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Factors Influencing the Size of Some Internal Organs in Raptors

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Factors Influencing the Size of Some Internal Organs in Raptors j RaptorRes. 30(4):219-223 ¸ 1996 The Raptor ResearchFoundation, Inc. FACTORS INFLUENCING THE SIZE OF SOME INTERNAL ORGANS IN RAPTORS NIGEL W. BARTON AND DAVID C. HOUSTON AppliedOrnithology Unit, Divisionof Evolutionaryand EnvironmentalBiology, Graham Kerr Building, Universityof Glasgow,Glasgow, G12 8Q• Scotland ABSTP•CT.--Thesize of the small intestine, stomach, kidney, liver and heart were compared among raptor speciesand considered in relation to hunting strategyand body size. Speciesrelying on rapid accelerationand maneuverabilityto captureprey in flight, suchas sparrowhawk (Accipiter nisus), goshawk (A. gentilis),and peregrine (Falcoperegrinus), had the smallestdigestive tracts for their size. Species depending more on soaringflight which do not need fast accelerationto capture prey,such as common buzzard (Buteobuteo), red kite (Milvus milvus), and European kestrel (Falco tinnunculus),had heavy digestiveorgans. In the Strigiformes,the same relationship wasfound, and specieswhich hunt by active flight, such as barn owl (Tyt0 alba), and 1ong-earedowl (Asio otus),had significantlylighter digestive tracts than the tawny owl (Strix aluco),a specieswhich mainly hunts from a perch dropping onto its prey from above.Body condition waspositively correlated with organ weights,including the heart, but to a lesserextent with linear measuresof size such as intestine length. KEYWOe. DS: internalorgans; predatory birds; condition. Factoresque infiuencian el tamafio de algunos6rganos internos en rapaces RESUMEN.--EItamafio del intestinodelgado, est6mago, riffones, higado y coraz6n,rue comparadoentre especiesde rapacesque difieren en estrategiade caza y tamafio corporal. Especiesbasadas en r•pida aceleraci6ny maniobrabilidadpara capturarpresas en vuelo, talescomo Accipiternisus, A. gentilisy Falco peregrinusten/an el tracto digestivom•s pequefio para su tamafio corporal. Especiesque dependen m•s bien de un vuelo de planeo y que no necesitande una r•pida aceleraci6n,tales como Buteobuteo, Milvus milvusy Falcotinnunculus, tenlan 6rganosdigestivos m:is pesados.En Strigiformesse encontr6 la misma relaci6n, especiesque cazan por vuelo activo,tales como Tytoalba, Asio otus, ten/an un tracto digestivo significativamentem•s liviano que Atrix aluco,una especieque caza esperandoa la presa desde una percha. La condici6n corporal estuvocorrelacionada positivamente con el pesode los 6rganos,inclu- yendo el coraz6n. [Traducci6n de Ivan Lazo] We previouslyreported that birds of prey show sociation between the length of the small intestine considerablevariation in the length of their small and the predatory strategyshown by a species,and intestines (Barton and Houston 1994) and sug- that after correcting for differences in body size, gestedthat these differences could be due to ad- those species that specialized in the capture of aptationsfor different stylesof predation. We hy- small birds in flight had small intestinallengths up pothesized that raptors which capture prey by to 50% shorter than specieswith a lessactive pred- pouncing on it from abovewould have the longest atory style. The small intestine is responsiblefor gut lengths for efficient digestion while species nutrient absorption and is therefore the organ which need to acceleraterapidly to capture active most likely to effect absorptionefficiency. We have prey, such as predators taking small prey in flight, shown that specieswith short small intestines di- would have very light organ systemsto enable gest their food less efficiently, and as a conse- them to acceleratemore rapidly. Reduction in the quence select only prey items of high energy con- length of the small intestine probably resultsin less tent (Barton and Houston 1993a, 1993b). efficient absorption of food but this could still be This paper considerswhether there may be as- selected for if this disadvantagewere more than sociations between predatory strategy and the compensatedfor by a higher rate of prey capture. weight of other organ systems.We compared the We were able to demonstrate that there was an as- massof the gizzard, proventriculus,small intestine, 219 220 N. BARTON AND D. HOUSTON VOL. 30, NO. 4 Table 1. Mean wet weights (_SD) of internal organs of various raptors expressedas a percentage of total body weight. Means are basedon a combination of the sizesof internal organsof both sexesand samplesizes are given in parentheses. SMALL BODY SPECIES LWER HEART KIDNEY STOMACH INTESTINE M_ASS Kestrel 3.24 -+ 0.46 (14) 1.13 -+ 0.14 (12) 1.14_+ 0.22 (10) 2.62 +- 0.34 (12) 0.75 _+0.42 (18) 180 Merlin 3.24 -+ 0.84 (3) 2.20 +- 0.31 (3) 1.1o (1) 1.56 ___0.32 (3) 0.56 (1) 190 Sparrowhawk 2.63 -+ 0.53 (86) 1.12 +_0.14 (75) 0.81 ___0.12 (59) 0.95 -+ 0.13 (38) 0.63 _+0.28 (22) 220 Peregrine 2.76 +_0.43 (3) 1.81 _+0.05 (3) 