Summary
Offspring production over the adult lifespan (b/M whereb is the yearly fledgling or offspring production and 1/M is the mean adult lifespan) is an approximate invariant within both birds and mammals. The two taxa differ, however, in that mammals have bothαM and αb as invariants (b/M = αb/αM) while birds do not (α is the age at first breeding). Birds have a surprising cancellation in that bothαM andαb are ∝α −0.25.
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Charnov, E.L. A note on dimensionless life histories for birds versus mammals. Evol Ecol 9, 288–291 (1995). https://doi.org/10.1007/BF01237774
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DOI: https://doi.org/10.1007/BF01237774