The breeding biology of Bullock's oriole (Icterus bullockii ) in the southern terminus of the Great Plains

During the Spring of 2006, efforts were made to find all Bullock's Oriole (Icterus bullockii) nests in two recreational parks in San Angelo, Texas. These parks are dominated by mature mesquite trees (Prosopis glandulosa). Observations of 26 nests investigated breeding biology of this oriole at the southern terminus of the Great Plains to compare this population to Bullock's Oriole in other regions of its breeding range and to Baltimore Oriole (Icterus galbula) in its breeding range. Orioles began arriving in the second week of April and nest construction began in the third week of April. Mesquites were the nest tree for 25 nests and one nest was built in a pecan tree (Carya illinoinensis). Nest construction took two weeks and the rate of oviposition was typically one egg/day. Average clutch size (x¯ =4.59, n=22, SD=1.18), egg dimensions (23.3 mm × 16.1 mm) and weight (2.9 g) were similar to Bullock's and Baltimore Oriole eggs. Asynchronous hatching occurred in 2-4 days and hatchlings grew at the rate of 3.0 g/day. Hatching and fledging success were both low at 43% and 39%, respectively. Nest placement was lower to the ground and in shorter trees at these study sites than at other sites and nests were larger at this site. Geographic variation in climate and floral assemblages explain these variations. Cowbird parasitism was observed in four nests. One nest hatched and fledged two cowbirds with no adverse affects to oriole hatchlings. Two cowbird eggs were rejected and one did not hatch.