossibly other taxa) have some control over inflation levels and that
the SPD is not integral for ventilating the gas-exchanging lung. This
interpretation is reinforced by experimental data in geese and mallard ducks, which show that the interclavicular sac is not the driving
ventilatory air sac sending oxygen-saturated inspired air through thebronchial tree but, rather, the receptacle for the air that has alreadypassed through the gas-exchanging parenchyma and is waiting to be
expired15,16.

Anatomical structure of the SPD
We examined two representative species of soarer—the red-tailed hawk
(B. jamaicensis) and Swainson’s hawk (B. swainsoni)—to visualize theanatomy of the SPD in three dimensions (Extended Data Table 1). The
specimens in this study were named after ancient Macedonians associated with Alexander the Great for identification purposes (for example,
Ptolemy, Seleucus; see ‘Anatomical and imaging methods’). When
inflated to maximum inspiratory capacity, the SPD of B. jamaicensis
is a large, bilaterally symmetric diverticulum (Fig. 1a–f) that branchesfrom the interclavicular air sac and leaves the coelomic cavity by meansof a small, constricted opening lateral to the coracoid and ventral tothe head of the humerus. It extends caudoventrally towards the sternalkeel, deep (dorsal) to the pectoralis muscle, and can be observed indeceased inflated specimens when the pectoralis muscle is detachedfrom its sternal origin and reflected cranially (Fig. 1d and Extended
Data Fig. 1). The morphology of the SPD is similar in B. swainsoni,
and both taxa show extensive intraspecific variation in diverticular
672 | Nature | Vol 630 | 20 June 2024

abCasuarius casuarius
Dromaius novaehollandiae
Struthio camelus
Eudromia elegans
Anas falcata
Anas acuta
Aythya americana
Somateria fischeri
Polysticta stelleri
Branta canadensis
Cygnus buccinator
Cygnus columbianus
Chauna chavaria*
Pauxi pauxi
Gallus gallus
Rollulus rouloul
Calypte anna (c)Podargus strigoides
Musophaga violacea
Caloenas nicobarica
Balearica pavonina (d)Grus americana*
Fulica atra*
Aechmophorus occidentalis
Phoenicopterus chilensis
Himantopus mexicanus
Uria aalge
Larus occidentalis (e)Gavia pacifica
Gavia immer
Gavia adamsii
Aptenodytes forsteri*
Aptenodytes patagonicus*
Pygoscelis adeliae*
Phoebastria immutabilis*
Diomedea exulans*
Puffinus griseus (f)Phalacrocorax penicillatus
Anhinga anhinga*
Fregata aquila*
Eudocimus ruber
Pelecanus erythrorhynchos
Pelecanus occidentalis (g)Nycticorax nycticorax
Cathartes aura (h)Vultur gryphus
Buteo jamaicensis (i)Buteo buteo
Buteo swainsoni (j)Haliaeetus leucocephalus (k)Haliaeetus albicilla*
Aquila chrysaetos
Terathopius ecaudatus
Pulsatrix perspicillata
Bubo virginianus
Bucorvus leadbeateri
Phoeniculus purpureus
Pteroglossus beauharnaesii
Ramphastos sulfuratus
Falco rusticolus
Falco peregrinus
Cacatua galerita
Ara ararauna
Psittacus erithacus
Eclectus roratus
Taeniopygia castanotis (l)Corvus corax
Entomyzon cyanotis

Pg

Ga

Ne

Gr

Ch

Pr
Su
Pe

Ac

In

Fa

Pa
0

PP of soaring

10

PP of SPD

c

d

e

f

g

h

i

j

k

l

1

Fig. 2 | Evolution of soaring flight and the SPD in Neornithes. a,b, The
inferred evolutionary histories of soaring flight (a) and the SPD (b) arenearly identical, with no fewer than seven independent acquisitions of the SPDwithin phylogenetically disparate soaring lineages. Branch colours indicate
posterior probabilities (PP) of an evolutionary transition, and asterisks denotespecies for which the absence or presence of the SPD was determined from theliterature rather than from direct observation of µCT scan data. Species names
in bold are those shown in panels c–l. c–l, Axial µCT slices at the level of thecoracosternal joints illustrate air sac morphology in species that lack the
SPD (outlines in white; c,d,l) and showcase the topologically consistent

Evolution of the SPD across Neornithes
To elucidate the phylogenetic breadth and functional relevance of theSPD, we tested for a correlation between soaring flight and the presenceof the SPD in a dataset of 68 extant bird species spanning 25 orders and
42 families, including almost all major soaring lineages. Both soaring
flight and the SPD were treated as discrete binary (that is, absence orpresence) characters. We define the SPD as a large projection of theinterclavicular air sac that passes deep to the pectoralis muscle andhas a long axis that courses subparallel to the cranioventral surface ofthe coracoid shaft, towards the sternal keel (Fig. 1 and Extended Data
Figs. 2, 4, 5 and 6). Using these criteria, we evaluated the presence of
the SPD in µCT and CT scans of 57 avian taxa (Extended Data Fig. 3),
and included data from the literature for 11 other species (Supplementary Table 1). We treat soaring as a locomotory behaviour that entails
sustained periods of non-flapping flight in which the humerus is held
in an extended position to maintain or gain altitude17–19. Given the lackof quantitative data for most species on the relative time spent using
different flight styles, scores do not depend on the regularity withwhich species soar or the mode of soaring, such that occasional soarers,(for example, the common raven; Corvus corax) and habitual soarers(for example, the turkey vulture; Cathartes aura) are both consideredsoarers (see Methods and Supplementary Table 1).

morphology of the SPD across at least four independent instances of its
evolution (outlines in purple) in gulls (Larus occidentalis; e), procellariiforms(Puffinus griseus; f), pelicans (Pelecanus occidentalis; g) and accipitrimorphs(Cathartes aura (h), B. jamaicensis (i), B. swainsoni ( j) and Haliaeetus
leucocephalus (k)). Note that the SPD passes dorsal to the pectoralis and ventral
to the supracoracoideus. Clade name abbreviations in a: Ac, Accipitrimorphae;
Ch, Charadriiformes; Fa, Falconiformes; Ga, Galloanserae; Gr, Gruiformes;
In, Inopinaves; Ne, Neoaves; Pa, Passeriformes; Pe, Pelecanimorphae;
Pg, Paleognathae; Pr, Procellariiformes; Su, Suliformes.

Using stochastic character mapping analysis20,21 on a time-calibrated
phylogeny (see Methods), we infer at least seven—and potentially as
many as nine—independent origins of soaring flight among our sampleof crown-group birds, and a single loss, in herons (Fig. 2a and Supplementary Information). Additional independent acquisitions of soaring flightare implied by its use in several lineages that could not be sampled forthis study, including tropicbirds22, swifts23 and bee-eaters24. The inferred
evolutionary history of the SPD closely matches that of soaring flight, withat least seven independent origins of the SPD and no losses (Fig. 2b–l).
The only exceptions to the close correspondence between soaring flightand the SPD are the soarers Eudocimus ruber (Extended Data Fig. 5ee) andC. corax (Extended Data Fig. 6w), both of which lack an SPD.
Phylogenetic comparative methods clearly support a correlation
between soaring flight and the presence of the SPD. Pagel’s25 test of
character state dependency strongly supports a model in which the SPD
evolves only in the context of soaring flight (mean Akaike information
criterion (AIC) weight across 1,000 phylogenies = 0.9997). An alternative approach that treats discrete character evolution as contingent on
an underlying, unobserved continuous trait26 likewise favours a strong
correlation between soaring flight and the SPD (the 95% highest posterior density for the value of the correlation coefficient (0.60–0.98)
is well-removed from zero) (Extended Data Fig. 8; see Methods andSupplementary Information for further discussion).
Nature | Vol 630 | 20 June 2024 | 673

Article
**

60

40

20

c

2,000

120
CranialNormalized fascicle length(mm per kg0.33)

b80

PCSA (mm2 per kg0.66)

