Modification and deterioration of old-growth forests by industrial forestry have seriously threatened species diversity worldwide. The loss of natural habitats increases the concentration of circulating glucocorticoids and incurs chronic stress in animals, influencing the immune system, growth, survival, and lifespan of animals inhabiting such areas. In this study, we tested whether great tit (Parus major) nestlings grown in old-growth unmanaged coniferous forests have longer telomeres than great tit nestlings developing in young managed coniferous forests. This study showed that the patches of young managed coniferous forests had lower larval biomass than old-growth forests. Since insect larvae are the preferred food for great tit nestlings, the shortage of food may divert energy resources away from growth, which can show up as physiological stress, often raising the heterophil/lymphocyte (H/L) ratio. The H/L ratio revealed a significant difference in stress levels, being the highest in great tit nestlings developing in young-managed pine forests. We also found that the development of great tit nestlings in young managed forests had significantly shorter telomeres than in old-growth forests. Although nestling survival did not differ between the habitats, nestlings growing up in old-growth forests had greater telomere lengths, which can positively affect their lifespan. Our results suggest that the forest habitats affected by industrial forestry may represent ecological traps as the development of young birds in deteriorated environments can affect the age structure of populations.

When selecting a habitat for breeding and offspring rearing, it is optimal for organisms to select a stable habitat with high amounts of quality resources. However, organisms may inadvertently select a habitat for breeding that would initially appear preferential and healthy but may, in reality, be detrimental to parent and/or offspring fitness. Such ecological traps may even be the result of human habitat modification and conservation efforts. In this study, we tested whether migratory European pied flycatcher (Ficedula hypoleuca) offspring cavity-nesting in patches of forest affected by outbreaks of the great web-spinning sawfly (Acantholyda posticalis) incurred fitness detriments characteristic of ecological traps when compared to flycatchers that nested in patches of unaffected forest. Outbreaking sawfly larvae overconsume Scots pine (Pinus sylvestris) needles and substrate, which decreases resource availability directly and indirectly – through cascading effects – for birds inhabiting the forest. In installing nest boxes to attract breeding pairs and potentially combat the outbreak, we found that flycatchers inhabiting areas of sawfly outbreaks had similar clutch sizes to pairs breeding in healthy forest patches. Contrarily, the fledgling number and body condition were significantly lower for those nesting in the damaged forests. Though larvae serve as the primary food source for nestlings, the forest patches damaged by sawflies also had lower larval biomass from rapid resource decline due to overconsumption. In provisioning nest boxes for migrating flycatcher pairs in forest patches that were subsequently impacted by a pest insect outbreak, an ecological trap arose for those pairs choosing to nest in what appeared to be an unaffected forest at first. Given the inability of breeding pairs to distinguish habitat quality on initial inspection, we suggest that nest boxes be used with caution in areas with unstable habitat conditions when attracting migratory birds given the trends of their declining global numbers.