METHODS Study species

Does inter-local variability in the timing of the wet season predicts the timing moult in a tropical passerine?

Common Bulbuls are resident across Nigeria (Elgood et al. 1973), they are capable of breeding year-round (Cox et al. 2013, Nwaogu et al. 2018) and typically long-lived (Stevens et al. 2013). Common Bulbuls are sexually monomorphic, usually weighing about 25 – 50g. They feed mainly on fruits and insects, whose availability vary with environmental conditions and phenology of plants. In central Nigeria, despite flexible breeding, wing moult at the population

level lasts 138 ± 5 days – usually from 1st_{May (± 15 days) to 26}th_{September (± 30 days) on}

average (Nwaogu et al. 2018a).

Figure 6.1: Map showing summary of moult records of Common Bulbul Pycnonotus barbatus,

from 17th_{of January to 8}th_{April 2017 across 15 locations along an aridity gradient in Nigeria}

where the wet season starts at different time in the year. Numbers in parenthesis against sampling locations indicate order in which sites where visited to assess moult.

Data collection

Within three months prior to the wet season, we travelled across Nigeria and mist-netted 308

Common Bulbuls across 15 locations between latitude 6 and 13o_{N (Figure 6.1). Mist netting}

was carried out between the 17th_{of January and 8}th_{of April 2017. All locations were visited}

before the wet season. We sampled from the southernmost location (Benin) and advanced northward (but not necessarily always consistent with latitude increase; see Figure 6.1 for sampling order), apart from Jos which was sampled on three occasions. The pattern of sampling was aimed at avoiding a temporal bias towards southern locations, because we predicted that moult will commence later in more arid locations, and these were more likely in the north. Precipitation of the driest quarter and onset of the wet season correlate negatively with latitude, hence by sampling south to north we set sampling date on a negative trend with latitude, onset of the wet season and precipitation of the driest quarter, allowing us to interpret a positive correlation between the extent of moult and precipitation of the driest quarter as an effect of rainfall rather than sampling date within a given year. Note that this sampling order is likely to weaken the predicted positive correlation because based on a regular feather deposition rate (Summers 1976), moult will advance as we move towards more northerly locations.

5

10

15

5

10

15 Longitude

Lat

itude

LOCATION KATSINA (16) NGURU (15) DUTSE (14) TORO (13) JOS (1,11,17) TULA (12) GUDI (10) LAFIA (7) PANDAM (6) MAKURDI (9) MONIYA (4) ILARA (5) AGENEBODE (3) CROSS RIVER (8) BENIN (2) Samplesize 10 20 30

Rainfall predicts inter-local variability in moult occurrence

6

Abstract

Rainfall is likely an important cue for species that live in seasonally arid environments, but because rainfall is often seasonal, it is difficult to distinguish its effect from other seasonal cues

and co-varying life history traits. In central Nigeria Common Bulbuls Pycnonotus barbatus

moult in the wet season, but this might be because other conflicting life history events like breeding are timed to occur at other periods, or just because moult is annual and occurs at the same time of the year in all locations regardless of environmental seasonality. To investigate the effect of local rainfall timing on moult, we examined variation in the extent of moult just before the wet season along a latitudinal gradient where the rains commence at different times. We predicted that moult should start earlier in sites where the rains commence earlier. We mist- netted 308 common bulbuls across 15 locations between latitude 6° N and 13° N within a three months period and modelled proportion of primary feather mass regrown as a function of precipitation of the driest quarter of the year which is a proxy for the timing of the wet season in each location. We found that moult advanced significantly with the timing of the wet season despite the absence of a similar pattern in the occurrence of breeding, suggesting that moulting is timed to local rainfall in each location rather than to the same period in the year across species range or to the timing of breeding. Thus, from one year of data along an environmental gradient, we show that the timing of moult may be organised to match suitable environmental conditions during the annual cycle of the Common Bulbul at each location independently of breeding. This pattern may apply to other species but will be less obvious if breeding is seasonal.

