Parasite and brood manipulation effects on telomere shortening
Telomere length (TL) was on average 7008±82 base pairs on day 5 (hereafter, bp±SE) and nestlings lost on average 264±10 bp over the 25-day measurement period (Table 1, Age effect). Within individuals, TL on day 30 was highly correlated with TL on day 5 (Fig. 1a; r=0.99, n = 322, P < 0.0001). TL correlated negatively with body mass, and this association was independent of age, as evidenced by a non-significant age x mass interaction (Table 1, telomere model; note that mass was Z-transformed by age group and brood size manipulation category). TL was on average 17±7 bp shorter per SD increase in body mass in the population (Table 1, Fig. 2).
Carnid flies were recorded in 142 out of the 175 nests and on 278 out of the 322 nestlings. Telomere shortening between day 5 and day 30 depended on parasite presence (Table 1, Fig. 1b, Age x Carnusinteraction): nestlings reared in infested nests shortened their telomeres by on average 100±33 bp more than nestlings reared in nests in which no parasite was recorded (Fig. 1b). Moreover, there was a significant three-way interaction between age, brood size manipulation and Carnus presence, indicating that the Carnus effect on telomere shortening depended on brood size. This was confirmed by post-hoc analyses in which we ran separate models for nestlings reared in reduced vs. enlarged broods. This revealed that Carnusinfestation accelerated telomere shortening in enlarged broods, but not in reduced broods (Table S2, Fig.3). The significant age x Carnuspresence interaction in the full dataset (Table 1) is thus driven by the effect in the enlarged broods. Nestlings reared in parasitized enlarged broods shortened their telomeres by on average 143±50 bp more than nestlings reared in non-parasitized enlarged broods (Table S2, Fig. 3).
Parasite prevalence varied between (χ25 = 13.02, p-value = 0.023) and within years, and to disentangle the effects of these different levels of variation on parasite-dependent telomere shortening, we separated within- and between-year parasite prevalence variation. We found a significant effect of parasite infestation on telomere shortening within- but not between-years (age x delta Carnus interaction, Tables S3 and S4), confirming that the parasite effect on telomere shortening is consistent within years and it is not dependent on parasite abundance variation between years (no between-year effect, age x average Carnus interaction, Table S2).