Consequences of spring arrival dates for the breeding phenology of migratory warblers

Abstract

ABSTRACT Capsule Early male arrival on the breeding grounds results in early pairing but not early nesting in Chiffchaffs Phylloscopus collybita, and Chiffchaffs can nest, fail, and re-nest before Willow Warblers Phylloscopus trochilus begin nesting. Aims To quantify the consequences of timing of arrival for the subsequent timing of pairing, nesting, and re-nesting of short-distance (Chiffchaff) and long-distance (Willow Warbler) migrants. Methods The arrival dates of 118 Chiffchaffs and 20 Willow Warblers were measured from March to June over 10 weeks in Foxley Wood nature reserve, Norfolk. Colour-ringing of 56 Chiffchaffs (55 males, 1 female) and 11 Willow Warblers (10 males, 1 female) was used to relate individual arrival dates to timing of male pairing, clutch initiation, and re-nesting. Results Male Chiffchaffs started to arrive in early March and increased rapidly in number until early April, while the arrival of male Willow Warblers began in early April. Early-arriving male Chiffchaffs paired earlier than later-arriving individuals, but timing of clutch initiation was unrelated to male arrival dates. Early nesting by Chiffchaffs allowed replacement clutches following nest loss, the earliest of which occurred 12 days after the first Willow Warbler male had paired. Conclusions Although early arrival in male Chiffchaffs does not translate into earlier nesting, timing of nesting in Chiffchaffs was sufficiently early to allow time for replacement clutches following nest loss. The later arrival of Willow Warblers is likely to mean fewer opportunities for replacement clutches following nest loss. This difference in breeding phenology could therefore contribute to differences in productivity between the species, especially if nest failure rates are high.