Puncturing an egg in order to estimate development is likely among the most reliable and direct of methods for determining nest age, and we have demonstrated that cowbirds vary the intensity of their attacks based on this developmental cue. Interestingly, the closely related shiny cowbird has been reported to puncture host eggs to gauge embryonic development to avoid laying in late-incubation stage nests, but that species apparently does not farm their hosts (Massoni & Reboreda, 1999). Whether brown-headed cowbirds also puncture to optimize their timing of parasitism is unknown. The Indirubin in our experiments also appear able to assess the age of a clutch indirectly by attending to absolute clutch size. Our results are consistent with those of White et al. (2007), who found that cowbirds preferentially parasitize nests containing three eggs over nests with one egg, but show no behavioural differences between nests with three versus six eggs. Accurately estimating nest age is important for both parasitism and farming behaviour, and cowbirds appear to use a similar method of doing so in both contexts. Using quantitative information is not unusual for birds (e.g. food hoarding; Hunt, Low, & Burns, 2008) and has been reported for other brood parasites (Odell & Eadie, 2010). Notwithstanding, we cannot yet be certain of a cowbird's numerical competency. Cowbirds, for example, could count individual eggs (e.g. Lyon, 2003) or attend to the volume of eggs in a nest.