Age-related associations between telomere length and metabolic pathways in an urban- and forest-dwelling songbird

Telomere length is an integrative marker of cumulative physiological stress, yet the metabolic mechanisms linking environmental conditions to telomere dynamics in wild birds remain largely unknown. Here, we tested whether plasma metabolomic profiles predict relative telomere length (RTL) in free-living great tits (Parus major), and whether these associations are modulated by age class and urban versus forest habitat. We profiled 85 targeted metabolites using liquid chromatography-mass spectrometry in 378 adults sampled across replicated urban and forest sites in southern Sweden during both breeding and non-breeding seasons. Using pathway-level principal component analyses, we tested associations between seven metabolic pathways (methionine and betaine metabolism, amino acid catabolism and the urea cycle, fatty acid and carnitine metabolism, purine and pyrimidine metabolism, and histidine metabolism) and RTL measured by qPCR. During the breeding season, RTL was significantly associated with metabolic variation across all seven pathways, but these associations were consistently reversed between first-year (2 yr) and older (3+ yr) birds. In older individuals, higher levels of amino acid catabolism end-products (arginine, urate, glutamate), methionine cycle intermediates, and purine degradation products were associated with shorter telomeres, suggesting an elevated oxidative burden and declining cellular maintenance capacity with reproductive experience. In younger birds, these same metabolites were positively associated with RTL, possibly reflecting ample antioxidant reserves during a first breeding attempt. Habitat type did not modulate any metabolite–RTL relationship, and no significant associations were detected outside the breeding season. These findings reveal age-structured metabolic correlates of telomere maintenance, specifically during reproduction, advancing our understanding of how life-history trade-offs are expressed at the biochemical level in a wild songbird.