M1
M2
M3

1,500

1,000

M4

500
Caudal

*

100

*

80
60

**
***

**
***

**
**
**

***
***
**

40
20
0

Bird group F(1) = 53.78, P < 0.001
Compartment F(3) = 11.216, P < 0.001

M1

So
a

Cranial

M2

M3

M4
Caudal

N

on
-s
oa

rin
g

rin
g

rin
g

So
a

on
-s
oa
N

rin
g

0

0

d

e

Seleucus SPD inflated

Mean moment arm (mm)Normalized fascicle length(mm per kg0.33)a

SPD inflated
Real > straight10

SPD deflated
Real < straight

f

8

6

Ptolemy real
(wrapped across the SPD)4

2

SPD function and the pectoralis
The strong correlation between the multiple independent instances ofsoaring flight and the presence of the SPD suggests that the diverticulum is a functionally important component of the flight apparatus insoaring birds. We observed considerable displacement of the pectoralis
muscle during artificial inflation and deflation of the SPD in deceased
Buteo specimens as well as during natural sedated ventilation in live
birds, indicating that there might be a functional interaction between
these structures. Given the close correspondence between flight ecology and the presence of the SPD across Neornithes (Fig. 2 and Extended
Data Figs. 5 and 6), we compared the muscle architecture of the pectoralis in the soarer B. jamaicensis and in four species of non-soaring
birds (see Methods), and used MDA models to quantify the effects ofthe presence and the relative inflation of the SPD on pectoralis function(Fig. 3 and Extended Data Figs. 8 and 9). We note that our artificial inflation of the SPD produces near-maximum expansion of the diverticulum(see Methods) and might not reflect the natural inflation levels of the
SPD in a live bird that is actively soaring, which would be extremely
challenging to measure and remains unknown.
In soarers, the relatively static, horizontal positioning of the wing
should favour specialization of the pectoralis for force generation,
which is accomplished by having relatively short muscle fascicles

R
St eal
ra
ig
ht
na
xa

ac
Ly

si

m

Ptolemy straight path
(passes through the SPD)

Ro

hu

s

n
ho
rc
pe
ly
Po

Fig. 3 | Functional interaction between the SPD and the pectoralis muscle
in soaring and in non-soaring birds. a,b, B. jamaicensis (n = 3 individuals)
has significantly shorter fascicle lengths (n = 80 fascicles per muscle) in the
pectoralis muscle (a) but similar PCSAs (b), compared with non-soaring birds(n = 4 species) when normalized to body mass (body mass0.33 in a, body mass0.66
in b). Significance was determined by an independent samples t-test. c, The
pectoralis muscle in soaring hawks (filled circles) and non-soaring birds(open circles) shows strong cranial-to-caudal variation in fascicle length, withsignificant increases in caudal regions (M3–M4; n = 20 fascicles per region ineach muscle). Error bars in a–c represent one standard deviation about themean. Soaring birds had significantly shorter fascicles in all regions, and theshortest fascicles measured here lie towards the furcula and cranial sternal
region in association with the SPD (M1–M2). Statistical significance was674 | Nature | Vol 630 | 20 June 2024R
St ealraig
htRSt ealraig
htRSt eal
ra
ig
ht
y
em
ol
Pt

Se

le

uc

us

In

fla
De tedfla
ted0
Seleucus SPD deflated

determined by two-way factorial ANOVA: *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001.
d–f, MDA models suggest that a large, inflated SPD in soaring hawks increases
muscle moment arms and lengths of the pectoralis muscle. This is shown bycomparison of mean moment arms in the same bird (Seleucus) when the SPD
is inflated versus deflated (d,e), and in other birds in which muscle paths
constructed directly from CT data (‘real’) are compared to hypotheticalstraight-line muscle paths (e,f). In birds in which the SPD is well-inflated, suchas Ptolemy (f), real muscle paths are convex and pass lateral to hypotheticalmuscle paths, which bisect the SPD and have smaller mean moment arms. In
birds with modestly inflated or deflated SPDs, such as Lysimachus and Roxana,
real muscle paths are relatively straight or concave and thus are similar or passmedial to straight-line muscle paths.

operating under isometric conditions27,28. By contrast, non-soaring
species should benefit from a pectoralis with a relatively greater
capacity for power and displacement: relatively long fascicles areadvantageous for increasing shortening velocity and allowing force
generation over longer contractile ranges27,28, which is importantfor decelerating the wing during the upstroke and increasing thepower of the downstroke28. We found that, at a whole-muscle level, thepectoralis muscle of B. jamaicensis had significantly shorter fascicle
lengths but similar physiological cross-sectional areas (PCSAs) tothose of non-soaring birds when normalized for body size (Fig. 3a–c
and Extended Data Fig. 8). This difference is consistent with thedisparate mechanical demands of force-based soaring versus morework- and power-based active flight styles. We also identify a clear
heterogeneous distribution in fascicle lengths throughout the pectoralis muscle in all birds. Muscle fascicle length increases caudally
from the pectoralis-pars sternobrachialis to the pectoralis-parsthoracobrachialis (Fig. 3a–c and Extended Data Fig. 8). Within each
subregion, the fascicles are consistently shorter in B. jamaicensis
than in non-soarers (reflecting the whole-muscle trends; Fig. 3a–c
and Extended Data Fig. 8), such that the shortest fascicles measured
here are found arising from the furcula and the cranial part of thesternum, where they surround the large SPD (Fig. 3a–c and Extended
Data Fig. 8).

Three-dimensional MDA models suggest that possessing a large,
inflated SPD exaggerates the disparate functional specialization in
the architecture of the pectoralis muscle that is seen in soaring birds(Fig. 3a–c and Extended Data Fig. 9) by increasing muscle lengths
and moment arms (Fig. 3d–f and Extended Data Fig. 9). The complex
topology of the SPD is shown to directly influence the moment arm andlength of the components of the pectoralis muscle with attachmentson the furcula and cranial sternum, contributing to regional (cranial–
caudal) heterogeneity in its leverage for depression and medial rotation of the humerus in the downstroke (Fig. 3d–f and Extended Data
Fig. 9). Birds that had a more inflated SPD also tended to have higher
absolute pectoralis moment arms (for example, Ptolemy versus Roxana), even when they were considerably smaller-bodied than were individuals with deflated SPDs (Fig. 3d–f and Extended Data Fig. 9). For one
B. jamaicensis individual (Seleucus), we constructed two models basedon µCT scans collected at varying levels of SPD inflation, which showthat the pectoralis moment arm is increased cranially when the SPD is
inflated (Fig. 3d,e and Extended Data Fig. 9j). In birds that were scanned
at a single level of SPD inflation (see ‘Anatomical and imaging methods’ and Extended Data Table 1), we compared real muscle momentarms with hypothetical ‘straight-line’ reconstructions that ignore theeffect of the SPD on the muscle’s path from origin to insertion. In birds
with an inflated SPD, these straight-line reconstructions tend to havelower moment arms than real muscle lines of action, which wrap in aconvex pattern across the surface of the SPD (Fig. 3e–f and Extended
Data Fig. 9). By contrast, in birds in which the SPD was more deflated,straight-line paths generally had higher moment arms than real muscle
lines of action, which take on a more concave shape across the surface ofthe deflated SPD (Fig. 3d–e and Extended Data Fig. 9). Therefore, thesehypothetical straight-line paths provide conservative estimates of theeffects of full SPD inflation–deflation on the moment arm of the pectoralis in our scanned specimens. Larger pectoralis moment arms caused
by SPD inflation will yield increased torque at the shoulder for a given
level of muscle contraction, thereby increasing the mechanical advantage of the lever system. The trade-off—reduced angular velocity—
is expected to be relatively inconsequential to soaring birds, which
typically need to maintain a fixed (non-flapping), horizontal humeral(or wing) position. In flapping flight, this mechanical trade-off could,
in theory, be circumvented by deflating the SPD. These results bearout speculative suggestions made in some of the earliest comparative
studies of the avian respiratory system. Both Strasser11 and Owen29
hypothesized that pulmonary diverticula could offer mechanical benefits for joints of the wing, but whereas the pneumatic bracing effectproposed by Owen is distinct from the mechanism we demonstratehere, Strasser appears to have independently suggested a role foraxillary diverticula in improving the mechanical advantage of flightmuscles.
Larger moment arms and longer muscle lengths induced by SPD
inflation will also induce relatively greater stretch in muscle fascicles
in the cranial part of the pectoralis muscle, where muscle fascicles areshortest (Fig. 3 and Extended Data Figs. 8 and 9). A large, inflated SPD
and the distinctive architecture of the pectoralis muscle observedin Buteo therefore potentially represent an example of a pneumatic
system enhancing pectoralis function during soaring flight, in which
there is limited downward movement of the wing to compress thespace deep to the pectoralis and deflate the SPD. The static horizontal
humeral position in soaring means that the pectoralis resists wing
elevation (and supports the body) through near-isometric contraction, which might be made even more economical by an inflated SPDthat increases the pectoralis moment arm, thereby increasing muscle
torque at the shoulder. The transmural pressure of an inflated SPD
might resist the shortening of the pectoralis muscle, reducing theshortening velocity and increasing the force-generating capacity.
This dynamic expansion of the SPD and reduced fascicle shortening would maintain sarcomere length on the plateau of the muscle’s

force–length curve, and might also function to modify the pennation angle of the fascicles and their multiple internalized tendons,
further modulating muscle gearing30 to improve performance. These
combined SPD–pectoralis interactions provide myriad mechanisms
through which to dynamically modify architecture to overcome
sarcomere-level constraints, and might theoretically contributeto reducing the ATP turnover rate31; however, these hypotheticalmechanisms fall outside the scope of the current study and warrant
further investigation.

