FORMAT OF THE ACCOUNTS

We provide statistical summaries of records when our files included more than eight breeding records for a species.  For species with eight or fewer records we summarize each record.  For species whose records are summarized statistically we use the following standardized format.

Records lists the number of records in the data file used for this report.  These are further subdivided by source to alert users to potential biases.

Counties lists the counties from which the records originate followed by the number of records for each county.

First-egg Dates lists the earliest and latest date for the laying of the first egg of a clutch.  To make differences in the timing of laying among species or among regions within species immediately apparent, distributions of first-egg dates are summarized in histograms, all of which start at day40 and end at day 275.  These are Julian days, numbered 1 through 365, starting at January 1.

First-egg dates are extrapolated from information associated with the record.  Thus, if the nest was first checked with young, then the date the first egg was laid is found by subtracting the estimated age of the young, the length of the incubation period, and one day for each egg of the clutch in excess of one (most species lay one egg a day).  Egg collectors usually assess the state of incubation, so extrapolations of first-egg dates were also possible for most collected clutches.  Without these extrapolations there would be very few records of first-egg dates because nests are rarely found early enough and checked frequently enough for the laying date of the first egg to be established directly.

Clutch Size is summarized either with a histogram or with a list of clutch sizes followed by the number of clutches observed of each size.  Only some records provide reliable measures of clutch size.  Nests with young were not used to estimate clutch size because the number of young in a nest is often less than the clutch size due to eggs failing to hatch and to nestling mortality.  Similarly, nests checked just once with eggs were excluded unless there was some indication that the clutch was complete.  Thus, for clutch size summaries, we indicate the number of usable records.

Most records of clutch size come from nests checked twice without a change in clutch size, or from collected egg sets with label notations indicating that incubation had begun.  Clutch size can also be inferred from dissected ovaries providing that the sizes of all enlarging pre-and post-ovulatory follicles were measured by the preparator.  Pearson and Rohwer (1998) explain this technique in detail and show that it gives accurate measures of clutch size for small birds that lay clutches no larger than about 6 eggs.

Season and Clutch Size summarizes changes in clutch size through the nesting season.  For these analyses only “usable clutch size records” were included.  For most species the number of usable clutch size estimates is far too small to reliably establish how clutch size changes through the season. For the cases in which change was significant, we present the regression equation summarizing the relationship between Julian day and clutch size, and state the number of eggs by which mean clutch size changes for the duration of that species’ laying season in Washington.

Parasitism summarizes what is known about interspecific and conspecific brood parasitism.  The Brown-headed Cowbird is the only obligate brood parasite in Washington, and accounts for most of the records of parasitism.  However, several species of ducks and geese, coots, and game birds occasionally or regularly parasitize the nests of other species, as well as nests of other members of their own species.  In most cases these species are facultative brood parasites, meaning that females may lay a series of eggs parasitically before going on to produce a clutch that they tend for themselves.  A section on parasitism is presented only if there is indication of parasitism in the records from Washington.

Photographs are included in the accounts in part to display the striking beauty of avian eggs, and also to show variability among clutches of the same species.  Less variable eggs may be represented by fewer egg sets, while others have photographs of several sets.

Unfortunately, the color of eggs preserved as specimens may vary somewhat from that of eggs you see in nests.  Small eggs, and especially small eggs that are white, become lighter when blown, and lose the pinkish glow of light transmitted through the yolk and albumen of fresh eggs.  Spectacularly blue eggs, or eggs with bluish backgrounds are notorious for fading to a dull bluish-gray with time, presumably as the blue pigments are oxidized. Thus many of the eggs represented in the Phenology are paler or even a different color from fresh eggs (see Swainson’s Thrush and American Crow accounts).

While most of the photos are of specimens from the Burke Museum (UWBM), we would like to thank the University of Puget Sound (PSM) for the use of their egg collections as well.  We believe that this is the most comprehensive set of egg photographs available on the web, and should help those interested in eggs identify them.