J. Avian Biol. 38: 564  572, 2007 doi: 10.1111/j.2007.0908-8857.04030.x # 2007 The Authors. J. Compilation # 2007 J. Avian Biol. Received 17 July 2006, accepted 24 November 2006 Carotenoid diet and nestling provisioning in urban and rural great tits Parus major Caroline Isaksson and Staffan Andersson C. Isaksson (correspondence) and S. Andersson, Dept. of Zoology, Go¨teborg University, Medicinaregatan 18, SE-413 90 Go¨teborg, Sweden. Email: caroline.isaksson@zool.gu.se, staffan.andersson@zool.gu.se Considering the importance of dietary constraints for the widely held view of carotenoid pigmentation as an honest quality indicator, there is surprisingly little data on carotenoid availability in different natural diets or along environmental gradients. Here we investigate the carotenoid availability in the main diet of breeding great tits Parus major , living in urban and rural environments with known differences in carotenoid pigmentation. Carotenoid availability for nestling great tits was investigated in two respects: (1) quantity and quality of diet (i.e., caterpillar abundance and their carotenoid concentration), and (2) parental feeding frequency. First, caterpillars were generally more abundant in the urban environment and the four common Lepidoptera (i.e., caterpillars) genera studied were also heavier here. Second, as determined by HPLC analysis of the caterpillar genera, carotenoid concentration was significantly lower in the urban caterpillars. Furthermore, all except one of the caterpillar genera had higher lutein/zeaxanthin ratio in urban areas, which is in accordance with earlier studies of carotenoid composition in great tit yolk. Third, parental feeding frequency was about twice as high to urban broods compared to rural broods. This result may simply reflect the higher caterpillar abundance (shorter search time) in the urban environment. Poor food quality (low carotenoid concentration) seems therefore to be compensated by quantity in the urban environment. As a consequence the carotenoid availability seems to be similar for nestlings in the two environments. Carotenoids are important as provitamins, antioxidants and pigments, synthesized de novo by plants, some fungi and microorganisms, while animals must obtain them through the diet (Goodwin 1984, Latscha 1990). In birds, carotenoids play many important roles by acting as colour pigments in feathers, free radical scavengers and immuno-stimulants (e.g., Lozano 1994, von Schantz et al. 1999). As free radical scavengers, carotenoids are assumed to protect lipids, proteins and DNA from oxidative damage (Bianchini et al. 2000), which can be induced internally (e.g., free radical production through elevated ATP synthesis during growth or infection) or directly by external factors (e.g., anthropogenic pollution). Due to trade- offs with these immunological functions, variation in plumage pigmentation may provide’honest’ informa- tion (to conspecifics) about individual health and condition (e.g., Lozano 1994, Johnsen et al. 2003, Saks et al. 2003). Considering that limited dietary access is the foundation (Endler 1983, Hill 1990) of the widely assumed quality advertisement of carotenoid colour signals (see e.g., Hill 1991, Olson and Owens 1998), the lack of quantitative data on carotenoid availability in the wild is surprising. Even among birds, where carotenoid signalling is particularly well documented (Hill 2006), there is only a handful species for which the natural dietary sources of carotenoids have been analysed. Moreover, these are mainly marine organisms (rich in ‘red’ keto-carotenoids such as canthaxanthin and astaxanthin) and some terrestrial plant parts (seeds, fruit, rich in’yellow’ lutein and zeaxanthin), whereas the enormous variety of prey of insectivorous songbirds is virtually unstudied in this respect (McGraw 2006, but see Olson 2006). One pioneering study is the analysis by Partali et al. (1987) of the caterpillar prey that great tits Parus major and other Parus species (both with and without carotenoid-pigmented plumage) rely on during 564