Chickadees and tits excel at identifying and exploiting novel food sources. One such food source in western North America is the larvae of Urophora gall flies (Diptera: Tephritidae), which were recently introduced to help control the spread of spotted knapweed (Centaurea maculosa). In winter, Black-capped Chickadees (Poecile atricapillus) of many western populations spend much of their time foraging exclusively on this new and rich food source. Because the number of gall flies within knapweed seedheads varies, I examined whether chickadees preferentially selected seedheads with high densities of prey. In large, semi-natural, outdoor aviaries, I presented bouquets of knapweed seedhead to chickadees and allowed them to forage until approximately half of the seedheads were removed. Seedheads rejected by chickadees had significantly fewer larvae than did seedheads not exposed to chickadee predation, indicating that chickadees had selected and removed seedheads with high densities of gall flies. Seedhead size was positively correlated with the number of insects housed within, and chickadees preferentially removed larger than average seedheads while foraging. These results indicate that size is one reliable cue that chickadees might use to select seedheads with high gall fly density. However, chickadees were more successful at selecting seedheads with higher larval density than expected if they used size alone, which suggests that these birds may also use other cues to further increase their foraging efficiency. This study demonstrates the types of subtle decisions chickadees and other birds make even when foraging on relatively novel food sources.