AcknowledgmentsWe thank the BBSRC for funding the work BB F

Attacks and Egg Number
The number of eggs in a nest may indirectly signal the approximate age of a nest to cowbirds (White et al., 2007). Whereas females prefer to parasitize nests with a clutch size indicative of a nest at the egg-laying stage (King, 1979, Trine, 2000 and White et?al., 2007), we expect that they 5-BrdU should preferentially attack nests that have a number of eggs representative of a complete clutch. The typical clutch size for cowbird hosts ranges from three to six eggs, but is most commonly four, and most hosts begin incubation after laying the last or second-to-last egg (Friedmann et?al., 1977 and Terres, 1980). Therefore, we presented 26 females with a nest containing four unfertilized yellow canary, Serinus flaviventris, eggs (21.1 × 14.3 mm), simulating a complete clutch, and a nest containing two canary eggs, simulating an early nest that was still in the egg-laying stage ( King, 1979 and White et?al., 2007).
While the absolute number of eggs in a nest may indicate approximate incubation timing, molecules is possible that cowbirds may simply attend to the relative number of eggs and target a larger clutch. To confirm whether cowbirds respond to absolute clutch size and to rule out a possible effect of our experimental manipulation, we presented 20 females simultaneously with a nest containing six eggs and a nest containing four eggs. In this experiment both nests contained a number of eggs that would typically represent the completion of egg laying, although a two-egg difference between treatments was maintained. If cowbirds do preferentially attack nests late in the incubation stage, we would not expect to see any behavioural differences between these two nest types because information on their relative developmental stage could not be gleaned from egg number.