A Comparison of the Reproductive Efficiency of Second-Year and After-Second-Year Male Northern Orioles (Icterus Galbula) near Hays, Ellis County, Kansas

Abstract

The relative success of nesting pairs of the Northern Oriole (lcterus galbula) with second-year (SYM) and after-second-year (ASYM) males was investigated on a 120- hectare area near Hays, Ellis County, Kansas during 1981 and 1982. Clutch size, fledging success, and fledging period were examined. Sixty-one nests studied had an overall success rate of 93.1%, with 83.3% for SYM nests and 95.6% for ASYM nests. Nests with SY males had significantly smaller clutch sizes (P < 0.01) and significantly fewer fledglings (P < .05). Nests with SY males had a mean clutch size of 2.7 +- 0.52 and fledged 1.6 +- 1.30 nestlings. Nests with ASY males had a mean clutch size of 5.0 +- 0.54 and fledged 3.7 +- 1.16 nestlings. In 01/01/1981 all nestlings fledged in a 19-day period, whereas 27 days were required in 1982. The less synchronous fledging in 1982 probably was due to inclement weather, causing some pairs to renest, and might have increased the success rate of SYM nests somewhat. Nests of SY males did not begin fledging until more than half the ASYM nests had fledged. Other factors discussed include first arrival dates, renesting, predation, cowbird parasitism, territoriality, the role of non-breeding individuals, and the effect of inexperience of first year breeders. Data were insufficient to determine whether SYM nests were more vulnerable than ASYM nests because of these factors.