Conclusions
The repeated evolution of soaring flight and the SPD across phylogenetically disparate lineages of birds strongly suggests that this structure
has an adaptive and functional relationship with a soaring ecology.
Although it is possible that the co-occurrence of these traits is instead
explained by other factors (for example, developmental mechanisms),
we suggest that our data provide the most parsimonious explanation.
Using two species of soaring hawk as a model, we traced the extent
of the SPD between and beyond the two primary flight muscles, thepectoralis and supracoracoideus, and showed that it is not integral
to ventilation. Our comparison of muscle fascicle architectural datain soaring hawks and non-soaring birds, and the application of MDA
biomechanical modelling, provides an explanatory mechanism forthis evolutionary pattern. We propose that the joint presence of the
SPD and a relatively short-fascicled pectoralis muscle optimizes theflight apparatus for soaring by influencing contractile dynamics andmuscle leverage, thereby improving the torque-generating capacity ofthe pectoralis when the wing is held in a static, horizontal position. It is
not clear whether soarers can trap air in the SPD voluntarily by mechanical means—that is, by positioning of the humerus or by muscularcompression of the opening between the diverticulum and the interclavicular sac32—and this is an avenue for future experimental investigation. The discovery that the respiratory system has a role in the flightmechanics of soaring birds suggests that avian pulmonary diverticula
might have other unknown secondary functions independent of ventilation and respiration. Our results also substantially modify the current
paradigm that describes the functional linkage between locomotion
and ventilation in birds, whereby the flapping movements of the wingbeat cycle enhance respiration33,34. We demonstrate the inverse: thepulmonary apparatus itself alters and enhances flight-muscle mechanics. These data provide a mechanistic explanation for the consistentappearance of the SPD in soaring groups, and suggest that the respiratory system has had a notable role in shaping the evolution of the flightapparatus in birds.

Online content
Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions
and competing interests; and statements of data and code availability
are available at https://doi.org/10.1038/s41586-024-07485-y.
1.
2.
3.
4.
5.
6.
7.

Duncker, H. R. The lung air sac system of birds. A contribution to the functional anatomyof the respiratory apparatus. Ergeb. Anat. Entwicklungsgesch. 45, 1–171 (1971).
King, A. S. in International Review of General and Experimental Zoology Vol. 2 (eds Felts, J. L.
& Harrison, R. J.) 96 (Academic, 1966).
Maina, J. N. What it takes to fly: the structural and functional respiratory refinements inbirds and bats. J. Exp. Biol. 203, 3045–3064 (2000).
Lockner, F. R. & Murrish, D. E. Interclavicular air sac pressures and vocalization in mallard
ducks Anas platyrhynchos. Compar. Biochem. Physiol. A 52, 183–187 (1975).
Plummer, E. M. & Goller, F. Singing with reduced air sac volume causes uniform decreasein airflow and sound amplitude in the zebra finch. J. Exp. Biol. 211, 66–78 (2008).
Akester, A. R., Pomeroy, D. E. & Purton, M. D. Subcutaneous air pourches in the Marabou
stork (Leptoptilos crumeniferus). J. Zool. 170, 493–499 (1973).
Rusli, M. A brief report on the development of dorsal air sacs in hand reared Von der
Decken’s hornbills (Tockus deckeni). Avian Biol. Res. 13, 87–91 (2020).

Nature | Vol 630 | 20 June 2024 | 675

Article
8.

Daoust, P.-Y., Dobbin, G. V., Ridlington Abbot, R. C. F. & Dawson, S. D. Descriptive anatomyof the subcutaneous air diverticula in the northern gannet Morus bassanus. Seabird 21,
64–76 (2008).
9.
Richardson, F. Functional aspects of the pneumatic system of the California brown
pelican. Condor 41, 13–17 (1939).
10. Groebbels, F. Der Vogel Vol. I (Borntraeger, 1932).
11. Strasser, H. Ueber die Luftsäcke der Vögel. Gegenbaurs Morphol. Jahrbuch 3, 179–225(1877).
12. Biewener, A. A. Muscle function in avian flight: achieving power and control. Philos. Trans.
R. Soc. B 366, 1496–1506 (2011).
13. Hamlet, M. P. & Fisher, H. I. Air sacs of respiratory origin in some procellariiform birds.
Condor 69, 586–595 (1967).
14. Ulrich, F. in Wissenschaftliche Ergebnisse der deutschenTiefsee-Expedition auf dem
Dampfer “Valdivia” 1898–1899 Vol. 7, 319–342 (1904).
15. Brackenbury, J. H. Airflow dynamics in the avian lung as determined by direct and indirect
methods. Respir. Physiol. 13, 319–329 (1971).
16. Bretz, W. L. & Schmidt-Nielsen, K. Bird respiration: flow patterns in the duck lung. J. Exp.
Biol. 54, 103–118 (1971).
17. Bruderer, B., Peter, D., Boldt, A. & Liechti, F. Wing-beat characteristics of birds recordedwith tracking radar and cine camera. Ibis 152, 272–291 (2010).
18. Lovette, I. J. & Fitzpatrick, J. W. (eds) The Cornell Lab of Ornithology Handbook of Bird
Biology (Wiley, 2016).
19. Pennycuick, C. J. Modelling the Flying Bird (Elsevier, 2008).
20. Huelsenbeck, J. P., Nielsen, R. & Bollback, J. P. Stochastic mapping of morphologicalcharacters. Syst. Biol. 52, 131–158 (2003).
21. Revell, L. J. Two new graphical methods for mapping trait evolution on phylogenies.
Methods Ecol. Evol. 4, 754–759 (2013).
22. Garde, B. et al. Thermal soaring in tropicbirds suggests that diverse seabirds
may use this strategy to reduce flight costs. Mar. Ecol. Progr. Ser. 723, 171–183(2023).
23. Hedrick, T. L., Pichot, C. & de Margerie, E. Gliding for a free lunch: biomechanics offoraging flight in common swifts (Apus apus). J. Exp. Biol. 221, jeb186270 (2018).

676 | Nature | Vol 630 | 20 June 2024

24. Sapir, N., Wikelski, M., McCue, M. D., Pinshow, B. & Nathan, R. Flight modes in migrating
european bee-eaters: heart rate may indicate low metabolic rate during soaring andgliding. PLoS One 5, e13956 (2010).
25. Pagel, M. Detecting correlated evolution on phylogenies: a general method for thecomparative analysis of descrete characters. Proc. R. Soc. B 255, 37–45 (1994).
26. Felsenstein, J. A comparative method for both discrete and continuous characters using
the threshold model. Am. Nat. 179, 145–156 (2012).
27. Charles, J., Kissane, R., Hoerhurtner, T. & Bates, K. T. From fibre to function: are we accurately
representing muscle architecture and performance? Biol. Rev. 97, 1640–1676 (2022).
28. Tobalske, B. W. Biomechanics of bird flight. J. Exp. Biol. 210, 3135–3146 (2007).
29. Owen, R. in The Cyclopædia of Anatomy and Physiology Vol. 1 (ed. Todd, R. B.) 265–358(Sherwood, Gilbert and Piper, 1836).
30. Azizi, E., Brainerd, E. L. & Roberts, T. J. Variable gearing in pennate muscles. Proc. Natl
Acad. Sci. USA 105, 1745–1750 (2008).
31. Smith, N. P., Barclay, C. J. & Loiselle, D. S. The efficiency of muscle contraction. Prog.
Biophys. Mol. Biol. 88, 1–58 (2005).
32. Casler, C. L. The air-sac systems and buoyancy of the anhinga and double-crested
cormorant. Auk 90, 324–340 (1973).
33. Boggs, D. F. Interactions between locomotion and ventilation in tetrapods. Compar.
Biochem. Physiol. A 133, 269–288 (2002).
34. Boggs, D. F., Jenkins, F. A. Jr & Dial, K. P. The effects of the wingbeat cycle on respiration inblack-billed magpies (Pica pica). J. Exp. Biol. 200, 1403–1412 (1997).
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to thisarticle under a publishing agreement with the author(s) or other rightsholder(s); author
self-archiving of the accepted manuscript version of this article is solely governed by theterms of such publishing agreement and applicable law.
© The Author(s), under exclusive licence to Springer Nature Limited 2024

