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Burung-burung sering beralih diet mereka secara musiman.Sebagai contoh, unggas air banyak beralih daritinggi karbohidrat makanan Že.g. benih. tinggi proteinŽe.g makanan. air serangga, jaringan tanaman baru.ketika mempersiapkan untuk pembiakan ŽKrapu dan Rei-necke, 1992. Juga, banyak penyanyi pemakan seranggaberalih ke makanan utamanya buah selama migrasiŽEvans, 1966; Herrera, 1984; Izhaki danSafriel, 1989; Bairlein, 1990, 1991; Bairlein danGwinner, 1994; Biebach, 1996. Switch ini Dietdari serangga untuk buah-buahan dapat menghemat energi karenabuah-buahan yang melimpah yang kurang penuh semangatmahal untuk mendapatkan dibandingkan dengan serangga.Buah-buahan mungkin gizi memadai jika hanya lemakcadangan harus diisi. Namun, buah-buahan mungkinmemadai jika burung harus mengisi lemak baik danprotein cadangan selama migrasi karena, dalamumum, buah mengandung relatif sedikit protein ŽBi-ebach, 1996. Perubahan yang dramatis di Diet substratdari, misalnya, kaya protein serangga untukbuah-buahan kaya karbohidrat, menawarkan signifikan fisiologistantangan bagi burung ŽAfik dan Karasov,1995; Karasov, 1996. Sini kami meninjau bagaimanaperubahan dalam diet kualitas mempengaruhi struktur pencernaandan fungsi dalam migrasi burung, dan bagaimana mungkinpengaruh tempo migrasi mereka.2. fenotipik plastisitas dan fleksibilitas dalamfisiologis sistemFisiologis adaptasi ini boleh disimpulkan darianalisis komparatif interspesifik sifat Že.g.ukuran organ dan fungsi, hara transportasi hargaand metabolic rate. with appropriate control forphylogeny. Accordingly, a central theme of evolutionaryphysiology involves conducting comparativestudies of physiological traits in vertebratetaxa with different life styles ŽFeder et al., 1987;Wainwright and Reilly, 1994.. However, manysuch physiological traits exhibit considerable variabilityin ecological time both within and amongindividuals which may make it difficult to detectimportant adapted patterns. The form of phenotypicvariation that involves a single genotypeproducing different phenotypes in response tovariation in some environmental variable is called‘phenotypic plasticity’ ŽTravis, 1994; Piersma andLindstrom, 1997.. Rapid reversible changes inbody composition, organ size, and digestiveprocesses provide examples of flexible norms ofreaction ŽStearns, 1989; Travis, 1994. or ‘phenotypicflexibility’ in that they may represent flexibleresponses to changes in the environment ŽPiersmaand Lindstrom, 1997.. Such phenotypic flexibilityin physiological traits may itself be a critical componentof the adaptive repertoire of animals thatmay influence diet diversity, niche width, feedingrate, and thus the acquisition of energy and essentialnutrients ŽKarasov, 1996; Kersten andVisser, 1996; Pigliucci, 1996; McWilliams et al.,1997; Piersma and Lindstrom, 1997..Understanding the patterns and consequencesof phenotypic plasticity and flexibility has importantimplications for animal ecology. Definingthese implications requires mechanistically linkingthe study of the trait in its various forms withsome ecologically relevant performance criteriaŽArnold, 1983; Wainwright and Reilly, 1994.. Ecomorphologistshave established a tradition of suchstudies although their usual focus is on comparingthe average response of many individuals,thus viewing variation in response or phenotypicplasticity as a nuisance to be avoided or at leastfor which control must be exercised. One of thecentral themes in this paper is that both bodycomposition and digestive features of migratorysongbirds are modulated in response to environmentalchange and this phenotypic flexibility hasimportant ecological consequences for birds duringmigration.3. Food intake and digestive performanceThe energetic gains that are realized by a birdwhen it eats more depend on interactions betweenfood intake rates and digestive efficiency ŽKara-sov, 1996.. In the absence of extensive sparedigestive capacity, if the absorptive surface of thegut or its capacity for absorption does not changewhen a bird eats more, then the increased flow ofdigesta may cause digestive efficiency to declineand thereby directly discount the potential energeticgains provided by hyperphagia. Alternatively,if gut size increases with food intake, thenS.R. McWilliams,W.H. Karaso Comparatie Biochemistry and Physiology Part A 128 (2001) 579593 581pencernaan efisiensi tidak dapat mengubah ketika burungmakan lebih banyak. Jika kadar enzim pencernaan jaringan-spesifikatau aktivitas transportasi gizi meningkat dengan makananasupan Žwith tidak ada perubahan dalam usus ukuran., maka pencernaanefisiensi juga tidak dapat mengubah ketika makan burunglain. Di bawah ini kita membahas efek jangka panjangdan jangka pendek peningkatan makan tarif di pencernaankinerja burung.3.1. efek jangka panjang kenaikan asupan makanan padadigesti e kinerjaBanyak studi burung laporan peningkatan permukaanluas dan volume usus dengan jangka panjang meningkatdalam ŽSavory asupan makanan dan lembut,1976a, b; Gurih, 1986; Dykstra dan
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