The results of the present study confirm and extend the findings of Guglielmo & Williams (2003), that juvenile Calidris mauri making their first southward migration have substantially enlarged SIs, relative to adult migrants. Guglielmo & Williams (2003) concluded that this age-dependent difference in SI size was associated with mass, as opposed to length. The present study demonstrates the opposite, that the mass difference is due primarily to a length difference and that there is no age-related difference in length-corrected mass when cestode infection is controlled statistically. This discrepancy was due to a difference in measurement methodology, and highlights the benefits of Brambell's method for measuring SI length, over Leopold's (Freehling & Moore 1987), as well as the importance of measuring SI size in several dimensions. A difference in mass could reflect a difference in length or circumference, which have different implications for digestive function and/or pathology.
We proposed three hypotheses to explain the postgrowth age-dependent difference in SI size previously observed in Calidris mauri by Guglielmo & Williams (2003), and the four associated predictions received either full or partial support. The prediction associated with hypothesis 1, that an enlarged SI is associated with the growth phase of ontogeny, was supported. Premigrant juveniles that had attained full structural size had SIs that were 10·0% longer than those of premigrant adults, even though the juveniles were 10·5% lighter than the adults. The prediction associated with hypothesis 2, that SI size is closely associated with intake rate, was also supported. In association with the commencement of migration, the length and circumference of the SI increased in both age-classes. Although we do not report any data on intake rate, these results indicate a marked change in SI size that is associated with the onset of migration and are consistent with the effects of migratory hyperphagia. There was partial support for the predictions associated with hypothesis 3, that the age-related difference in SI size is associated with cestode infection. Although the length of the SI was independent of cestode infection, the length-corrected mass of the SI was 9·7% heavier in infected migrants, and juveniles had a higher prevalence of infection than adults in one of two years.
Guglielmo & Williams (2003) suggested five hypotheses to account for their observation of a larger SI in refuelling juvenile Calidris mauri. In light of further research aimed at exploring this observation (Stein et al. 2005; this study), we are now able to evaluate their hypotheses, which fall into four general categories: growth, foraging proficiency, digestive strategy and endoparasites. Their first hypothesis, that larger SIs in juveniles indicate continued growth during migration, was not supported by their morphometric data or by ours. Juveniles achieve full structural size before leaving the breeding grounds. Their second hypothesis, that juveniles might be consuming a lower-quality diet, remains untested. Although it is possible, and in some regards likely, that diet quality declines seasonally and that the later-migrating juveniles have a lower-quality diet (Stein et al. 2005), there is no direct evidence to support this at Boundary Bay. If this was true, then we would expect Julian date to be a significant predictor of SI size within each age class and it was not. Their third hypothesis, that juveniles ingest more sediment than adults, and that this might explain their heavier but not longer SIs, was not supported. In the present study we demonstrate that the age-dependent difference in SI mass is due to length, not length-corrected mass. In addition, Stein et al. (2005) demonstrated that faecal samples from juveniles contained a smaller proportion of ash, which would include any ingested sediment, than those from adults. Their fourth hypothesis, that juveniles employ a digestive strategy that maximizes extraction efficiency by having large guts and slow passage rates, also remains untested. Stein et al. (2005) suggest, however, that refuelling juveniles might be processing large volumes of food quickly, thereby sacrificing efficiency, with the aim of maximizing the net rate of energy gain. Stein et al. (2005) report data that are consistent with this idea, but are not able to rule out the possibility that diet quality is lower for the later-migrating juveniles. In addition to having a longer SI, refuelling juveniles also have a substantially smaller (27%) proventriculus, and this led Stein et al. (2005) to conclude that juveniles probably do process prey differently from adults. Guglielmo & Williams (2003) fifth hypothesis, that the enlarged SIs of juveniles might be associated with higher intestinal parasite loads, receives some support from the present study. Although cestode infection had no influence on SI length, which is the primary age-related difference in SI size, the length-corrected mass of the SI was heavier in infected individuals.
