Letters • Why so many kinds of passerine birds? Raikow and Bledsoe (2000), in em- Slud 1976). It is unreasonable to assume the list of possible reasons for passerine bracing the null model of Slowinski that there is no underlying biological rea- success, but I would place more empha- and Guyer (1989a, 1989b), may perhaps son for this pattern and for the major sis on it than he did. be said literally to have added nothing turnover in avifaunas in the Northern It is difficult to discuss the nest-bufld- to the Kst of suggestions for why there are Hemisphere in favor of passerines after ing capabilities of passerines without re- so many species of passerine birds. When the Oligocène. sorting to anthropomorphisms such as Raikow (1986) addressed this problem Reproductive adaptations presumably "clever" or "ingenious." Is it not mar- previously and could find no key mor- made the holometabolous insects velous, however, that a highly special- phological adaptations to explain the di- (Coleóptera, Díptera, Hymenoptera, ized aerial feeder such as a cliff swallow versity of Passeriformes, the so-called Lepidoptera, and so on) the dominant (Hirundo pyrrhonota), with tiny, weak songbirds or perching birds, he despaired clade of organisms on earth. Likewise, it bill and feet, can fashion a complex nest and suggested that the problem may be appears that reproductive adaptations, out of gobs of mud fastened to a flat, only "an accident of classificatory his- not morphology, are responsible for the vertical surface? What adjective suffices tory," which brought on a storm of protest dominance of passerine birds over other to describe the nest of tailorbirds {Systematic Zoology 37: 68-76; 41: 242- orders of birds. (Orthotomus), which actually stitch the 247). Now, Raikow and Bledsoe have sub- Most arboreal nonpasserine birds are edges of two leaves together with plant stituted one form of nihilism for another. obligate cavity nesters that rely on holes fibers to provide a concealed cup in The nuH model formulation deals merely in trees, earthen banks, or termite nests. which to place their nest? The long wo- with numerical probability and lacks any This automatically restricts them to habi- ven bags of oropendulas and relatives explanatory value in this case, unless per- tats where those features are present and (Icteridae) and the various complex wo- haps it could be shown that no evolu- at the same time makes competition for ven structures made by weaverbirds tionary forces influenced patterns of nest sites severe. Cavities cannot be read- (Ploceidae) are well-known examples of speciation in birds. ily concealed and numerous predators the complexity of passerine nest types. What is known about the dynamics exploit the vulnerability of cavity-dweUing The delicate cup nests of gnatcatchers and timing of the radiation of the order organisms (snakes, certain hawks, and so (Poliopttla), made of spider webs cov- Passeriformes does not support the suc- on). On the other hand, passerine birds, ered with lichens and fastened to a hor- cess of these birds being due to random both oscines and suboscines, have an ex- izontal limb, or the cup nest of the processes. Similar early and middle traordinary abñity to fashion nests out of Australian sitellas (Sitella), placed in an Eocene avifaunas from the United States a wide variety of materials, to place them acutely angled crotch and covered with (Wyoming), England, and Germany show in an equally wide variety of situations, minute, verticafly-oriented strips of bark, that by about 50 million years ago, Europe and, when need be, to camouflage or con- are exquisite uses of camouflage among and North America were inhabited by a ceal them with unusual "resourcefijlness." passerines. diverse array of arboreal birds, some of That the ability of passerines to protect Baptista and Trafl (1992) emphasized very small size, but not one of which was their eggs and young in this manner may the abflity of passerines to move into new a passerine (Olson 1989, Mayr 1998a, have led to their success as a group is not environments as a key factor in the suc- 1998b, 2000). Although passerines oc- a new idea, having been advanced clearly cess of the order. If there is any single at- cured in the early Eocene of Australia and in some detail by Irwin (1962) in an tribute that would make this possible, it (Boles 1997), the earliest evidence of the otherwise obscure publication that has is the ability of passerines to adapt their order in the Northern Hemisphere is since been widely overlooked. CoUias breeding regimen to locally avaflable nest from the latest Oligocène (about 25 mil- (1997) added nest-buflding behavior to sites and nesting materials. The brain in lion years ago) of France (Mourer- the entire order appears to be "hard- Letters to the Editor Chauviré et al. 1996). Passerines do not wired" for nest-building inventiveness. BioScience