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Aim: Climate oscillations are known to influence the reproductive phenology of birds. Here, we quantify the effects of cyclic climatic variation, specifically El Nino Southern Oscillation (ENSO), on birds that breed opportunistically. We aim to show how inter-decadal climate fluctuations influence opportunistic breeding. This knowledge is essential for tracking the phenological responses of birds to climate change.

Location: Temperate and arid Australia.

Methods: We assessed variation in egg-laying (start, peak, conclusion, length) during the three phases of ENSO (El Nino, La Nina and Neutral) for 64 temperate and 15 arid region species using similar to 80,000 observations. Linear mixed-effect models and analysis of variance were used to (1) determine if, on average within each region, egg-laying dates differed significantly among species between Neutral-El Nino and Neutral-La Nina phases, and (2) assess how La Nina and El Nino episodes influence egg-laying in birds which breed early in the year.

Results: During La Nina phases, which are characterized by mild/wet conditions, most bird species in the temperate and arid regions exhibited longer egg-laying periods relative to Neutral phases. However, there was substantial variation across species. This effect was strongly seasonal; species breeding in spring experienced the greatest increases in egg-laying periods during La Nina. Further, we found only small differences in peak egg-laying dates during Neutral and La Nina in the arid region; suggesting that hot temperatures may constrain breeding regardless of rainfall. The effects of El Nino on breeding phenology were not consistent in the temperate and arid regions and may be confounded by highly mobile species opportunistically moving and breeding with localized rainfall during dry periods.

Main conclusion: In both arid and temperate regions, increased rainfall associated with La Nina phases positively influences avian breeding, and likely recruitment. However, dry El Nino phases may not have the dramatic impacts on breeding phenology that are commonly assumed.