Lack's Principle

Content of the principle

Lack (1947, 1954) argued that the more eggs a bird lays, the less food each chick receives and the lower its survival. Natural selection should therefore favor the clutch size that maximizes the number of young surviving to leave the nest, a size set by the parents' ability to supply food. He used this idea to explain why tropical birds tend to lay fewer eggs than temperate birds, a latitudinal pattern that Moreau (1944) had documented for African birds. Lack's principle challenged the then-common view that reproduction is regulated for the good of the species or group, explaining breeding strategy through individual-level natural selection.

Williams's refinement and theoretical extensions

In practice birds often lay somewhat fewer eggs than the optimum Lack predicted. Williams (1966) refined the principle by noting that reproduction carries a cost to the parents' own survival and future breeding, so the clutch size maximizing lifetime reproductive success is smaller. Charnov and Krebs (1974) formalized this, and Smith and Fretwell (1974) modeled the trade-off between offspring number and size. Stearns (1992) and Godfray, Partridge and Harvey (1991) placed these questions at the center of life history theory.

Experimental tests and broad patterns

Perrins (1965) analyzed the relationship between clutch size and chick survival in the long-term great tit study at Wytham Woods near Oxford. Pettifor, Perrins and McCleery (1988) reported that when clutch sizes were altered experimentally by moving eggs or chicks, pairs showed individual optimization, each choosing a clutch suited to its own capacity. A meta-analysis by VanderWerf (1992) confirmed that in many species actual clutch sizes are smaller than Lack's optimum. Jetz, Sekercioglu and Böhning-Gaese (2008) used data on more than 5,000 species to show that seasonality, latitude and nest predation pressure explain variation in clutch size.

FAQ

What is Lack's principle in simple terms?

Birds have evolved to lay as many eggs as the parents can feed and raise to independence, because laying too many would leave fewer surviving chicks.

Do real birds lay clutches as Lack predicted?

In many species actual clutches are slightly smaller than Lack's optimum, which Williams's refinement explains by the cost of reproduction to the parents' own survival and future breeding.

Sources reviewed: 2026-10-26

References

1. Lack, D. (1947). The significance of clutch-size. Ibis, 89(2), 302–352.
2. Lack, D. (1954). The Natural Regulation of Animal Numbers. Oxford: Clarendon Press.
3. Lack, D. (1966). Population Studies of Birds. Oxford: Clarendon Press.
4. Williams, G. C. (1966). Natural selection, the costs of reproduction, and a refinement of Lack's principle. American Naturalist, 100(916), 687–690.
5. Perrins, C. M. (1965). Population fluctuations and clutch-size in the great tit, Parus major L. Journal of Animal Ecology, 34(3), 601–647.
6. Charnov, E. L., & Krebs, J. R. (1974). On clutch-size and fitness. Ibis, 116(2), 217–219.
7. Stearns, S. C. (1992). The Evolution of Life Histories. Oxford: Oxford University Press.
8. Smith, C. C., & Fretwell, S. D. (1974). The optimal balance between size and number of offspring. American Naturalist, 108(962), 499–506.
9. Godfray, H. C. J., Partridge, L., & Harvey, P. H. (1991). Clutch size. Annual Review of Ecology and Systematics, 22, 409–429.
10. VanderWerf, E. (1992). Lack's clutch size hypothesis: An examination of the evidence using meta-analysis. Ecology, 73(5), 1699–1705.
11. Pettifor, R. A., Perrins, C. M., & McCleery, R. H. (1988). Individual optimization of clutch size in great tits. Nature, 336(6195), 160–162.
12. Moreau, R. E. (1944). Clutch-size: A comparative study, with special reference to African birds. Ibis, 86(3), 286–347.
13. Jetz, W., Sekercioglu, C. H., & Böhning-Gaese, K. (2008). The worldwide variation in avian clutch size across species and space. PLoS Biology, 6(12), e303.