Journal article

Local timing of rainfall predicts the timing of moult within a single locality and the progress of moult among localities that vary in the onset of the wet season in a year-round breeding tropical songbird

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Publication Details

Author list: Nwaogu CJ, Cresswell W


Publication year: 2021


Volume number: 162

Start page: 265

End page: 276

Total number of pages: 12

ISSN: 2193-7192

eISSN: 2193-7206



Rainfall seasonality is likely an important cue for timing key annual cycle events like moult in birds living in seasonally
arid environments, but its precise effect is difficult to establish because seasonal rainfall may affect other covarying annual
events such as breeding in the same way. In central Nigeria, however, Common Bulbuls Pycnonotus barbatus moult in the
wet season but only show weak breeding seasonality. This suggests that moult is more sensitive to rainfall than breeding, but
a similar outcome is possible if moult is simply periodic. We tested the relationship between rainfall and moult in Common
Bulbuls at a single location over 18 years: on average moult started 5th May (± 41 days: 25th March–15th June), being on
average later than the onset of the rains which is usually mid-April. The likelihood of finding a moulting Common bulbul
was best predicted by rainfall 9–15 weeks before moult was scored. We then tested the generality of this across populations:
the progress of moult should, therefore, correlate with the average timing of the wet season along a spatial environmental
gradient where the rains start at different times each year south-to-north of Nigeria. To test this, we modelled moult progress
just before the rains across 15 localities 6°–13° N as a function of the onset of the wet season among localities. As predicted,
moult progressed further in localities with earlier wet seasons, confirming that the onset of moult is timed to the onset of the
wet season in each locality despite weak breeding seasonality in the Common Bulbul. This strategy may evolve to maintain
optimal annual cycle routine in seasonal environments where breeding is prone to unpredictable local perturbations like nest
predation. It may, however, be less obvious in temperate systems where all annual cycle stages are seasonally constrained,
but it may help with explaining the high frequency of breeding–moult overlaps in tropical birds.


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Last updated on 2021-16-02 at 23:58