Near ubiquitous reproductive trends across taxa follow a pattern in which output increases with latitudes and decreases with calendar date. Research grounded in life history theory provides separate ultimate explanations for latitudinal and seasonal trends. Here we frame these dual trends as a Simpson's paradox and attempt to gain insights into proximate cues that might account for both simultaneously. Using citizen science data on Eastern bluebirds, we found highest support for a model of clutch size based on change in day length at clutch initiation. Describing reproductive trends based on non-biologically relevant constructs of latitude and calendar date obscured links between proximate and ultimate explanations. For birds, our findings are consistent with an internal coincidence model of circadian rhythmicity as a proximate control of clutch size. Other avian studies might benefit from viewing clutch size as a circadian behavior of clutch initiation and termination rather than a quantified trait.