Methods
Anatomical and imaging methods
To visualize the extent and variability of the SPD intraspecifically, we
examined seven red-tailed hawks (B. jamaicensis) and two Swainson’s
hawks (B. swainsoni) (Extended Data Table 1). The birds were named forancient Macedonian individuals to protect the anonymity of the client
birds and for ease in identifying the specimens. The wild specimens
were collected as salvage specimens on US Fish and Wildlife ScientificCollecting permits to E.R.S.: MB90135C (Louisiana State University)and MBPER8930896 (University of Florida). They were obtained from awildlife sanctuary, frozen immediately post-mortem and exhibited nosigns of pulmonary decay or disease; the imaging data were evaluatedby a board-certified avian clinician (M.S.E.) for the latter. Sample size
was based on specimen availability. Owing to small sample size andstudy design for the entire study, no randomization or blinding wasdone. One B. jamaicensis individual (Alexander) was imaged at the
Research Service Centers of the Herbert Wertheim College of Engineering at the University of Florida (RRID:SCR_025135) on a Zeiss Versa 620
X-ray microscope and two (Ptolemy and Lysimachus) were imaged at
the University of Southern California Molecular Imaging Center on a
Nikon XT H 225 ST. Two of these birds (Ptolemy and Alexander) wereintubated with polyethylene tubing via a tracheotomy. A horizontal cut
was made on the ventral aspect of the trachea approximately 2 cm distal
to the glottis, the tubing was inserted and sutured in, a 60-cc syringe
was connected to the tubing through a stopcock and the respiratory
system was carefully inflated until slight resistance was felt. The third
B. jamaicensis specimen, Lysimachus, was imaged at a natural end tidal
volume with no intubation. Before imaging, all birds were placed vertically in a polymer tube and held in place with bubble wrap. The remaining Buteo specimens were scanned at Parrish Creek Veterinary Hospitalon an Epica Vimago 3030 for clinical purposes unrelated to this study.
Imaging data (DICOM stacks) were segmented into surface modelsof the lungs and skeleton in the scientific visualization program Avizo(v.7.1 and 2020.1; Thermo Fisher Scientific) with a Wacom Intuos Pro
tablet, using a combination of established thresholding and manual
methods35,36. The SPD, pectoral girdle and humeri were separated out
in six individuals of B. jamaicensis and B. swainsoni, exported as .stl
files and used for the modelling portion of this study (see ‘Momentarms and muscle architecture’).
To visualize the SPD grossly, three B. jamaicensis individuals (Ptolemy,
Lysimachus and Harpalus) were dissected. These birds were intubatedfollowing the methods described above. The skin was removed along
the chest wall and reflected cranially to expose the pectoralis muscles, and then the pectoralis muscles were reflected from their originsites along the sternal keel, revealing the caudoventral margin of theinflated SPD (Fig. 1 and Extended Data Fig. 1c,d). The birds were thencarefully inflated and deflated to visualize air moving in and out of thediverticulum.
Aside from three of the naturally deceased red-tailed hawks (Ptolemy,
Lysimachus and Alexander), all of the birds were imaged for reasons
unrelated to this study. For a complete list of taxa, their scanning site,
status at the time of scan (live or deceased) and provenance, see Supplementary Table 1. The scans of live (and a few deceased) birds wereimaged for clinical purposes from 2019–2021 at Parrish Creek Veterinary Hospital by M.S.E. and thus no Institutional Animal Care and Use
Committee was required. Some of the scans of deceased birds wereimaged at the University of Utah South Jordan Medical Center in 2012
for a separate project. These specimens were originally obtained fromdonations to the Society for the Prevention of Cruelty to Animals in
Monterey County, and all specimens have since been skeletonizedand housed in the Museum of Vertebrate Zoology at the Universityof California, Berkeley. Evaluation of the presence or absence of the
SPD across Aves was performed from CT or µCT imaging data in the
DICOM editing programs OsiriX MD and Horos using the Multiplanar

Reconstruction (MPR) viewer36. To select and export the axial image
for each bird, the viewer was aligned directly parallel to the thoracic
vertebrae in both the coronal and the sagittal view (Extended Data
Fig. 4b–d,f). The axial slice was selected at the juncture at which thesternal keel emerges from the sternal plate and the coracoids articulate(Extended Data Fig. 4a,e), a scale bar was added and the image wasexported as a TIFF file. To ensure that a possible collapsed SPD was not
misinterpreted as absent, we scrolled through the entire image stack,from the cervical region to the pelvis, and checked three orthogonalviewing planes. In birds in which the SPD was voluntarily collapsed(for example, in some of our live hawks) at the time of imaging or had
naturally collapsed post-mortem, evidence of the diverticulum remains
visible on the scan either as a clear demarcation of intermuscular fascia
or as a small bubble of trapped air. In addition, in some of the captive
birds that were diagnosed as obese by a board-certified avian specialist(M.S.E.), the SPD was evident even when completely collapsed (see, forexample, the Vultur gryphus in Extended Data Fig. 6e).

Character correlation analysis
Sources of character data. In most cases (57/68 species), the absenceor presence of an SPD was assessed directly from µCT or CT image
stacks by two of us (E.R.S. and A.J.M.; see above). For the remaining
species (11/68), the presence of an SPD was determined by recourse to
published studies with explicit anatomical descriptions and/or clear
illustrations of the diverticula surrounding the shoulder joint and adjacent bones (Supplementary Table 1).
Our dataset comprises nearly all major lineages of soaring birds.
Soaring lineages that could not be sampled for this study include tropicbirds22, swifts23 and bee-eaters24, which together suggest that there areat least three other instances of soaring flight beyond those identified
here (see above). Like other authors interested in the morphofunctionalcorrelates of flight mode37,38, we do not discriminate between modesof soaring (that is, slope soaring, thermal soaring and dynamic soaring18). We scored species in our dataset as exhibiting soaring flight ifthey have been observed making use of any mode of soaring, because
for this study, we predicted that the general use of soaring flight would
have a role in the development of the SPD. Species-specific sources forflight mode are provided in Supplementary Table 1.
Phylogeny. All phylogenetic comparative analyses were conductedusing time-calibrated phylogenies that closely match the hypothesis ofneornithine relationships produced in a previous study (Prum et al.)39.
Following R code provided previously40, subclades from 1,000 trees
extracted from the pseudo-posterior distribution of taxonomicallycomprehensive phylogenies of Jetz et al.41 were grafted onto the Prumet al.39 backbone phylogeny, combining the species-level resolution of
the former with the rigorously estimated topology and timescale of thelatter. The resulting trees of 9,993 species were pruned to match the
68 species in the dataset using the treedata function in the R package
geiger42.
Reconstruction of ancestral state. We reconstructed the evolutionary histories of both SPD development and soaring flight using stochastic character mapping analysis20,21. For each trait, we calculated amodel-averaged instantaneous transition rate matrix on the basis of
the Akaike weights of the four competing models of evolution applicable to binary characters (that is, equal rates, different rates and twoalternative forms of irreversible transitions), and used this matrix to
produce a sample of 1,000 discrete character histories. We visualized
the evolutionary history of each character by mapping the posterior
density of the character state from the aggregated character histories
onto the maximum clade credibility tree of the 1,000 grafted phylogenies in the R package phytools43. We used pie charts to illustrate theposterior probability of a character-state transition along brancheswith probabilities greater than 0.1 (Extended Data Fig. 7 only).