INTRODUCTION

Life history traits should be timed to coincide with suitable environmental conditions or at different times from other traits that may compete for common resources (Barta et al. 2006, McNamara and Houston 2008, Wingfield 2008, Visser et al. 2010, McNamara et al. 2011). Although the seasonality of many such traits are well documented, identifying the environmental cues that govern their occurrence are difficult because they are often correlated with multiple environmental variables, that are then themselves correlated with time (Dawson 2008). For example, in birds, breeding and moult are often timed to spring and summer conditions in temperate environments and to the wet season in most tropical environments (Baker. 1939, Hau 2001, Repenning and Fontana 2011) but it is unclear whether the timing of each life history event is related to independent cues, flexible to the timing of other events or is dominated by the need to time the most important annual life history event to a seasonal optimum.

Rainfall is an important determinant of seasonal environmental condition in the tropics (Poulin et al. 1992, Araujo et al. 2017), and is likely an important cue for species that live in seasonally arid environments (Chapman 1995, Lloyd 1999, Hau 2001, Houston 2012, Mares et al. 2017). However, rainfall is often seasonal, and so its occurrence correlates with seasonal

life history traits. In central Nigeria, for example, we know that Common Bulbuls Pycnonotus

barbatus specifically moult in the wet season despite weak breeding seasonality (Nwaogu et al. 2018a), but this may be because moult is unlikely to occur during breeding, or because moulting is controlled endogenously (Gwinner et al. 1995) or in response to variation in day length or other periodic cues like sunrise time (Goymann et al. 2012, Shaw 2017). Detailed analyses of the annual routine of the Common Bulbul at a single site in central Nigeria, suggests that timing of moult to the wet season may be important for optimising its annual cycle, since individuals are resident and capable of breeding year-round (Cox et al. 2013, Nwaogu et al. 2018a). However, we cannot conclude that rainfall exerts a direct effect on the timing of moult, because moult and rainfall are periodic, and vary little between years within a single location. However, because timing of the wet season varies over the geographic range of the Common Bulbul, we can test the effect of rainfall on the timing of moult using differences in the relative amount of feather material replaced by a moulting bird at a given time in its moult cycle (Summers 1976, Underhill and Joubert 1995). Such geographic variation in rainfall is similar to latitudinal temperature variation in temperate systems (Baker. 1939), which exerts predictable effects on the timing of life history events (Pimentel and Nilsson 2007, Bourgault et al. 2010).

The start and end of the wet season in Nigeria follows an annual predictable pattern from south to north. There is a single period of rainfall and one of drought each year, but the amount of rain and duration of the wet season varies latitudinally - the wet season is later and shorter going from south to north. The rains do not end abruptly, hence the commencement of the dry season and associated drying out of the environment is much more prolonged in more northerly locations. However, in all locations, the months before the onset of the rains are usually the driest, but more southerly locations where the wet season commence earlier remain relatively wet compared to more northern because they experience longer periods of

Chapter 6

precipitation leading into the quarter before the wet season. This creates an aridity gradient from the edge of the Sahara Desert in the north to the coast of the Atlantic Ocean in the south and this is consistent with the timing of the wet season (Nwaogu et al. 2018c).

Having established that moult is timed almost exclusively to the wet season, we test whether inter-local variability in the timing of the wet season specifically predicts the timing of moult in the Common Bulbul. We analyse the extent of primary moult across locations where onset and duration of rainfall vary over a four-month period. We predict that moult will be advanced in sites which experience earlier rains, modelled as precipitation of the driest quarter of the year per location. The amount of precipitation in the driest quarter of the year corelates negatively with the timing of the wet season. We modelled the proportion of primary feather mass grown by each bird as a function of the amount of precipitation in the driest quarter at capture locations ranging over 785 kilometres north to south.

METHODS Study species