TREE vol, 2, no. 1, January 1987 d e ta ile d mapping of ecotypes and hyp o the sis must be treated w ith flo w e ring tim e s. c a utio n. C o nc lusio ns Acknowledgements The se e xa m p le s illustra te two p o ints: ra the r fe w sup p o se d c a se s o f re info rc e m e nt have been de- te c te d , a nd re info rc e m e nt is d iffi- c ult to d e m o nstra te une q uivo c a lly. No ne o f the stud ie s ha s d ire c tly te ste d p re d ic tio ns o f the re info rc e - me nt m o d e l a g a inst a lte rna tive e x- p la na tio ns. Fo r e xa mp le , the re a re fe w a tte m p ts to d isting uish re in- fo rc e m e nt fro m re p ro d uc tive c ha r- a c te r d isp la c e m e nt. Mo re c a re fully p la nne d stud ie s, e spe c ia lly in te n- sio n zo ne s, a re c le arly needed. Pro g re ss is a lso ha m p e re d by a g e ne ra l la c k o f info rm a tio n o n the func tio na l a nd g e ne tic struc ture of m a te re c o g nitio n syste m s and on the o pe ra tio n o f se xua l se le c tio n w ithin p o p ula tio ns. O nly when the se p ro b le ms have been re - so lve d w ill it b e p o ssib le to a sse ss the im p o rta nc e o f re info rc e m e nt in e vo lutio n. It may be a common mode of speciation and yet be only rarely o b se rve d , e spe c ia lly if it c an o c c ur ra p id ly in a p p ro p ria te c ir- c um sta nc e s. Ho we ve r. the d a ta c ur- re ntly a va ila b le , w hilst no t d e nying the p o ssib ility of re info rc e m e nt, g ive little p o sitive sup p o rt fo r its ro le in the e vo lutio n o f p re zyg o tic iso la tio n. Fo r the p re se nt, the I am grateful to Godfrey Hewitt, Richard Nichols and Michael Ritchie for helpful dis- cussions and to a referee for comments on the manuscript. Re fe re nc e s 1 Paterson, H.E.H. (19781 S. 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Sot LondonSer.B211,83--117 44 McNeilly, T. and Antonovics, 1 j 1968 Heredily 23.205-2 I8 45 Coyne, I A. ( I974 I Evolution 28. 505-506 - A decade ago it was suggested that much of the plumage variability exhibited by flocking birds can be explained by ‘status signalling’, plumage varia6ility 6eing used to signal agonistic sfatus’. As a result of this suggestion, a number of studies have examined the social significance of plu- mage d iffe re nc e s, 6ut fhe sta tus signalling hypothesis has not received unequivocal support. Other factors. such as the fatilita- tion of individual recognition, also appear to 6e important in explaining plumage varia6ilify. Plumage Variability, Status Signalling and Individual Recognition in AvianFlocks D. Philip Whitfield titula r so c ia l p o sitio n, o r sta tus, re - la te d to its fig hting a b ility, w ith o rd e r o f p rio rity to re so urc e s in- c re a sing w ith hig he r sta tus. As the number and p a rtic ula rly the in- te nsity of inte ra c tio ns inc re a se s, ho w e ve r, the re w ill b e inc re a sing c o sts in te rm s o f e ne rg y e xp e nd i- ture , time a nd risk o f injury. Thus, a ny ind ivid ua l whic h p o sse sse d a ttrib ute s fa c ilita ting a re d uc tio n in the num b e r o f c o stly inte ra c tio ns it w a s invo lve d in, witho ut suffe ring a re d uc tio n in its sta tus, would pre- sum a b ly be at an advantage. One suc h a ttrib ute could be an external sig na l, suc h as a plumage typ e , re fle c ting fig hting a b ility: if a sig na l- le r’ s sta tus c a n b e a sse sse d by con- sp e c ific s the n the sig na lle r w ill b e ne fit. Sig na lling sta tus would be p a rtic ula rly b e ne fic ia l if flo c ks a re unsta b le w ith re g ard to mem- b e rship , sinc e it wo uld re duc e the c o st o f a sse rting sta tus e a c h time a ne w flo c k w a s jo ine d . Ma ny no n-b re e d ing b ird s fo rm flo c ks and compete fo r p rio rity of a c c e ss to re so urc e s suc h a s fo o d a nd she lte r thro ug h a g o nistic in- te ra c tio ns. The re sult o f re p e a te d inte ra c tio ns b e tw e e n flo c k mem- b e rs is tha t e a c h b ird g a ins a p a r- Philip Whitfield is at the Department of Zoology, Downing Street, Cambridge CB2 3Ej, UK. 13