Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Parasite infection in young animals can affect host traits related to demographic processes such as 19 survival and reproduction, and is therefore crucial to population viability. However, variation in 20 infection among juvenile hosts is poorly understood. Experimental studies have indicated that 21 effects of parasitism can vary with host sex, hatching order and hatch date, yet it remains unclear 22 whether this is linked to differences in parasite burdens. We quantified gastrointestinal nematode 23 burdens of wild juvenile European shags (Phalacrocorax aristotelis) using two in situ measures 24 (endoscopy of live birds and necropsy of birds that died naturally) and one non-invasive proxy 25 measure (faecal egg counts). In situ methods revealed that almost all chicks were infected (98%), 26 that infections established at an early age, and that older chicks hosted more worms, but faecal egg 27 counts underestimated prevalence. We found no strong evidence that burdens differed with host sex, 28 rank or hatch date. Heavier chicks had higher burdens, demonstrating that the relationship between 29 burdens and their costs is not straightforward. In situ measures of infection are therefore a valuable 30 tool in building our understanding of the role that parasites play in the dynamics of structured 31 natural populations. The costs that parasite infection can impose on their hosts can influence key demographic traits, 45 such as reproductive success and survival, which are crucial to the growth rate and hence viability 46 of populations (Albon et al., 2002; Newey et al., 2005; Redpath et al., 2006, Tompkins 2011. 47 Between-individual variation in nematode burden among juveniles in a wild hostHowever, parasitism is unlikely to affect all individuals in a population in the same way. Firstly, 48 individuals may host different burdens as a result of differences in exposure to parasites, 49 susceptibility to infection and resistance to its impacts. This contributes to parasite abundance 50typically showing a skewed distribution among hosts, which is particularly well documented in 51 macroparasites (Randolph et al., 1999; Shaw and Dobson, 1995). Secondly, once parasitized, the 52 relationship between parasite load and host fitness may vary between individuals due to differences 53 in tolerance for a given parasite load. Siblings, for example, may vary in the level of maternal 54 antibodies they receive (Pihlaja et al., 2006), males may be affected more than females due to 55 immunosuppressive effects of testosterone (Mougeot et al., 2009), and the relative benefits of 56 allocating resources between fighting infection and reproduction may vary with age (Adamo et al., 57 2001). These factors may lead to different types of host responding differently to infection, with 58 consequenc...