0.86_+0.2 (3) 1.24 (1) 0.53 +_0.05 (3) 760 Buzzard 2.15 +_0.47 (5) 0.86 +_0.15 (31) 0.92 +_0.05 (3) 1.48 _+0.41 (4) 0.99 _+2.62 (7) 850 Red Kite -- 1.13 _+ 0.26 (7) -- -- -- 1040 Goshawk 1.65 _+0.36 (50) 0.86 -+ 0.09 (44) -- -- 0.46 (1) 1110 Long-earedowl 2.10 (2) 0.98 -+ 0.28 (6) 0.84 (2) 1.93 (1) 0.75 (2) 270 Barn owl 2.83 (40) 0.87 (38) 0.78 (36) 1.94 (12) 0.42 (2) 310 Short-Pared owl 3.17 (2) 1.22 (2) 0.98 (1) 2.48 (1) 1.64 (1) 350 Tawnyowl 2.55 _ 0.19 (3) 0.71 -+ 0.10 (3) -- 1.91 (2) 1.26 _+0.38 (3) 460 liver, and kidney between eight speciesof Falconi- significant loss in tissue weight (Barton and Houston formes and four Strigiformes.We also examined 1992); we therefore discardedany birds which had obvi- ously lain for some time before being frozen. heart weight in thesespecies to see if this is heavier In order to test our hypothesis,we first needed to cat- in specieswith an energetic form of prey capture. egorize the predatory styleof each raptor species.For the Finally, we considered the correlation between Falconiformes we did this based on literature review of body condition and the size of these organs. the proportion of avianprey in each speciesdiet. Species taking more than 75% avian prey were categorizedas METHODS attackers and included peregrine falcons (Falcoperegn- Most of the birds used in this study had been found nus), sparrowhawks(Accipiter nisus), and goshawks(A. dead and sent to the Institute of TerrestrialEcology as a gentilis),while speciestaking predominantly small mam- part of their program to monitor pesticideresidues. In mals and carrion were categorizedas searchersand in- addition, a sampleof goshawkswas examined which had cluded buzzard (Buteo buteo), red kite (Milvus milvus), been shot under licensefor game preservationand im- and kestrel (Falco tinnunculus)(Brown1978). Attackers ported under license (CITES 038371A). Birds were were specieswhich capture prey by high-speed aerial stored frozen at -20øC for up to 6 wk. It is known that chase,requiring great accelerationand agilitywith an av- freezing has little effect on gut morphology, but pro- erage hunting successof 13% (TemPles1985). Searchers, longed periods between death and freezing can lead to however,were speciescapturing mammalian prey mainly Table 2. Correlation betweenorgan size (g) and index of body condition of variousspecies of raptors.Sample sizes given in parentheses. SPECIES SEX GIZZARD HEART Sparrowhawk M r = 0.71, P= 0.0018 (16) r = 0.89, P= 0.02 (6) F r = 0.53, P= 0.0018 (32) r = 0.63, P= 0.02 (13) Peregrine M r = 0.96, P = 0.03 (4) r = 0.96, P = 0.01 (5) Kestrel M r = 0.55, P = 0.09 (10) -- F -- r = 0.80, P = 0.03 (7) Buzzard M r = 0.93, P = 0.0001 (10) r = 0.87, P= 0.0001 (16) F -- r = 0.77, P= 0.0003 (17) Red Kite M r = 0.69, P = 0.31 (4) r = 0.95, P = 0.004 (6) Goshawk M r = 0.33, P= 0.12 (23) r = 0.58, P= 0.002 (25) F r = 0.44, P = 0.07 (18) r = 0.79, P = 0.0001 (19) Barn Owl r = 0.34, P= 0.19 (16) r= 0.78, P= 0.02 (8) TawnyOwl -- r = 0.72, P = 0.28 (4) DECEMBER 1996 INTERNAL ORGAN SIZE IN RAPTORS 221 by dropping on it from above and not depending so and sternum diagonal length (from base of sternum to heavily on high-speedpowered flight, with higher rates distal point of coracoid) as the best predictors.We used of prey capture (Temeles 1985). For the owl speciesex- the product of these two measuresas our body sizefactor amined, the main difference in hunting strategybetween and used this factor as the covariatein ANCOVA analysis specieswas that barn owls( Tytoalba) and long-earedowls to correct for body size differences.We log-transformed (Asiootus) have an active flight mode with prey mainly data on both axes. The Bonferroni method was used for located while in flight, while tawnyowls (Styixaluco) are pairwise comparison of adjusted treatment means (Day more passivepredators, locating prey from perch sites and Quinn 1989). (Cramp and Simmons 1980). Since our study dealt with evolutionary adaptation of There was some variation in the condition of the birds digestiveorgan size with relation to hunting strategy,we availablefor this study.We used dry weights to record considered including a phylogenetic component in our organ massbecause we did not know the extent to which analysis.However, since the speciessample was small and carcasseshad become dehydrated after death. Wet we had specieswith contrastinghunting strategiesin the weightswere used only from birds known to have died same genus,we decided there would be no advantageto accidentallyand whose carcasseswere collected shortly adding a phylogeneticcomponent. after death. To considerthe contribution that each organ makesto The proventriculus, gizzard, small intestine, liver, kid- total body mass,we used a smaller sample of fresh car- ney, and heart were dissectedfrom 583 carcasses.Stom- cassesto obtain wet weightsof liver, kidney,proventncu- achswere separatedinto the proventriculus,whose func- lus, gizzard, small intestine, and heart.
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