Bigger chicks pay off for great tits, but only in harsh years
How many chicks should a bird raise, and how big should they be? In our new paper in the Journal of Evolutionary Biology we revisit this classic question with 58 years of data from the great tits of Wytham Woods, near Oxford. Larger broods always meant lighter chicks, but heavier chicks had a better chance of breeding later only in harsh years.
David Lack, who started the Wytham great tit study in 1947, argued that each female should raise the number of young that gives the most surviving offspring, because more chicks means less food for each. Louis Bliard, who led the study, used data from 1961 to 2018: 7,287 broods of 5,032 females and 53,753 chicks. A recently developed statistical model let the links between brood size, chick weight and later breeding change from year to year, with spring temperature and rainfall, the number of great tits, the beech seed crop, and how well breeding was timed to the peak of caterpillars, a key food for the chicks.
Mothers with larger broods had lighter chicks in all kinds of years: this trade-off was strong and barely affected by the environment. The benefit of heavy chicks, however, depended on the year. In harsh seasons, with a cold or dry spring or many great tits competing, heavier chicks were more likely to join the breeding population. In good seasons this advantage disappeared. Contrary to Lack’s expectation, brood size itself was not linked to how many young went on to breed, though this estimate was uncertain.
We did the study with the University of Oxford’s Edward Grey Institute, the University of Sheffield, Eawag and the Estación Biológica de Doñana. The approach could be applied to the many long-term bird studies that share their data through SPI-Birds.