Table 1.

Possible functions of allopreening in socially monogamous, colonially breeding species, including fitness benefit, recipient, predicted relationships with preen rate and which were tested in this study (see electronic supplementary material). (Note that other functions have been put forward in species that live in hierarchical cooperatively breeding groups (reviewed in Radford & Du Plessis 2006).)

function	fitness benefit	recipient	prediction	tested	reference
1. reciprocity	to be preened in return to benefit self, e.g. parasite removal, feather maintenance, stress reduction (see below)	mate/neighbour	positive correlation between preen rates of players	yes	Seyfarth & Cheney (1984); Hart & Hart (1992)
2. parasite removal/feather maintenance	to improve condition of mate or breeding neighbour (reduces offspring predation if neighbouring pairs breed successfully)	mate/neighbour	negative/positive correlation between preen rate and parasite load/feather condition	no	Clayton (1991); Hart & Hart (1992)
3. mate choice (signalling and assessment)	preening as a sexually selected honest signal of quality; as means of assessing quality of potential mate	mate	positive correlation between preen rate and mate selection	no	Zahavi (1975); Roberts (1998); Stopka & Graciasova (2001)
4. pair-bond maintenance	preening as ritualized behaviour to reaffirm pair bond after prolonged separation	mate	positive correlation between preen rate and length of the pair bond	yes	Black (1996)
5. stress reduction	tactile stimulus to reduce stress levels and aggression	mate/neighbour	negative correlation between fight rate and preening rate	yes	Terry (1970); Feh & Demazieres (1993)
6. parental care stimulation	to promote production of hormones such as prolactin that encourage care	mate	a) positive correlation between preen rate and hormone levels; b) positive correlation between preen rate and care	no	Buntin (1986); Keverne et al. (1989)