[    13  ] 
III.  Observations  on  the  Laws  which  appear  to  influence  the  Assumption  and  Changes  of 
Plumage  in  Birds.    By  William  Yarrell,  Esq.,  F.L.S.  &f  Z.S. 
Communicated  February  26,  and  April  23,  1833. 
The  changes  of  plumage  observable  in  birds  at  certain  seasons,  have  of  late  years  oc- 
cupied much  of  the  attention  of  ornithologists ;  and  among  others  of  our  own  country- 
men, we  are  greatly  indebted  to  the  late  Colonel  Montagu  for  a  long  series  of  obser- 
vations, by  which  the  difficulties  of  tracing  specific  identity  in  many  instances  were  suc- 
cessfully cleared  up,  and  various  periodical  appearances  distinguished  and  described. 
These  interesting  changes  have  their  origin  in  various  causes,  and  are  produced  by 
diiFerent  means :  some  are  the  effect  of  age,  others  of  sex,  season,  or  disease ;  some- 
times they  are  produced  by  moulting,  or  discharging  the  old  feathers  and  obtaining  new 
ones,  but  more  frequently  by  one  or  other  of  two  different  processes. 
Before,  however,  proceeding  to  explain  the  manner  in  which  changes  are  otherwise 
produced  in  the  plumage  of  birds,  it  may  be  considered  necessary  to  say  a  few  words  on 
the  feathers  themselves. 
The  bulb  or  pulp,  which  is  the  foundation  of  each  feather,  has  its  origin  in  a  gland  or 
follicle  of  the  skin  ;  and  as  the  pulp  lengthens,  this  gland  or  folhcle  is  itself  absorbed. 
The  pulp  still  lengthening  becomes  invested  on  its  outer  surface  with  several  concentric 
layers  of  condensed  cellular  membrane,  from  which  the  shaft,  the  filaments  of  both 
lateral  webs,  the  colouring  matter  and  the  horny  quill  are  severally  produced  ;  but  ana- 
tomists appear  to  differ  a  little  in  opinion  as  to  the  exact  manner  in  which  the  growth 
of  the  various  parts  takes  place.  The  pulp,  which  nearly  fills  the  barrel  of  the  quill 
while  the  feather  is  forming,  is  connected  with  the  body  of  the  bird  by  an  aperture  at 
that  end  of  the  quill  which  is  fixed  in  the  skin,  through  which  aperture  or  umbilicus  a 
portion  of  the  pulp  is  extended.  The  whole  of  the  pulp,  within  as  well  as  without  the 
quill,  is  the  only  part  of  the  feather  which  appears  to  be  vascular,  and  the  large  feathers 
of  the  wing  may  be  injected,  while  growing,  from  the  humeral  artery  ;  but  the  feathers 
once  perfected,  the  injection  can  no  longer  be  sent  even  into  the  pulp.  The  membranes 
of  which  it  was  composed,  the  former  nidus  of  vessels  now  obliterated,  dry  up,  contract, 
and  ultimately  separating  transversely  into  funnel-shaped  portions,  (which  remain  in 
the  barrel  of  each  quill,)  are  well  known  by  the  familiar  term  of  pith. 
A  perfect  feather  presents  many  points  of  interest,  if  we  consider  its  various  parts, 
form,  colour,  strength,  lightness,  durability,  and  the  peculiar  manner  in  which  the 
fibres  of  the  web  lock  in  with  each  other  to  afford  continuity  of  surface.  The  accessory 
plume  also  requires  to  be  noticed.    This  is  usually  a  small  downy  tuft,  which  not  only 