Article
Analytical approach. Previous work has shown that character correlation tests are susceptible to high rates of type I error, caused by a
failure of existing methods to adequately account for phylogenetic
pseudoreplication44,45. Particularly troubling are instances in whichsuch tests yield strong statistical significance for two discrete traits
that are perfectly co-distributed but share only a single point of origin (‘Darwin’s scenario’), or in which one trait exhibits a single origin
and the other arises numerous times within that clade (‘unreplicatedbursts’)44. There is at present no quantitative solution to this problem,
but vigilance about selecting appropriately complicated scenariosthat provide compelling prima facie evidence for correlation—that is,selecting case studies well-removed from the boundary condition represented by Darwin’s scenario—remains important. Our dataset implies
that there are at least seven origins of the SPD within phylogeneticallydisparate soaring lineages; this pattern most closely resembles the‘replicated co-distribution’ scenario of Maddison and FitzJohn44, andprovides a reasonable a priori expectation of a statistically significant
relationship.
We tested for a correlation between soaring flight and the presenceof the SPD using two methods of discrete character correlation: Pagel’s
method25 and Felsenstein’s26 implementation of the threshold model.
Pagel’s approach25 tests whether the rate of evolution in one binary
character depends on the state of another binary character. We hypothesize that the evolution of the SPD is biomechanically advantageousin species that make occasional or regular use of soaring flight, andtherefore only considered a scenario in which the rates of SPD evolution
depend on the presence of soaring flight (and not the reverse). We used
the AIC to compare the fit of four possible models; that is, independentand dependent models of trait evolution in which transition rates were
either symmetrical (‘SYM’) or differed across all possible transitions(‘ARD’). Model fitting was conducted using the fitPagel function in the
R package phytools (ref. 43). To incorporate phylogenetic uncertainty
in topology and branch length, we compared competing models acrossthe full distribution of 1,000 phylogenetic trees.
Pagel’s test25 has been shown to be particularly susceptible to type I
error, including in Darwin’s scenario44, and thus on its own provides aweak test of character correlation. We therefore also used the threshold approach of Felsenstein26, which is rather distinct from Pagel’s25
method. Whereas the latter model depends on the instantaneous
rates of discrete character-state changes under a continuous-timeMarkov process, the threshold approach characterizes the covariationof discrete characters by assuming that the state of each character isdetermined by an underlying, unobserved continuous trait called
liability, the value of which causes a character-state transition whenever it crosses a threshold. The threshold method models liability
under Brownian motion; thus, unlike the ‘memoryless’ Markov process that underlies Pagel’s25 test, lineages that have recently evolved
a new character state under the threshold model are more likely torevert to the ancestral condition early in their history. The thresholdmodel therefore has the benefit of being more biologically realistic(at least for many traits46) and has been demonstrated in simulation to
be unbiased26. However, modelling unobserved continuous variationin liabilities based only on discrete trait scores at the tips necessarily
results in a substantial loss of information about phenotypic covariation. For this reason, the threshold model suffers from limited power,
as has been demonstrated in simulation for relatively small datasets(n = 100 tips) with modest (r = −0.51–0.56) correlation26. The method is
also computationally intensive, requiring the use of Bayesian Markov
chain Monte Carlo machinery to estimate the correlation coefficientof the liabilities.
We implemented the threshold model in the R package phytools43.
Because of the greater computational burden, we performed the threshold test only on the maximum clade credibility tree of the 1,000 grafted
phylogenies. We ran four independent chains for 10,000,000 generations each, sampling every 1,000 generations, with the first 20% of eachchain discarded as burn-in. Post-burn-in chains were combined, andthe significance of the correlation was assessed by demonstrating thatthe 95% highest posterior density interval of the correlation coefficient
did not include zero.

Moment arms and muscle architecture
Muscles were dissected from seven birds: four non-soaring taxa (barnowl (Tyto alba), Eurasian blackbird (Turdus merula), chicken (Gallus gallus domesticus) and grey partridge (Perdix perdix)) and threesoaring red-tailed hawks (B. jamaicensis; Harpalus, Lysimachus andPtolemy; see Extended Data Table 1). The four non-soaring species
were donated to the Liverpool World Museum for use in research andeducation. Whole pectoralis muscles were weighed and then fixed in
4% paraformaldehyde for 24 h at resting muscle length for muscle fascicle processing47. The pectoralis muscle of birds comprises a complex
neuromuscular network48 that allows the differential recruitment ofmuscle compartments during different flight strategies48. This complex
neuromuscular arrangement gives rise to multiple muscle compartments across the cranial–caudal axis of the pectoralis muscle. Subsequently, we divided the pectoralis into four compartments (M1–M4;
Extended Data Fig. 8) running along the length of the muscle, with M1
corresponding to the most cranial pectoralis-pars sternobrachialis andM4 the caudal pectoralis-pars thoracobrachialis. The four fixed muscle
compartments were subsequently immersed in 30% nitric acid to digest
the connective tissue and free muscle fascicles. Digestion was terminated by transferring the fascicles to a 50% glycerol solution. A totalof 80 muscle fascicles were measured from each pectoralis (20 fromeach compartment: 10 superficial, 10 deep), allowing for the calculation of whole-muscle and compartmental fibre architecture, the latter
considered along the cranial–caudal axis and the superficial–deep axis.
PCSAs were calculated using the standard formula27
PCSA = (muscle mass × cosine θ)/(density × fascicle length),where θ is the pennation angle and muscle density is assumed to be
1,056 kg m3 (ref. 27). Statistical differences in fascicle architecture were
quantified using either a two-way factorial ANOVA (SPSS v.28.0.1.1, IBM)or a one-way ANOVA, with a Bonferroni post-hoc test for significant
difference at the P < 0.05 level. Differences in fascicle length across thenon-soaring and the soaring groups were compared using an independent samples t-test (Supplementary Table 2).
MDA models were constructed for the hawks in the open-source
forward dynamics simulation package GaitSym, following our previous
methods49,50. Models consisted of a pectoral girdle segment (sternum,
furcula and scapula) and a left and/or right humeral segment, whichwere segmented from μCT scans (see ‘Anatomical and imaging methods’). The mass properties of these segments were represented byminimum convex hulls51; these parameters did not directly influence
the specific numerical metrics analysed here, but were required by
the simulator. The SPD of each bird was visualized within the modelling software to directly guide muscle strands (using via points49) that
represented the deepest portions of the pectoralis muscle that interface
with the SPD. We also constructed hypothetical straight-line muscle
strands to compare with our accurate representations of deepest portions of the pectoralis muscle, to demonstrate the qualitative effect thatvarying levels of SPD inflation have relative to hypothetical ‘neutral’muscle paths that ignore the real influence of the SPD. In other words,
hypothetical straight-line reconstructions provide an approximate
estimate of what pectoralis moment arms would be in these birds if
the SPD were highly reduced or absent.
Typically, MDA models are constructed after body segments havebeen articulated into a predefined anatomically neutral posture, inwhich orthogonal functional axes (for example, flexion–extension,
abduction–adduction and so on) are agreed, by consensus in the anatomy literature, to be at zero values49,50. This facilitates the systematiccomputation of moment arms (and other parameters) across specificjoint-angle ranges and standardized comparisons across taxa49,50. However, in this study we were interested in the effect of the SPD on muscle moment arms and lengths. Because these are determined by the
inflation and deflation of the SPD directly, they can only be accurately
measured in the specific posture in which the birds were scanned. Thatis, rotation of the wing will be likely to lead to soft-tissue deformationthat alters the relationship between muscle paths, the inflated–deflated
diverticulum and the joint centre, which cannot be accurately predictedby rigid-body MDA models. We therefore constructed MDA models inthe specific postures in which the birds were scanned, and we present
muscle data only at these specific scanned postures. This carries two
limitations. First, posture is qualitatively similar but is not quantitatively standardized across birds, which limits absolute comparisons ofmuscle moment arms across individuals. However, such comparisons
are not the goal of this study, and thus this limitation is of little consequence here. Second, it makes the specification of traditional joint functional axes (for example, flexion–extension and abduction–adduction)
challenging, and this situation was exacerbated by the fact that theentire humerus could not be scanned for all specimens owing to scannersize constraints. We therefore adopted a pragmatic approach to defining the shoulder joint axis of rotation about which pectoralis muscle
moment arms were calculated. For birds in which the complete humerus
was scanned, we used a shoulder joint axis of rotation parallel to theaxis plane running through the centre of the medial and lateral distal
condyles of the humerus, thereby realizing an orientation of motion
in which the distal condyles would be elevated–depressed at 90° to
the ground and the torso of the bird. In birds in which the distal end
was missing from the scan, we attempted to replicate the same joint
axis orientation manually. To assess the effect of alternative joint axis
orientations on our qualitative findings, we experimented with different joint axis orientations in all of our models. In all cases, our relative
conclusions remained unaffected: for example, an inflated SPD yields
higher pectoralis moment arms than does a deflated SPD and/or hypothetical straight-line paths for the pectoralis (mimicking a situation inwhich the SPD is absent and does not alter the path of the pectoralis).

Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

Data availability
All high-resolution images of the axial and transverse µCT slices, codefrom the character correlation analysis and MDA models have been
uploaded to Data Dryad and are available here: https://doi.org/10.5061/
dryad.0k6djhb64.
35. Lawson, A. B., Hedrick, B. P., Echols, S. & Schachner, E. R. Anatomy, variation, andasymmetry of the bronchial tree in the African grey parrot (Psittacus erithacus). J. Anat.
282, 701–719 (2021).

36. Schachner, E. R. et al. Perspectives on lung visualization: three-dimensional anatomical
modeling of computed and micro-computed tomographic data in comparative
evolutionary morphology and medicine with applications for COVID-19. Anat. Rec. https://doi.org/10.1002/ar.25300 (2023).
37. Lowi-Merri, T. M., Benson, R. B. J., Claramunt, S. & Evans, D. C. The relationship between
sternum variation and mode of locomotion in birds. BMC Biol. 19, 165 (2021).
38. Lowi-Merri, T. M. et al. Reconstructing locomotor ecology of extinct avialans: a case studyof Ichthyornis comparing sternum morphology and skeletal proportions. Proc. R. Soc. B
290, 20222020 (2023).
39. Prum, R. O. et al. A comprehensive phylogeny of birds (Aves) using targeted nextgeneration DNA sequencing. Nature 526, 569–573 (2015).
40. Cooney, C. R. et al. Mega-evolutionary dynamics of the adaptive radiation of birds. Nature
542, 344–347 (2017).
41. Jetz, W., Thomas, G. H., Joy, J. B., Hartmann, K. & Mooers, A. O. The global diversity ofbirds in space and time. Nature 491, 444–491 (2012).
42. Pennell, M. W. et al. geiger v2.0: an expanded suite of methods for fitting macroevolutionarymodels to phylogenetic trees. Bioinformatics 30, 2216–2218 (2014).
43. Revell, L. J. phytools: an R package for phylogenetic comparative biology (and otherthings). Methods Ecol. Evol. 2, 217–223 (2012).
44. Maddison, W. P. & FitzJohn, R. G. The unsolved challenge to phylogenetic correlation
tests for categorical characters. Syst. Biol. 64, 127–136 (2015).
45. Uyeda, J. C., Zenil-Ferguson, R. & Pennell, M. W. Rethinking phylogenetic comparative
methods. Syst. Biol. 67, 1091–1109 (2018).
46. Revell, L. J. Ancestral character estimation under the threshold model from quantitative
genetics. Evolution 68, 743–759 (2014).
47. Kissane, R. W. P., Egginton, S. & Askew, G. N. Regional variation in the mechanicalproperties and fibre-type composition of the rat extensor digitorum longus muscle.
Exp. Physiol. 103, 111–124 (2018).
48. Boggs, D. F. & Dial, K. P. Neuromuscular organization and regional EMG activity of thepectoralis in the pigeon. J. Morphol. 218, 43–57 (1993).
49. Bates, K. T. & Schachner, E. R. Disparity and convergence in bipedal archosaur
locomotion. J. R. Soc. Interface 70, 1339–1353 (2012).
50. Dempsey, M., Maidment, S. C. R., Hedrick, B. P. & Bates, K. T. Convergent evolution ofquadrupedalism in ornithischian dinosaurs was achieved through disparate forelimbmuscle mechanics. Proc. R. Soc. B 290, 20222435 (2023).
51. Macaulay, S. et al. Decoupling body shape and mass-distribution in birds and theirdinosaurian ancestors. Nat. Commun. 14, 1575 (2023).
Acknowledgements We thank the Tracy Aviary for allowing us to use retrospective imaging
data of birds that had been scanned for clinical purposes unrelated to this study; J. Stevens forher time aggregating and processing the DICOM data for this project; R. Wilhite and P. Laporte
for assisting with specimens; J. Ziermann and W. Klein for assistance with German translations;
T. Skorka for assistance with imaging; and E. Stanley for intubating and inflating one of thehawks. Funding for this study was provided by the Louisiana State University Health Sciences
Center Research Enhancement Program Fund to E.R.S. and B.P.H. and the University of Florida
Gatorade Award Allocation to E.R.S.
Author contributions E.R.S. conceptualized the study. E.R.S., A.J.M. and K.T.B. designed thestudy. E.R.S. did the anatomical dissections. E.R.S., R.E.D., M.S.E., B.P.H. and J.A. acquired the
CT and µCT data. E.R.S., A.J.M. and A.M. segmented the 3D anatomical models. E.R.S and
A.J.M analysed and scored the CT and µCT data for phylogenetic analysis. A.J.M. completed
the character correlation analysis. K.T.B. completed the biomechanical models, with
anatomical input from E.R.S. R.K performed the muscle fascicle analysis. E.R.S., A.J.M., K.T.B.,
R.K. and B.P.H. wrote the original draft. All authors critically reviewed and contributed to thefinal draft of the manuscript. All authors approved the final version of the manuscript.
Competing interests The authors declare no competing interests.
Additional information
Supplementary information The online version contains supplementary material available athttps://doi.org/10.1038/s41586-024-07485-y.
Correspondence and requests for materials should be addressed to Emma R. Schachner.
Peer review information Nature thanks Daniel Field, Bret Tobalske and the other, anonymous,
reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports areavailable.
Reprints and permissions information is available at http://www.nature.com/reprints.

Article

Extended Data Fig. 1 | Details of intubation and anatomy of the subpectoral
diverticula in the red-tailed hawk (Buteo jamaicensis). a, Adult malespecimen (Ptolemy) in ventral view. b, View of the tracheotomy with a medium
sized polyethylene tube sutured into the cut trachea with silk Ethicon suture,and pectoralis muscle (p) exposed by dissection. c, Magnified view of b with thepectoralis reflected cranially on the left side; arrow indicates the inflated SPD.
d, Right inflated SPD (indicated by the arrow) is exposed as the pectoralis isreflected cranially with forceps. Scale bars, 1 cm.

Extended Data Fig. 2 | Segmented surface models of the SPD in cranial andright lateral views. a–u, The right (turquoise) and left (blue) subpectoral
diverticulum, pectoral girdle and humeri of the red-tailed hawk (B. jamaicensis)(a–e,h–l,o–s) and Swainson’s hawk (B. swainsoni) (f,g,m,n,t,u) in ventral (a–g),left lateral (h–n) and cranial (o-u) views in various states of inflation.
a,h,o, Ptolemy: male, deceased, and artificially inflated via a tracheotomy.

b,i,p, Lysimachus: male, deceased, and deflated (imaged at natural end tidal
volume). c,j,q, Seleucus: male, deceased, and artificially inflated via a
tracheotomy. d,k,r, Seleucus: male, deceased, imaged deflated. e,l,s, Roxana:
female, live, sedated, natural apnoea. f,m,t, Polyperchon: male, deceased, andartificially inflated a tracheotomy. g,n,u, Eurydice: female, live, sedated,natural apnoea with vascular contrast selectively removed from the model.

Article

Extended Data Fig. 3 | Methods for selecting homologous axial (transverse)
slices across Aves. a–c, Volume rendered models of a red-tailed hawk
(B. jamaicensis; Ptolemy) in ventral view with a pink line showing where theaxial/transverse slice is taken on the surface of the bird (a), a keel-billed toucan(Ramphostos sulfuratus) in dorsal view demonstrating the location of thesagittal slice (b) and a Brandt’s cormorant (Phalacrocorax penicillatus)
showing the location of the coronal slice aligned to the thoracic vertebrae (c).

d–f, In all birds, the axial–transverse images are selected using the multiplanarreconstruction (MPR) viewer in the DICOM viewers OsiriX MD or Horos.
The windows are aligned so that the axial slice lines up with where the coracoid
processes articulate with the sternal plate, and the coronal and sagittal slices
are aligned with the thoracic vertebrae. The axial/transverse slice (e) is thenenlarged and exported as a TIFF for analysis. Abbreviations: ax, axial/
transverse; cor, coronal; sag, sagittal.

