Avian Hatching Asynchrony: Brood Classification Based on Discriminant Function Analysis of Nestling Masses

Studies of hatching asynchrony in altricial birds have used several methods to determine if a brood hatches synchronously or asynchronously. The most commonly used methods include making frequent nest visits during the time of hatching and using nestling mass or other measures of size on a given day to estimate nestling age or to describe the extent of the size hierarchy within broods. We made daily nest visits to determine if House Wren (Troglodytes aedon) broods hatched synchronously or asynchronously, and then compared classification of these broods based on discriminant function analysis with that based on mass differences of nestlings. The best misclassification rate based on discriminant function analysis was 17.5%, whereas the rates based on mass differences (23.1%) and estimates of nestling age (27.5%) were higher, but not significantly higher. The most frequent error occurring in discriminant function analysis was misclassifying asynchronous broods as synchronous broods, whereas the most frequent error based on mass differences of nestlings was misclassifying synchronous broods as asynchronous ones. Discriminant function analysis based on coefficient of variation of nestling mass and number of nestlings was about as effective (error rate = 17.5%) as was classification using individual masses. We recommend that investigators who wish to categorize broods objectively as synchronous or asynchronous based on nestling masses or other measures of size should use a discriminant function analysis.