A METHOD TO ISOLATE ENVIRONMENTAL-EFFECTS ON NESTLING GROWTH, ILLUSTRATED WITH EXAMPLES FROM THE GREAT TIT (PARUS-MAJOR)

1. Fledging weight has been shown to correlate with survival in many b ird species and, therefore, is an important component of fitness. Fled ging weight results from growth during the nestling stage. Hence, envi ronmental effects on nestling growth in altricial bird species play a key role in proximate explanations of selection after fledging. 2. We describe and test a method to quantify environmental effects on (daily ) increments in nestling weight. It accounts for the known genetic com ponent of growth and simultaneously detects the effects of short-term environmental fluctuations on growth. 3. The method is illustrated wit h data on daily weights of individual great tit nestlings from a rich and from a nutritionally marginal study area near Basel, Switzerland. However, the method can also be applied to other traits. 4. The observ ed weight increment is expressed as a ratio through division by the in crement expected under good conditions. The expected increment is calc ulated from a Richards growth curve with a shape parameter derived fro m a fit on nestlings growing under favourable conditions and an asympt otic weight based on parental weight to incorporate the genetic compon ent. 5. Under good conditions the ratio of realized over expected grow th increment is on average close to 1.0 irrespective of nestling age. Under poor conditions, however, there is a decrease in ratio with an i ncrease in age due to a larger influence of the environmental conditio ns later in the growth period. 6. The method is only useful when the e xpected growth increments are greater than the measurement error, in o ur examples about 0.1 g. 7. Monte Carlo simulations confirm that our i nterpretations are realistic. 8. We demonstrate that mean parental win ter weight as an estimator of the asymptote is a practicable way of in corporating genetic variance in final body weight into growth models.