Extended Data Fig. 4 | Axial µCT images of the SPD in B. jamaicensis andB. swainsoni, showing variation and capacity for differential and asymmetricinflation and collapse of the structure. Each axial slice was selected in themultiplanar reconstruction (MPR) viewer in Horos or OsiriX MD with thewindow aligned perpendicular to the thoracic (dorsal) vertebrae and sternalkeel, and the axial slice was selected at the location where the coracoids
articulate with the sternal plate. a, Ptolemy (B. jamaicensis): adult male,deceased, artificially inflated to maximum inspiratory capacity. b, Lysimachus(B. jamaicensis): adult male, deceased, natural end tidal volume. c, Alexander(B. jamaicensis): sub-adult female, deceased, artificially inflated to nearmaximum inspiratory capacity. d, Seleucus (B. jamaicensis): adult male,deceased, artificially inflated to near/at maximum inspiratory capacity.
e, Roxana (B. jamaicensis): adult female, live and sedated, natural apnoea.
f, Olympias (B. jamaicensis): adult female, live and sedated, natural apnoea;
note the total collapse of her SPD. g, Polyperchon (B. swainsoni): adult male,deceased, artificially inflated to near/at maximum inspiratory capacity, wings
extended. h, Eurydice (B. swainsoni): adult female, live and sedated, naturalapnoea.

Article

Extended Data Fig. 5 | Axial CT and µCT images of the SPD across Aves (part 1).
Each axial slice was selected using the MPR as in Extended Data Fig. 3. Purple
text and arrows indicate presence of the SPD. a, Southern cassowary (Casuarius
casuarius). b, Emu (Dromaius novaehollandiae). c, Common ostrich (Struthio
camelus). d, Elegant crested tinamou (Eudromia elegans). e, Falcated duck(Anas falcata). f, Northern pintail (Anas acuta). g, Redhead (Aythya americana).
h, Spectacled eider (Somateria fischeri). i, Steller’s eider (Polysticta stelleri).
j, Canada goose (Branta canadensis). k, Trumpeter swan (Cygnus buccinator).
l, Tundra swan (Cygnus columbianus). m, Helmeted curassow (Pauxi pauxi).
n, Chicken (Gallus gallus). o, Crested partridge (Rollulus roulroul). p, Anna’s

hummingbird (Calypte anna). q, Tawny frogmouth (Podargus strigoides).
r, Violet turaco (Musophaga violacea). s, Nicobar pigeon (Caloenas nicobarica).
t, Black crowned crane (Balearica pavonina). u, Western grebe (Aechmophorus
occidentalis). v, Chilean flamingo (Phoenicopterus chilensis). w, Black-necked
stilt (Himantopus mexicanus). x, Common murre (Uria aalge). y, Western gull
(Larus occidentalis). z, Pacific loon (Gavia pacifica). aa, Common loon (Gavia
immer). bb, Yellow-billed loon (Gavia adamsii). cc, Sooty shearwater (Puffinus
griseus). dd, Brandt’s cormorant (Phalacrocorax penicillatus). ee, Scarlet ibis
(Eudocimus ruber).

Extended Data Fig. 6 | Axial CT and µCT images of the SPD across Aves (part 2).
Each axial slice was selected using the MPR as in Extended Data Fig. 3. Purple
text and arrows indicate presence of the SPD. a, White pelican (Pelecanus
erythrorhynchos). b, Brown pelican (Pelecanus occidentalis). c, Black crownednight heron (Nycticorax nycticorax). d, Turkey vulture (Cathartes aura).
e, Andean condor (Vultur gryphus). f, Common buzzard (Buteo buteo). g, Baldeagle (Haliaeetus leucocephalus). h, Golden eagle (Aquila chrysaetos).
i, Bateleur (Terathopius ecaudatus). j, Spectacled owl (Pulsatrix perspicillata).
k, Great horned owl (Bubo virginianus). l, Southern ground hornbill (Bucorvus

leadbeateri). m, Green woodhoopoe (Phoeniculus purpureus). n, Curl-crested
aracari (Pteroglossus beauharnaesii). o, Keel-billed toucan (Ramphastos
sulfuratus). p, Gyr/saker falcon hybrid (Falco rusticolus/cherrug). q, Peregrine
falcon (Falco peregrinus). r, Sulphur-crested cockatoo (Cacatua galerita).
s, Blue-and-yellow macaw (Ara ararauna). t, African grey parrot (Psittacus
erithacus). u, Moluccan eclectus parrot (Eclectus roratus). v, Australian zebra
finch (Taeniopygia castanotis). w, Common raven (Corvus corax). x, Blue-faced
honeyeater (Entomyzon cyanotis).

Article

Extended Data Fig. 7 | Ancestral-state reconstruction and statistical testsof a correlation between soaring flight and the SPD. a, The inferred
evolutionary histories of soaring flight (left) and the SPD (right), with pie charts
illustrating the posterior probability (PP) of a character-state transition forbranches with probabilities >0.1. b, AIC values associated with Pagel’s18 test of
discrete character correlation under four competing models. The best-fit
model is one in which the evolution of the SPD depends on the presence ofsoaring flight and character transition rates are asymmetrical (ARD_dep);

in order of decreasing fit, the remaining models include: dependent character
evolution with symmetrical transition rates (SYM_dep); independent character
evolution with symmetrical transition rates (SYM_dep); and independent
character evolution with asymmetrical transition rates (ARD_indep).
c, Posterior distribution of the value of the correlation coefficient (r) for
Felsenstein’s19 implementation of the threshold model. The 95% highest
posterior density (marked by red dashed lines) is well-removed from zero.

Extended Data Fig. 8 | Additional data on the architecture of the pectoralis
muscle in the red-tailed hawk and in non-soaring birds. a, Three specimensof soaring birds (B. jamaicensis) do not differ significantly from each other intheir mean pectoralis fascicle lengths (n = 80 fascicles per muscle), but eachhave significantly shorter fascicle lengths than all four non-soaring species
measured here. b, Functional morphospace (normalized mean fascicle length
versus normalized PCSA) plots of the pectoralis muscle in soaring and nonsoaring birds illustrating their similar PCSAs, but shorter fascicle lengths insoaring birds. This suggests a more force-based architectural specialization
in soaring muscles versus a more power/displacement specialization in

non-soaring species. c,d, The strong cranial-to-caudal variation in fascicle
length noted at the whole-muscle level (see Fig. 3c in main text), with significant
increases in fascicle length in caudal regions, is present in both the superficial(c) and the deep (d) (whiskers denote min and maximum data range, n = 40 per
bird, n = 10 per region) layers of the pectoralis (although note that the overall
fascicle lengths are consistently longer in superficial than in deep parts of themuscle). Open circles indicate non-soaring birds, whereas filled circles indicate
the red-tailed hawks in c,d. Two-way factorial ANOVA test statistics are presented
in c,d, with statistical significance described as *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001.

Article

Extended Data Fig. 9 | See next page for caption.

Extended Data Fig. 9 | Muscle moment arm data from the nine modelledstrands from the deep part of the pectoralis muscle in five hawks.
a–i, Comparisons of real muscle paths (orange bars) that are guided across theinterface between the deepest part of the pectoralis, the SPD and hypotheticalstraight-line paths (blue bars) that deliberately ignore the contribution of the
SPD to pectoralis geometry. Eurydice (a), Lysimachus with the shoulder joint
axis defined as described in the main text (b), Lysimachus with alternative
shoulder joint axis of rotation (c), Polyperchon with the shoulder joint axis
defined as described in the main text (d), Polyperchon with alternative shoulder
joint axis of rotation (e), Ptolemy with the shoulder joint axis defined as

described in the main text (f), Ptolemy with alternative shoulder joint axis ofrotation (g), Roxana based on the right side (h) and Roxana based on the left
side (i). Comparison of moment arms in Lysimachus (b,c), Polyperchon (d,e)
and Ptolemy (f,g) calculated about different joint axis angles illustrates that thequalitative comparisons and conclusions presented here are not sensitive tothe precise angle of shoulder rotation prescribed in the models. j, Comparisonof pectoralis moment arms in Seleucus in the nine deep muscle strands
wrapping across the surface of the SPD when the SPD is inflated versus
deflated.