ontogeny of intestine size in association with migration
In birds that have self-feeding precocial chicks, the relative size of the SI (to body mass) is larger during growth than it is in adults (Konarzewski et al. 1990; Starck 1998). In addition to growth-specific energy requirements, precocial chicks have high thermoregulatory and activity costs, which result in higher total metabolizable energy requirements than predicted by allometry (Schekkerman & Visser 2001), and this deviation is particularly large in long-distance migrants that breed at northern latitudes (Schekkerman et al. 2003). The self-feeding precocial chicks of Calidris canutus meet the high mass-specific energy requirements of growth and self-maintenance through hyperphagia, and are able to achieve extremely high growth rates despite high thermoregulatory and foraging costs (Schekkerman et al. 2003). For long-distance migrants with precocial young, such as Calidris sandpipers, an enlarged SI in fledglings that have attained full structural size is probably the result of selection for high intake rates during growth. Consistent with this idea, the sample of premigrant juvenile Calidris mauri reported on here had SIs that were 10·0% longer than those of premigrant adults. These premigrant juveniles were in the final stages of growth at the time of capture. They were approximately 22 days of age, had achieved full structural body size, and their primary feathers were fully grown (R. W. Stein, personal observation). However, the juveniles had not yet reached the predeparture mass exhibited by adults. The results presented here indicate that premigrant juvenile Calidris mauri retain an elongated SI that was initially associated with growth, and that there is an additional increase in length associated with the onset of migration.
Age-dependent differences in SI size have received little attention in studies of refuelling physiology and organ dynamics during migration (but see Guglielmo & Williams 2003; Stein et al. 2005); this is somewhat surprising because migration-related mortality can be quite high (Owen & Black 1989; Sillett & Holmes 2002), particularly for juveniles (Owen & Black 1991). Juvenile Calidris mauri initiate southward migration 1 month after adults, and navigate the same route but under a more severe predation regime (Lank et al. 2003). The costs of carrying an enlarged SI during their first migration could be substantial for juveniles, but the benefits of doing so have not been considered until recently. Stein et al. (2005) investigated the possible benefits of maintaining an enlarged SI to refuelling juvenile Calidris mauri, and determined that juveniles exhibited: (1) larger volumetric digestive capacity, (2) lower total enzymic digestive capacity and (3) a higher proportion of residual dietary energy in their faeces. This led Stein et al. (2005) to suggest that refuelling juveniles might be rapidly processing large volumes of food with the aim of maximizing the net rate of energy gain, rather than digestive efficiency. Their results, however, could not rule out the alternative explanation that juveniles were simply consuming a lower-quality diet (Stein et al. 2005). In either case, the later-migrating juveniles are confronted with additional energetic costs associated with maintaining an enlarged SI and it appears that the only benefit from doing so is increased volumetric digestive capacity. An enlarged SI may reflect a somewhat counter-intuitive response to selection acting to minimize the duration of migration for the later-migrating juveniles.
endoparasitic infection and intestine size
In addition to being confronted with increased predation pressure (Lank et al. 2003), later-migrating juveniles may also be at a higher risk of contracting food-borne parasites, such as cestodes, than adults. Consistent with this idea, controlled experiments on age-dependent susceptibility of Gallus gallus (domestic chickens) to cestode infection demonstrate that host resistance increases with age, even under benign conditions (Gray 1972). Juvenile Calidris mauri migrants had a higher prevalence of cestode infection in one year, and this is probably the more typical situation. Annual variation in cestode prevalence in juveniles could result from variation in the abundance of the intermediate invertebrate hosts or the infection rates of these prey items. Once ingested, immature cestodes anchor into the intestinal wall with rostellar hooks, thereby producing pathological effects that impair the host's health and energy assimilation. In the most extreme case, cestode infection leads to intestinal obstruction and can result in the death of the host (Bailey et al. 1996). Sub-lethal pathologies associated with cestode infection include localized inflammation at the site of attachment, raised lymphoid nodules, denuded mucosal epithelium accompanied by hyper-regenerative response, and hypertrophy of the muscularis (Jones et al. 1996a,b). In Calidris mauri, sub-lethal effects of cestode infection were apparent in the length-corrected mass of the SI, which was 9·7% heavier in infected individuals; it is likely that this was the result of hypertrophy of the muscularis and inflammation of the SI. Cestode infection, however, was not associated with the post-growth age-dependent difference in SI length.
Another important sub-lethal effect of cestode infection is competition between the host and parasite for food energy, which impairs nutrient assimilation by the host. Munger & Karasov (1989) demonstrated that cestode infection can have negative impacts on the energy budget of Peromyscus leucopus (White-Footed Mouse), and suggested that the magnitude of the energy losses could compromise host performance during periods of peak energy demand, such as reproduction. For Calidris mauri, sub-lethal effects of cestode infection might decrease the overall speed of migration by increasing the time required to refuel. A decrease in the overall speed of migration could result in increased mortality, and this would have a disproportionate impact on juveniles owing to a higher prevalence of cestode infection in some years. The present study is the clearest example of a cestode-related effect on the morphology of the SI in a wild bird (see Shutler, Alisauskas & McLaughlin 1999), and we were able to resolve its effect on length-corrected mass because we measured the SI size in several dimensions. The interaction between age-dependent susceptibility to intestinal parasites and refuelling performance warrants further consideration in studies of refuelling physiology and organ dynamics during migration.