SIBLEY: ORNITHOLOGY 643 



cal studies (e.g., 1931). In 1935 Lorenz proposed the "releaser" concept to ex- 

 plain the initiation of instinctive behavior patterns. In an English version of 

 the 1935 paper Lorenz (1937, p. 249) defined a "releaser" as follows. 



The means evolved for the sending out of key-stimuli may lie in a bodily character, 

 as a special color design or structure, or in an instinctive action, such as posturing, 

 "dance" movements and the like. In most cases they are to be found in both, that is, in 

 some instinctive acts which display color schemes or structures that were evolved exclu- 

 sively for this end. All such devices for the issuing of releasing stimuli, I have termed 

 releasers (Ausloser) , regardless of whether the releasing factor be optical or acoustical, 

 whether an act, a structure or a color. 



The releaser concept found general acceptance among students of behavior 

 and was quickly applied to other studies. It was the unifying principle which 

 had been lacking and which greatly simplified much of the complicated termi- 

 nology that had enmeshed the study of animal behavior. Owing largely to Lo- 

 renz the problem of innate behavior has received a great deal of attention 

 in the past fifteen years. 



There have been many ethological studies utilizing the "releaser" concept. 

 The principal contributor has been Nikolas Tinbergen, formerly of the Uni- 

 versity of Leiden, now Lecturer in Animal Behavior at Oxford. Tinbergen has 

 successfully developed the objectivistic approach to the analysis of animal be- 

 havior. His work has included study of the orientation mechanism of the digger 

 wasp (Philanthus) , territory and breeding behavior of the three-spined stickle- 

 back {Gasterosteus aculeatus) , and numerous investigations of avian behavior. 

 Among the latter his study (1939a) of the spring behavior of the snow bunting 

 {Plectrophenax nivalis), and the analysis of the releaser for the begging re- 

 sponse in herring gull (Larus argentatiis) chicks (with H. C. Perdeck, 1950) 

 are examples. Tinbergen's ability to synthesize has been of great value to other 

 ornithologists. His extensive knowledge of this complex field has made possible 

 several valuable "review" papers (1936, 1939b, 1942, 1948) and recently (1951) 

 has resulted in a book which summarizes the present state of knowledge of in- 

 stinctive behavior. 



Fossil Birds 



The history of paleornithology is nearly coterminal with the span of our 

 Century of Progress. Few discoveries of importance were made before 1861, 

 when the remains of Archeopteryx were found in the lithographic limestone 

 quarry at Solenhofen, Bavaria. The skeleton of this Upper Jurassic link be- 

 tween reptiles and modern birds was described by Owen in 1863. In 1877 a 

 second Jurassic bird was found near Eichstatt, Bavaria. It was described by 

 Dames in 1884 as Archeopteryx siemensi. In 1921 Petronievics made this second 

 fossil the type of the genus Archeornis. Both specimens combine numerous rep- 

 tilian characters with the presence of feathers. There is general agreement that 

 these Jurassic fossils represent the first birds although Lowe (1944) believes 

 that they should be considered flying reptiles. 



In the preparation of his four volumes on the fossil birds of France (1867- 

 1871) Alphonse Milne-Edwards (b. 1835, d. 1900) visited all the large geological 

 collections in Europe. He assembled more than 4,000 fossil bones and the skele- 