become an important component of the Attn: Science Editor The next most successflfl order of birds in avifaunas of the Northern Hemisphere 1444 Eye St., NW Suite 200 terms of number of species is the until the Miocene (about 20 to 5 million Washington, DC 20005 Apodiformes (swifts and hummingbirds), years ago), when they radiated explo- The Stan of BioScietice reserves the right to edit species which also have complex and di- sively. Passerines are now the numeri- letters for length or clarity without notifying verse nest-building habits. This is un- cally dominant group of birds in virtually the author. Letters are published as space likely to be coincidental. becomes available. all geographical areas of the globe (see 268 BioScience • April 2001 I Vol. 51 No. 4 » Letters As a morphologist, I am quite willing . 2000. Tiny hoopoe-like birds from the To understand the value of null to concede that morphology is not going Middle Eocene of Messel (Germany). Auk model approaches, consider the ques- to provide the answer to the numerical 117:964-970. tion "How different do sister taxa need dominance of the Passeriformes. Mor- Mourer-Chaviré C, Hugueney M, Jonet P. 1996. to be in species richness to cause a phology WÜ1 stiU be useful, in tandem Paleogene avian localities of France. Pages search for a deterministic explanation with molecular studies, in providing 567-598 in Mlikovsky J. Tertiary avian locali- for the difference?" Would sister groups much-needed improvements in our un- ties of Europe. Acta Universitatis Caroliniae of, say, 100 and 110 species, respectively, derstanding of relationships within the Geológica 39: 519-846. differ sufficiently to warrant a search for Passeriformes, which, in contrast to the Olson SL. 1989. Aspects of global avifaunal an explanation like the one Olson pro- impression given by Raikow and Bledsoe, dynamics during the Cenozoic. Acta XIX poses for passerine diversity? Exacfly how is still very imperfect. At the same time, Congressus Internationalis Ornithologici 2: different must groups be in species rich- a detailed analysis of nest types could 2023-2029. ness before one would be willing to in- also contribute materially to a better phy- Raikow RI. 1986. Why are there so many kinds of voke a deterministic, as opposed to logenetic fi-amework and lead to fasci- passerine birds? Systematic Zoology 35: stochastic, cause for the difference? nating insights into divergence and 255-259. As noted in our article, and contrary to convergence in nest-building behavior. Raikow RI, Bledsoe AH. 2000. Phylogeny and evo- Olson's implication at the end of his first Another fruitful avenue of investigation lution of the passerine birds. BioScience 50: paragraph, we are not arguing here that would be descriptive and experimental 487^99. the number of species in a given clade has studies of the neuroanatomy and neuro- Slowinski JB, Guyer C. 1989a. Testing null models no antecedent causes. It of course must. physiology of nest-building behavior, in questions of evolutionary success. System- Instead, we merely ask by how much sis- along the lines of the studies of vocaliza- atic Zoology 38: 189-191. ter groups must differ in species rich- tions that have generated so much at- . 1989b. Testing the stochasticity of pat- ness to prompt a search for an tention in ornithology in recent years. It terns of organismal diversity: An improved explanation for the difference in the adap- is time for the analysis of passerine suc- null model. American Naturalist 134: tations possessed by the groups. We reit- cess to progress beyond disheartened sor- 907-921. erate that the null model approach is ties into semantics and probability Slud P. 1976. Geographic and climatic relation- designed precisely to answer the ques- statistics and enter the realm of enthusi- ships of avifaunas with special reference to tion of whether such a search is required. astic biological exploration. comparative distribution in the Neotropics. Gotelli and Graves (1996) identify five STORRS L. OLSON Smithsonian Contributions to Zoology 212: important features of null models. With Department of Vertebrate Zoology 1-149. respect to Olson's statement that the nuH National Museum of Natural History model we used "lacks any explanatory Smithsonian Institution Response from Raikow and value," we believe its most important fea- Washington, DC 20560 Bledsoe tures are (1) that it allows for the possi- Olson makes two main points in his bility that no deterministic mechanism is References cited letter. He criticizes our appKcation of operating to produce an observed result; Baptista LF, Trail PW. 1992. The role of song in a null model to the study of the diversity (2) that stochastic processes may be re- the evolution of passerine diversity. Systemat- of passerine birds, and he offers the hy- sponsible for a result; and (3) that parsi- ic Biology 41: 242-247.