The unctions of the little-studied avian spleen are reviewed and compa
red with those of its better known mammalian counterpart, which is gen
erally larger in proportion to body size than in birds. A role in immu
nity similar to that in mammals is evident, but the organ's contributi
on do oxygen supply seems less extensive; splenic storage of erythrocy
tes, for example, is unrecorded for birds. The spleen is a principal o
rgan of systemic immunity, and its importance in disease resistance is
presumably accentuated by the scarcity of avian lymph nodes. The stri
king intraspecific: variation in size partly reflects seasonal changes
in spleen morphology and activity. Several explanations, principally
based on changing oxygen demand, have been proposed previously for the
se periodical cycles. But seasonally small spleens could sometimes sim
ply stem from a combination of (1) a cessation of active splenomegaly
as seasonally patent infections recede, and (2) a seasonal lymphoid in
volution, occurring even if an individual has not recently responded t
o, and recovered from, an infection. Possible determinants for these a
nd other processes are discussed from evolutionary and ecological pers
pectives. There is a pressing need for a thorough investigation of bot
h hematological and immunological functions, using a phylogenetically
and ecologically broad range of species, as well as modern histologica
l and experimental techniques.