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Climate change has a great impact on wildlife, which needs to be considered when designing conservation and management practices in habitats rich in biodiversity, such as wetlands and forests. Varied responses to climate change of species with different environmental preferences can be used to monitor different aspects of the environment. Since 2000, we have observed contrasting changes in the numbers of two related forest bird species, Goldcrest and Firecrest, monitored during autumn migration (14 August–1 November) over 1976–2024 by mist netting at 2 bird ringing stations on the Polish coast of the Baltic Sea. At both stations of the Operation Baltic project (Bukowo-Kopań and Mierzeja Wiślana), located ca 200 km apart, we observed a long-term decline in the number of Goldcrest since the 1980s but an increase in the number of Firecrest since the year 2000. These two species with ranges overlapping in Europe slightly differ in their preferences for breeding habitat and migration strategies. We found that in both species, the changes in migrating bird abundance in autumn at both stations were correlated with the Pan-European breeding population trends from the Pan-European Common Bird Monitoring Scheme (PECBMS) over 1982–2022. These correlations suggest that some Europe-wide factors underly these changes. Thus, we analyzed the effects of seven climate factors: the European Temperature Anomaly and the North Atlantic Oscillation Index in spring (March–April), summer (May–July) and autumn (October), and the Eurasian Snow Cover in October, on bird abundance indexes during breeding and autumn migration, using multiple regression models. Both species were more abundant during breeding and migration after warm springs, which conduce their early breeding and two broods per season, thus improving their breeding success. But with warm summers, Goldcrest were less abundant during breeding in Europe and on migration at Bukowo-Kopań. We attribute that pattern to Goldcrest’s tendency to overwinter in Scandinavia after warm summers. We suggest that this strategy became an ecological trap, as the climate change in Europe involves the increase of summer temperatures by 2 °C over the last 42 years but also earlier snow in October, which impedes Goldcrest’s feeding. These factors, combined with the decreasing proportion of coniferous forests in Scandinavia, which are Goldcrest’s main breeding habitat, assumably caused the species’ decline, which we expect to continue. Firecrest showed a weaker response to snow in October, but their autumn migration at both stations shifted earlier since the late 1980s, alleviating any effect of early winters. Warm spring and summers favor Firecrest abundance during breeding and migration. Firecrest also benefit from more broadleaf trees in Fennoscandia, thus we expect this species to expand its breeding range farther northeast and to continue the population increase. The change in forest management policy in Scandinavia by planting more broadleaf trees aims at increasing biodiversity and might considerably influence the forest environment. Thus, it is imperative to identify the methods suited for early detection of such changes. We showed that monitoring the numbers of migrating birds provides good indicators of the effects of short- and long-term environmental changes on bird populations.