Article
Extended Data Table 1 | Specimen table for the Buteo species included in this study

All birds were adults except for Alexander, who was a sub-adult. Abbreviations: D, deceased; F, female; L, live; M, male; MIC, maximum inspiratory capacity; NETV, natural end tidal volume.

Last updated by author(s): April 25th, 2024

Reporting Summary
Nature Portfolio wishes to improve the reproducibility of the work that we publish. This form provides structure for consistency and transparency
in reporting. For further information on Nature Portfolio policies, see our Editorial Policies and the Editorial Policy Checklist.
Please do not complete any field with "not applicable" or n/a. Refer to the help text for what text to use if an item is not relevant to your study.
For final submission: please carefully check your responses for accuracy; you will not be able to make changes later.

nature portfolio | reporting summary

Corresponding author(s): Emma R. Schachner

Statistics
For all statistical analyses, confirm that the following items are present in the figure legend, table legend, main text, or Methods section.
n/a Confirmed
The exact sample size (n) for each experimental group/condition, given as a discrete number and unit of measurement
A statement on whether measurements were taken from distinct samples or whether the same sample was measured repeatedly
The statistical test(s) used AND whether they are one- or two-sided
Only common tests should be described solely by name; describe more complex techniques in the Methods section.

A description of all covariates tested
A description of any assumptions or corrections, such as tests of normality and adjustment for multiple comparisons
A full description of the statistical parameters including central tendency (e.g. means) or other basic estimates (e.g. regression coefficient)AND variation (e.g. standard deviation) or associated estimates of uncertainty (e.g. confidence intervals)
For null hypothesis testing, the test statistic (e.g. F, t, r) with confidence intervals, effect sizes, degrees of freedom and P value noted
Give P values as exact values whenever suitable.

For Bayesian analysis, information on the choice of priors and Markov chain Monte Carlo settings
For hierarchical and complex designs, identification of the appropriate level for tests and full reporting of outcomes
Estimates of effect sizes (e.g. Cohen's d, Pearson's r), indicating how they were calculated
Our web collection on statistics for biologists contains articles on many of the points above.

Software and code
Policy information about availability of computer code
Data collection

Data were collected via microCT scanning and dissection

Data analysis

Data were analyzed in R, Horos (https://horosproject.org/), OsiriX MD (https://www.osirix-viewer.com/), Gaitsym (https://www.animalsimulation.org/) and Avizo v. 7.1 and 2020.1 (https://www.thermofisher.com/us/en/home/electron-microscopy/products/
software-em-3d-vis/avizo-software.html) and SPSS v28.0.1.1. GaitSym software and associated tools for viewing and manipulating the
MDA models presented in the manuscript are available from GitHub <http://github.com/wol101>.

For manuscripts utilizing custom algorithms or software that are central to the research but not yet described in published literature, software must be made available to editors andreviewers. We strongly encourage code deposition in a community repository (e.g. GitHub). See the Nature Portfolio guidelines for submitting code & software for further information.

Data
Policy information about availability of data
All manuscripts must include a data availability statement. This statement should provide the following information, where applicable:
April 2023

- Accession codes, unique identifiers, or web links for publicly available datasets- A description of any restrictions on data availability- For clinical datasets or third party data, please ensure that the statement adheres to our policy
All high-resolution images of the axial/transverse μCT slices, code from the character correlation analysis, and MDA models have been uploaded to Data Dryad and1Research involving human participants, their data, or biological material
Policy information about studies with human participants or human data. See also policy information about sex, gender (identity/presentation),and sexual orientation and race, ethnicity and racism.
Reporting on sex and gender

N/A

Reporting on race, ethnicity, orother socially relevantgroupings

N/A

Population characteristics

N/A

Recruitment

N/A

Ethics oversightN/A

nature portfolio | reporting summary

2
nature portfolio | reporting summary April 2023
are available here: https://doi.org/10.5061/dryad.0k6djhb64

Note that full information on the approval of the study protocol must also be provided in the manuscript.

Field-specific reporting
Please select the one below that is the best fit for your research. If you are not sure, read the appropriate sections before making your selection.
Life sciences

Behavioural & social sciences

Ecological, evolutionary & environmental sciences

For a reference copy of the document with all sections, see nature.com/documents/nr-reporting-summary-flat.pdf

Ecological, evolutionary & environmental sciences study design
All studies must disclose on these points even when the disclosure is negative.
Study description

The study is composed of three sections which are mapped out in detail in the methods section. 1) An anatomical part, with
dissection images, 3D segmented surface models from microCT data, and cross-sections of CT data. 2) A phylogenetically-informed
character correlation analysis with select CT images. 3) Data from multibody dynamics analysis models and muscle fascicle analysis.

Research sample

The research samples consisted of red-tailed hawks (n=7; Buteo jamaicensis), Swainson's hawks (n=2; B. swainsoni), and then a broad
phylogenetic survey of 68 species derived from microCT/CT scans. Of the red-tailed hawks, n=3 were dissected and used for tissue
analysis. The four non-soaring specimens were found by members of the public and donated to the Liverpool World Museum for use
in education and/or research.

Sampling strategy

See above regarding hawks. For the phylogenetic survey, n=1 was used for each species due to the difficulty in acquiring scans of live
sedated birds. For the MDA models, we modeled six hawk specimens because each animal had to be segmented by hand which takes
approximately 30-60 hours each.

Data collection

Live imaging data were collected by author MSE for clinical purposes prior to the study. Three of the hawks were scanned
postmortem by ERS as high resolution microCT scans. Three were part of the live clinical dataset imaged by MSE.

Timing and spatial scale

Live birds were imaged for clinical reasons unrelated to this study from 2019 - 2021. DICOM images were donated for this project. A
few of the deceased birds were scanned in 2012, also for a separate project. Hawks that were microCT scanned for this project were
scanned between 2019 and 2022. Dissections were completed in 2022 and 2023.

Data exclusions

N/A

Reproducibility

Code, surface models, and high resolution TIFF files (images of the scans) have been uploaded to Data Dryad. The SI includes the raw
data for the muscle fiber analysis in table format so that it may be reanalyzed by reviewers and others.

Randomization

N/A

Blinding

N/A
Yes

No

April 2023

Did the study involve field work?

2

We require information from authors about some types of materials, experimental systems and methods used in many studies. Here, indicate whether each material,
system or method listed is relevant to your study. If you are not sure if a list item applies to your research, read the appropriate section before selecting a response.

Materials & experimental systems

Methods

n/a Involved in the study

n/a Involved in the study

Antibodies

ChIP-seq

Eukaryotic cell lines

Flow cytometry

Palaeontology and archaeology

MRI-based neuroimaging

Animals and other organisms
Clinical data
Dual use research of concern

nature portfolio | reporting summary

Reporting for specific materials, systems and methods

PlantsAnimals and other research organisms
Policy information about studies involving animals; ARRIVE guidelines recommended for reporting animal research, and Sex and Gender in
Research
Laboratory animals

No laboratory animal were used in this study.

Wild animals

The wild hawk specimens (Buteo jamaicensis and B. swainsoni) were obtained as salvage specimens on US Fish andWildlife Scientific Collecting permit to ERS #MB90135C, as donations from a wildlife sanctuary after they died of naturalcauses. All of the hawks were either sub-adults or adults. Additional deceased birds were imaged in 2012 and were
donated to the Museum of Vertebrate Zoology at the University of California, Berkeley from the Society for thePrevention of Cruelty to Animals in Monterey County.

Reporting on sex

Where possible, sex was identified for our exemplar hawk specimens.

Field-collected samples

No birds were collected from the field.

Ethics oversight

All of the imaging data from live birds included in this study were from clinical scans obtained by veterinary staff at Parrish Creek
Veterinary Hospital in Utah for purposes unrelated to this study, and the DICOM image stacks were donated to the project.

Note that full information on the approval of the study protocol must also be provided in the manuscript.

PlantsSeed stocks

N/A

Novel plant genotypes

N/A

Authentication

N/A

April 2023

This checklist template is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in
the article's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons license and your intended use is not permitted bystatutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

3