Morphometric variation and deteriorating food availability both explain seasonally declining reproductive success of Dicrurus hottentottus (Hair-crested Drongo)

Seasonally declining reproductive performance in avian populations may be attributed to high-quality individuals breeding earlier than low-quality individuals (the ‘quality hypothesis’), to deteriorating environmental conditions (the ‘date hypothesis’), or both. By comparing the reproductive performance among different perennial pairs, and the same perennial pairs across different years, we tested the influence of timing of breeding on reproductive performance in the Hair-crested Drongos (Dicrurus hottentottus) from 2010 to 2022. Breeding pairs that typically laid earlier relative to the population mean (i.e., early-breeding pairs) exhibited a significantly higher fledging rate compared to breeding pairs that often bred later relative to the population mean (i.e., late-breeding pairs). For a given pair, the fledging rate of early-breeding pairs increased when they bred later compared to other years, while the fledging rates of late-breeding pairs were significantly lower in years when they bred later than usual. We found that male morphological traits were associated with both the timing of breeding and the feeding rate. Pairs with long-billed (indicator of high quality) males bred earlier than pairs with short-billed males, and pairs with larger males (indicator of high quality) had a higher feeding rate than pairs with smaller males. Female age had reversed effects on the timing of breeding and fledging rate: older females tended to breed earlier than younger females; but there was a sharp decline in fledging rate after 7 years of age in late-breeding pairs. Finally, our results showed that the feeding rate and the amount of invertebrate biomass decreased with laying date, indicating deteriorating feeding conditions later in the breeding season. Overall, our findings support both the ‘quality hypothesis’ and the ‘date hypothesis’. Low-quality Hair-crested Drongos are likely to be most adversely affected by the phenological mismatches induced by climate change, which, in turn, may affect population dynamics through reduced fecundity.

Methods
We collected all the data from the field. We used the within subject model to test whether the 'individual quality hypothesis' or 'the date hypothesis' could explain the seasonal declining of the breeding performance.