The individual optimization of fitness: variation in reproductive output, including clutch size, mean nestling mass and offspring recruitment, in manipulated broods of great tits Parus major

1. The functional prediction of the Individual Optimization Hypothesis (Pet tifor et al. 1988) is explicitly tested, namely that 'the optimal clutch si ze is n for birds that choose to lay n eggs, regardless of the actual value of n' (Hogstedt 1980). 2. Over a 10-year period, we examine the recruitment from 1392 nests of gre at tits, 575 of which had their brood size experimentally manipulated. We d efined final manipulation as the difference between the number of young rin ged and the original clutch size. 3, Mean nestling mass declined with final manipulation: those broods from w hich young were removed had a significantly higher mean hedging mass than c ontrol broods, which in turn fledged at a higher mass than those from nests to which young had been added. There was a significant difference in the s trength of this effect between years. 4, Addition nests (final manipulation greater than or equal to2 young) fled ged significantly more off-spring ((x) over bar = 10.96, SE = +/- 0.16, n = 167) than control nests (final manipulation = -1, 0 or +1 young; (x) over bar = 8.68, SE = +/- 0.07, n = 646), which fledged significantly more young than removal nests (final manipulation less than or equal to 2 young; (x) over bar = 5.68, SE = +/- 0.09, n = 579). 5, Recruitment per nest (the key measure upon which selection would act sin ce parental survival was independent of manipulation in this study) was gre atest for parents rearing a brood size equal to their own clutch size. Aver aged over the 10 years, a Poisson model of the number of young recruited pe r nest showed maximal recruitment to occur at a final manipulation of 0.42 extra young added to the original clutch size. This final manipulation valu e was not significantly different from zero manipulation (i.e. when parents rear the same size of brood as their original clutch size). 6, Using a restricted data set, where there was no loss of young through eg g-hatch failure and/or loss of very small young, and examining recruitment per nest with respect to experimental manipulation, indicated maximal recru itment to occur at a manipulation of an additional 0.44 young. The recruitm ent from this experimental manipulation was not significantly different fro m zero manipulation (i.e. control broods). 7, The original size of clutch laid by a female was indicative of the 'cond ition' of the parents. Manipulation of brood size showed that birds natural ly laying larger clutches were 'fitter' than birds laying smaller clutches in that, when comparing birds that were all made to rear the same brood siz e, those with young removed recruited more young than control parents, whic h in turn recruited more young than those parents to which young had been a dded to the nests. 8, There were significant differences between years in the shape of recruit ment per nest relative to the extent of both final and, separately, experim ental manipulation, but in no year did addition broods recruit significantl y more offspring than control broods. 9. All these results are consistent with the Individual Optimization Hypoth esis - individual optimization via phenotypic plasticity is the most parsim onious functional explanation for the observed variation in the range in si ze of clutch laid by individuals.