Life history trade-offs between survival, moult and breeding in a tropical season environment
Abstract
The trade-off between current and future investment in reproduction lies at the heart of life
history theory. The need to differentially allocate resources between these two options arises
generally as a result of environmental pressures. Higher risk of mortality in adults is linked
with increased investment in current reproduction, whereas the opposite is true where adults
are long-lived (the r- K selection paradigm). Perhaps the most obvious factors influencing the
environment stem from seasonality of the climate, since rainfall and temperature affect food
availability, resulting in a higher risk of mortality. The available trade-offs that an organism
can make will therefore be constrained by environmental variability potentially resulting in
general adaptation and so ultimately influencing evolution of biome-specific life-history traits.
In this thesis, I examine how the seasonality of a West African tropical savannah environment
influences moult and breeding timing and duration, and survival in West African tropical
savannah bird species. I show that moult in tropical birds follows the same basic descendant
pattern through the wing feathers, but is a much lengthier process than for temperate species
(mean = 131 ± 11 days, N = 29 species), and that it frequently overlaps with breeding
activities. This suggests either that either the feathers of tropical species take longer to grow;
that it is a relatively low-cost activity and has little influence on life history trade-offs; or that
individuals further aim to reduce mortality risk by attempting to maintain high flight capability
at all times. Breeding also occurred over a longer season than for temperate species,
although an obvious peak in occurrence was identified to coincide with the food-abundant
period of the late rains and early dry season. Lengthy breeding seasons may indicate an
increased tendency to re-nest (possibly as a result of higher nest predation levels), and we
also identified a prolonged immature plumage phase – potentially indicating an extended
duration of parental care. Survival rates were calculated from mark-recapture models based
on mist-netting data. Previous work has focussed on the use of incorporating mark-resighting
data alongside that obtained by standard mark-recapture techniques. Here, I assess the
models applied in those methods, identify problems associated with over-paramaterisation,
goodness of fit and the generation of biologically unrealistic estimates, and so provide
suggestions on how to improve the protocol. Average survival from my study (40 species:
0.63 ± 0.02) was higher than previous estimates obtained from this site and were comparable
with estimates from other Afrotropical and Neotropical areas, although rates varied greatly
between species. Juvenile survival (13 species) was similar or possibly lower than adult
survival. I then used my empirically derived estimates of moult, breeding and survival life
history traits to identify potential trade-offs between traits. Overall I was unable to identify
significant relationships between any of the life history trait estimates, other than between
adult survival and clutch size. In this, the results followed those of previous researchers in
identifying a pattern of lower investment in current reproduction (clutch size) and
maximisation of adult survival in tropical species. My study, however, demonstrates for the
first time how moult and breeding duration are likely to be less constrained in tropical
environments.
Type
Thesis, PhD Doctor of Philosophy
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