Laying gaps in the New Zealand Stitchbird are correlated with female harassment by extra-pair males

Abstract

Nutritional constraints during formation and laying of eggs can result in laying interruptions, that is a day during the laying period when no egg is produced. In previous studies of wild birds such laying gaps are almost exclusively correlated with cold weather (<6°C), though this relationship has only been studied in northern temperate species exposed to such conditions. I studied laying gaps in the Stitchbird (or Hihi, Notiomystis cincta) of New Zealand during three summers from 2000–01 to 2002–03, where minimum temperatures remained above 11°C. Female Stitchbirds interrupted laying in 22 of 163 clutches, and of these there was no relationship between the occurrence of a laying gap and temperature, rainfall, proximity to supplementary food, age of the female, clutch number, egg number within the clutch, reproductive output, or year. Instead, laying gaps were correlated with territorial intrusions by extra-pair males in the days before laying. I hypothesise that such harassment of the female, which involves vigorously resisted attempts at forced copulation, limits optimal patterns of nutritional intake during egg-formation by altering foraging behaviour. Females lost significantly more weight immediately before the laying gap compared to the loss of weight during a normal laying sequence, while gaining weight on the day of the laying gap, providing further support that laying gaps in this species result from nutritional constraints. The importance of factors in limiting access to key nutrients that result in laying gaps is likely to reflect the ecological and physiological constraints specific to the species and populations being studied.