Breeding density, male age and mixed reproductive strategies in the Northern oriole (Icterus galbula bullockii)

1) This study investigated the relationship between breeding density, male age and the reproductive strategies employed by northern orioles (Icterus galbula bullockii) in central coastal California, USA, 1994-1996.;2) A nesting association was found to occur between northern orioles and yellow-billed magpies (Pica nuttalli). Northern orioles often chose to nest close to magpie nests (< 50m) and, apparently as a result, suffered significantly lower nest predation (13.5%, 5/37) than did northern orioles nesting further away from magpies (43.3%, 13/30). The clumping of northern orioles nests that occurs in some areas is suggested to be largely a result of northern orioles attempting to nest close to the less abundant, semi-colonial magpies to gain from the predator protection they afford.;3) Single-locus DNA profiling revealed that overall 32.2% of chicks were extra-pair young (EPY) and that 46% of nests contained EPY. EPY was shown to be distributed across nests in a highly bimodal fashion. Paternity was assigned for 44.6% (29/65) of EPY. Low levels of intra-specific brood parasitism were also detected (1.0% [2/202] of chicks in 4.6% [2/48] of nests).;4) Male age/plumage status was shown to have a considerable affect on the frequency of extra-pair paternity. On average, first-year (subadult plumage) males lost 55.3% of their paternity while older (adult plumage) males lost only 20.6% of their paternity. Furthermore, paternity assignment indicates that it is nearly always adult males that gain the extra-pair paternity (EPP). The uneven distribution of paternity resulted in adult males having a significantly higher annual reproductive success rate than subadult males (4.3 v 1.9 fledglings, respectively). The bimodal distribution of EPP and the evidence that adult males gain higher levels of both within and extra-pair paternity are, in the light of female control of extra-pair fertilisations (EPFs), best explained by the 'good genes' hypothesis. Females appeared to prefer fertilisations from older 'better quality' males and it is suggested that they base this choice on the plumage differences that occur between adult and subadult males. This result has considerable implications for the evolution of delayed plumage maturation (DPM) and a new hypothesis (the honest plumage hypothesis) is suggested that may explain the evolution of DPM in the northern oriole and other bird species.