The energetic cost of mate guarding is correlated with territorial intrusions in the New Zealand stitchbird

Male stitchbirds (hihi) Notiomystis cincta attempt to minimize paternity losses by engaging in intense mate guarding centered on the fertile period of their social mate. While this strategy is directed at preventing intruding extrapair males from successfully achieving forced copulation with their female, it is likely to be energetically costly because of the large amount of time spent chasing intruders. In this study, I measured the energetic cost of mate guarding by recording the daily body weight of 28 breeding male stitchbirds during first clutch attempts and used a classification and regression tree (CART) analysis to assess the relationship between their weight fluctuations and (1) measures of mate-guarding intensity, (2) the total duration of extrapair male intrusions into their territory, (3) the age of the focal male, (4) the local population density, and (5) proximity to supplementary food. The CART model divided resident males into two groups based on the level of intrusions by extrapair males. For the low-intruder group, resident males maintained a stable weight during their female's fertile period, whereas, for the high-intruder group, males lost an average 4.3% of their body weight, with their weights reaching a minimum on days - 1 and 0 relative to the date of first egg lay. This pattern of weight loss mirrored the pattern of extrapair territorial intrusions into the focal male's territory. While the costs of harassment associated with forced extrapair copulation have previously focused on females, this study shows that these costs can also be significant for the resident male. Copyright 2006.