Within-population variation in breeding group composition (breeders and nonbreeding helpers) among 3 Acorn Woodpecker populations. Listed are means (range).

 
  Hastings Reservation, California Water Canyon, New Mexico Research Ranch, Arizona
Mean breeding group size 4.60 (2–15) 2.64 (2–5) 2.15 (2–3)
Male breeders/group 1.82 (1–8) 1.23 (1–3) 1.05 (1–2)
Female breeders/group 1.30 (1–3) 1.00 1.00
Male nonbreeding helpers/group 0.87 (0–8) 0.24 (0–2) 0.10 (0–1)
Female nonbreeding helpers/group 0.61 (0–7) 0.08 (0–1) 0.00
Percent of groups a pair only 16.2 59.8 85.4
Percent of adult nonbreeding helpers 32.2 12.0 5.0
Sample size (no. of group years) 1641 164 33