Tracking small passerines using miniaturized location tags is a rapidly expanding field of study. In a 1-year study, we tested whether there were any short-or longer-term effects of fitting geolocators weighing 3% of body mass on male Pied Flycatchers Ficedula hypoleuca. In the deployment year, we compared adult provisioning rates to nestlings, nestling growth and nest success between nesting attempts in which adult males were fitted with a geolocator, with control nests where males had the same capture history but were not tagged. We found no difference between treatments in provisioning effort by males or their associated female 2 days after geolocator fitting, in terms of nestling growth, subsequent brood reduction or nest success. Return rate, arrival date on territories, nest timing and breeding parameters were compared between tagged and untagged males in the following breeding season. We found no difference in return rate or arrival date, and no difference in nest timing, fecundity or outcome. Our study suggests that fitting lightweight tags to small passerines need not affect behaviour, breeding or apparent between-year survival. However, tagging new species should still require assessment and comparison with well-matched control cohorts, and it should be recognized that tag effects could vary between years and populations, mediated by environmental conditions.

Songbird populations are in decline all over the world, and our understanding of the causal mechanisms remains surprisingly limited. It is important to identify the extent of individual variations in migratory behaviour to better understand species' ability to respond to environmental change. We describe the annual migratory behaviour of British breeding European Pied Flycatchers Ficedula hypoleuca by using light‐recording geolocators. During both autumn and spring migrations, individuals used previously unknown staging areas on the Iberian Peninsula and northern West Africa. Furthermore, partial sex‐specific segregation in the location of non‐breeding areas was observed within West Africa, with females located west of males. We also found sex‐based phenological differences, with females staying longer in non‐breeding areas and undertaking the spring Sahara Desert crossing later than males. Irrespective of sex, multiple use of the two predominant staging regions was identified during both migrations, with 63% of individuals stopping more than once in these regions. We also identified instances of migratory behaviours rarely documented in individually tracked songbirds. These include making daytime landfall during the Sahara crossing, and a case of a temporary retreat migration, with an individual aborting a spring Sahara crossing before making a second successful crossing 14 days later. Together, our results show variability in migratory behaviour both between sexes and between individuals. For Pied Flycatchers, such flexible migratory behaviour may increase their resilience to environmental change.

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Temperature plays an important role in determining the breeding phenology of birds in temperate climates, with higher spring temperatures associated with earlier breeding. However, the effect of localised territory-scale temperature variations is poorly understood, with relationships between temperature and breeding phenology mostly studied using coarse-grained climatic indices. Here, we interpolate spring temperatures recorded at 150-m 2 grid intersections encompassing 417 ha of forest to examine the influence of territory-scale temperature, and its interaction with mean annual temperature, on territory selection, breeding phenology, clutch size and fledging success for three co-occurring single-brooded passerine birds using data from 672 nests over four years. All species exhibited significant trends in reproductive traits associated with territory-scale temperature. Pied flycatchers Ficedula hypoleuca settled in cooler territories first, where they raised more fledglings. Blue tits Cyanistes caeruleus laid larger clutches in warmer territories in warm years and always laid earlier at warmer territories irrespective of annual temperature variation. Contrastingly, pied flycatcher and wood warbler Phylloscopus sibilatrix breeding phenology was earlier at warmer territories in cool years and cooler territories in warm years, with wood warbler clutch size responding similarly to this interaction. Greater previous breeding experience and higher rates of historical territory occupancy (territory quality) also predicted earlier breeding phenology and higher fledging success for pied flycatchers. We suggest that the migratory pied flycatcher and wood warbler are best synchronised with their prey availability in cooler years at a local population level. However resident blue tits match local phenology across all years, which is potentially advantageous under warmer predicted climate change scenarios. We conclude that temperature at the territory scale can be an important driver of settlement and breeding phenology and influence reproductive traits.