Breeding performance of Common Terns (Sterna hirundo) does not decline among older age-classes

Declines in reproductive performance among older age-classes have been
reported in many bird and mammal species, and are commonly presented as
demonstrating reproductive senescence. However, no declines in performance
could be demonstrated in studies of several bird species. We measured
reproductive performance in Common Terns (Sterna hirundo) of known age
(2–28 yr) during a 19-yr period at a site in Buzzards Bay,
Massachusetts, USA. We measured 6 components of reproductive performance
and used Generalized Additive Mixed Models (GAMMs) in a Bayesian framework
to analyze dependence of each measure on parental age, while controlling
for variations among years and indices of individual quality. Four
measures of performance improved (earlier laying date, higher values of
clutch size, fledging success and productivity) with age, most rapidly
between ages 2–10 yr; egg mass and hatching success varied only slightly
with age. No measure of performance showed reversals among the older
age-classes; fledging success and productivity continued to increase
through at least age 22 yr. These findings are consistent with results
from an earlier study of the same species. Continued increase in
reproductive performance through the oldest age-classes is not
incompatible with ‘reproductive senescence’ (decline in physiological or
other functions required for successful reproduction) if either
reproductive effort or efficiency continue to increase. Studies within our
population have yielded no evidence for age-related increase in
reproductive effort, but three studies have suggested that older Common
Terns can raise chicks more successfully than younger birds without
increasing reproductive effort, probably by more efficient foraging and
chick provisioning. Our findings suggest that Common Terns offset
reproductive senescence by continuing to improve efficiency through at
least age 22 yr. Age-related changes in efficiency should be investigated
in other species with similar life-history traits.