Western Tanager - Major Life Events

Major Life Events

Western tanagers arrive on their breeding grounds in spring. Breeding primarily occurs among ≥2 years old, beginning in May and continuing into July, although some 1st-year western tanagers also breed. In the Sandia Mountains of north-central New Mexico, western tanagers were heard singing beginning in late May, and the 1st nest was found in early June. In public open-space areas in Boulder County, Colorado, the start of the western tanager breeding season was estimated as 28 May, and the peak of the breeding season, defined as at least 50% of western tanager nests active, was from 6 June to 1 July. In the Southwest brooding generally begins in early May, while in the Northwest brooding starts typically in mid-June. Brooding can begin earlier in British Columbia and Alberta than in the northwestern United States. An egg-laying date as early as 16 May in British Columbia was estimated by back calculation, and a complete egg set was observed as early as 26 May in Alberta.

Cup nests are built by the female, take about 4 or more days to construct, and are made from twigs, rootlets, grasses, and pine needles. There is no evidence for 2nd broods in western tanagers. However, a review notes a nesting attempt after a failed nest in west-central Idaho and suggests that renesting is a substantial source of late nesting attempts. In addition, renesting was suggested as the explanation for a few late nests observed in Boulder County, Colorado.

Clutch size is typically 3 to 5 eggs. Average clutch size in 10 nonparasitized nests in Boulder County was 3.8 eggs. Average clutch in the Southwest may be smaller than that of western tanagers nesting in the North. Egg laying generally takes about 1 day per egg. The female incubates the eggs for approximately 13 days, although shorter incubation periods have been reported. The young are fed by both parents and typically fledge 11 to 15 days after hatching. Immature western tanagers have been observed with the parents at least 2 weeks after fledging.

Immature western tanagers initiate migration later than adult birds. Generally western tanagers leave more northerly locations in late summer or early fall while those in more southerly areas may stay as late as early November.

Reproductive success of western tanagers varies widely between studies and across years. An average annual nest success probability estimate is 0.186 over 3 years, with a low of 0.035 and a high of 0.349. In a northern Arizona study area, an average of 43% (n=7) of nests succeeded to the nestling stage. In Boulder County, nesting success varied from 11.3% to 75.3%, with an average of 51.8% over a 3-year period. Daily nest survival rate on ungrazed sites in northeastern New Mexico was 0.955, which was not significantly (p<0.05) different from the 0.973 daily nest survival rate found on grazed sites. Nest predation is the leading cause of nest failure. Predation rates ranged from 30% (n=48) in a study in New Mexico pinyon-juniper woodland to 86% (n=14) in a mixed-conifer forest in Idaho.

Western tanagers can live several years. The annual average survival rate is 0.753 and a return rate is 30.1% for western tanagers in west-central Idaho. A wild western tanager 7 years and 11 months old has been documented from banding data.