LG. Nutritional importance of seeds and arthropods to painted spiny pocket mice (Lyomis pictus): the effects of season and forest degradation

Temporal and spatial fluctuations in food abundance may affect the feeding habits of vertebrates in tropical dry forests. We explored the effects of season and forest degradation in dietary patterns of the painted spiny pocket mouse ( Lyomis pictus (Thomas, 1893)) (Heteromyidae) in a Mexican tropical dry forest. We used carbon (13C, 12C) and nitrogen (15N, 14N) stable isotope analyses to test the hypotheses that (i) L. pictus would increase its use of arthropods during the rainy season when seeds are less available on the forest floor and (ii) that L. pictus would increase its use of arthropods in degraded forest compared with conserved forest. Our hypotheses were wrong because assimilated biomass was derived almost exclusively from seeds in both seasons and the importance of arthropods was marginal in both sites. Examination of food remains in feces and cheek pouches confirmed these trophic patterns. Seed hoarding during the season of high seed availability probably allows L. pictus to subsist on a seed-based diet throughout the year in conserved and disturbed forests. This behavioral trait would enable L. pictus to maintain its specialized feeding habit in environments threatened by habitat degradation.

Forest fragmentation and body condition in wintering black-capped chickadees

Winter represents a critical energy management challenge for northern birds. Subzero temperatures may persist almost without interruption for months, food supply steadily decreases, day length greatly limits time available for foraging, and they must endure long fasting at night. Body condition represents a potentially important concept to our understanding of how severe winters impact individual fitness and populations. Many researchers have addressed body condition in small wintering birds. However, studies published so far have rarely been placed into a landscape context, despite reported effects of landscape structure on the ecology of birds exposed to severe winters. We investigated whether forest cover affects body condition in a population of black-capped chickadees ( Poecile atricapillus (L., 1766)) during winter. We measured residual body mass, fat score, and rate of growth of induced feathers to assess body condition in 12 landscapes with forest cover ranging from 10% to 87%. Forest cover was not associated with any of these measures of body condition, suggesting that highly deforested and fragmented landscapes were not energetically more costly to chickadees. However, we may not rule out the possibility that this result was biased against by a higher emigration or mortality rate of individuals in poor body condition.