Rainfall, leafing phenology and sunrise time as potential Zeitgeber for the bimodal, dry season laying pattern of an African rain forest tit (Parus fasciiventer)

Phil Shaw

doi:10.1007/s10336-016-1395-6

Abstract

Recent studies have documented a mismatch between the phenology of leaf production, prey availability and the nestling food requirements of north temperate songbirds, attributed to climate change effects. Although tropical forest species have often been regarded as relatively aseasonal breeders, similar disruptive effects can be expected at equatorial latitudes, where comparatively little is known of the links between weather, leafing phenology, food availability and bird breeding activity, particularly in complex rain forest habitats. During a 19-year study at 1°S in Bwindi Impenetrable Forest, Uganda, Stripe-breasted Tits Parus fasciiventer showed a strongly bimodal laying pattern, breeding mainly in the two dry seasons, with 50 % of breeding activity occurring in January–February and 19 % in June–July. Individual females bred in both dry seasons, laying their first and last clutches up to 28 weeks apart. Breeding activity was linked to leaf production, which peaked mainly in November–December, following the September–November wet season. Increased leaf production is likely to have stimulated a rise in caterpillar numbers during December–February, coinciding with peak food demands by nestling tits. Laying was thus positively correlated with increased leaf production in the preceding calendar month, but was also linked to day length and a change in sunset time. To investigate possible links between egg laying and photic cues I compared the median date of first clutches laid by marked females in each half of the breeding year (October–March and April–September), with annual changes in photoperiod (varying by 7 min p.a.) and sunrise time (varying bimodally, by 31 min p.a.). The two median laying dates fell 138–139 days after the last date on which sunrise had occurred at 07:05 in August and January, suggesting the potential for sunrise time to act as a cue, or Zeitgeber, for breeding in tropical birds. Further work is required to establish whether the relationship is causative or coincidental.

Highlights

ESM Table 1 Tree and shrub species monitored as part of the Gorilla Food Plant Study
Taxa that could not be identified to species
fewer than 10 observations were made per calendar month

Summary

Zanthoxylum gilletii

ESM Table 2 Time intervals (‘lag-times’) separating the median laying date of first clutches in each half-year, from the preceding minimum and maximum sunrise and sunset times. Differences in the median lag-time evident in the first- and second half of the breeding year were tested using general linear mixed-models, in which study year and female identity were entered as random effects. First clutch dates were recorded for 17 marked females in 12 years (n = 46 female-years). Lag times differed significantly between the two half-years

Second First

Proportion of maximum