Records

531. Burke Museum nest record cards=380 (Class cards=143); egg sets=38; field notes=45; no source code=68.

Counties

Asotin=1; Benton=4; Chelan=4; Clallam=8; Douglas=7; Ferry=2; Grant=1; Island=10; King=202; Kitsap=5; Kittitas=8; Okanogan=2; Pacific=19; Pend Oreille=2; Pierce=31; San Juan=1; Skagit=6; Skamania=2; Snohomish=6; Spokane=105; Thurston=2; Walla Walla=2; Whatcom=43; Whitman=45; Yakima=13.

First-egg dates

Earliest 28 March (Julian day 87); latest 3 August (Julian day 215).

Over 70% of records fall between mid-April and mid-May (median 5 May, Julian day 125). However summer records are surely seriously under-represented because virtually all of the 143 class cards are for April and May, and because birders are more active in spring than mid summer. Thus the strong right skew in the distribution, and the secondary peak of laying in late June suggest that successful birds attempt second broods.
First-egg dates vary with geography and elevation. For eastern Washington (N=196), and for areas above 2,000’ (N=109) first eggs are a few days later (both medians 8 May, Julian day 128) than for western Washington (N=335, median 4 May, Julian day 124)). However, robins, are such habitat generalists that separating records just by elevation fails to segregate high and low elevation birds. For example, they breed early in open country above 2,000’ throughout eastern Washington. When the subset of high elevation records is restricted to the Olympic and Cascade Mountains, first-egg dates are later by more than 3 weeks (N=21; median 31 May, Julian day 151).

Clutch size

231 usable records. Mean 3.5 eggs. The two large clutches of 6 and 7 eggs may signify laying by more than one female.

Season and clutch size

There was not even a hint of clutch size changing through the nesting season. The line was almost perfectly flat despite the large sample of nests.