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Capsule: A playback survey comprising two visits to woodland in early spring can reliably detect Marsh Tits Poecile palustris and permit reasonable estimation of the number of territories. Aims: To assess the efficacy of an efficient survey method for detecting and estimating populations of Marsh Tits. Methods: Detection probability of colour-marked Marsh Tits, surveyed using playback, was assessed with Cormack–Jolly–Seber models. Reliability of territory estimates was compared between colour-marked and largely or wholly unmarked populations. Results: Playback surveys over two visits in early spring were highly effective in detecting individual Marsh Tits. Territory estimates were similar for woods in years where less than 20% of the population was marked compared to years when a higher proportion of birds were colour-marked, although territories may be underestimated in larger woods with unmarked populations. Conclusion: A playback survey comprising two visits in early spring is recommended as an efficient method of surveying Marsh Tit populations and locating individuals. A survey protocol is suggested.

Biometrics are commonly used to compare bird species. For Marsh Tits and Willow Tits in Britain there are few biometric data from birds of known age and sex, despite their value for population analyses in estimating the proportion of males and females in samples. Comparing measurements between the two species could also aid identification and the monitoring of these declining species in Britain. We present biometrics for a large sample of Marsh Tits of known age and sex, and new data for Willow Tits, which act as reliable reference material. Overall, adults of both species were larger than first-years and males were larger than females, but not among first-year Willow Tits. Marsh Tits were slightly larger and heavier than Willow Tits, but Willow Tits had proportionately longer tails. Discriminant analyses produced new equations for separating the species based on wing length and the measurement between the shortest and longest tail feathers. Probabilities were generated for estimating Marsh Tit population structure from samples of ringing data, but there was a greater overlap between sexes in Willow Tit measurements. We conclude by discussing issues of measurement accuracy and consistency in the collection and analysis of biometric data.

Capsule: All British Marsh Tits belong to subspecies Poecile palustris dresseri, being smaller than nominate P. p. palustris of central Europe. Aims: Determining the subspecies of Marsh Tit in Britain to test whether ssp. P. p. palustris occurs in northern England and Scotland, by assessing regional variation in size compared with central European birds. Methods: 1147 wing length and 250 tail length measurements from 953 Marsh Tits were compared between eight British locations to test for regional variation. Biometrics were compared between birds from Britain and six locations within the continental European range of ssp. palustris. Results: There was no regional variation in wing or tail lengths among British Marsh Tits, indicating that all resident birds belong to ssp. dresseri. There was no evidence supporting the existence of ssp. palustris in northern England. British birds were significantly smaller than those from continental Europe, with proportionately shorter tails, consistent across all age and sex classes. Conclusion: All British Marsh Tits should be considered as ssp. dresseri, with ssp. palustris being limited to continental Europe. With no evidence of regional variation in size within Britain, reliable sexing methods based on biometrics could be applied in demographic studies throughout the country.