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Washington Birds Breeding Phenology Project


Interpreting the Breeding Phenology Data

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What is a species account?

We present our breeding phenology data as "species accounts"—summaries of the timing of laying and how it is affected by geography and elevation, clutch size variation and how it changes with season, and a list of the counties from which we have records of breeding for Washington birds. The accounts that follow summarize one or more nesting records for 220 of the 244 species listed as Washington breeders in the recent Washington State Gap Analysis (Smith et al. 1997). We list the species for which we have no records to alert observers to the special need for filing Nest Record Cards for these species.

A cursory examination of the accounts presented here reveals great variability in the numbers of records and in their distribution across Washington. Much of this variability is explained by the ease with which nests may be found and by their accessibility to observers. For example, early egg collectors from the late 1800s and early 1900s were master nest finders. As a result, for many species most records come from this era, because contemporary observers are rarely as skilled at finding nests.


What is striking is how few and spotty the records are for most species. Partly, of course, this is simply reflective of people not knowing that the Burke Museum was a repository for such records. For example, common species that breed over large areas of Washington may have records for only a small subset of the counties included in their Washington range. In contrast, certain seasons and areas are over-represented in the accounts because they have been intensively studied by students or professionals. For example, between 1975 and 1980, students at the University of Washington filled out Nest Record Cards as part of their course requirements in Ornithology. These records are noted in the species accounts because they are concentrated at the beginning of the breeding season in April and May (before spring quarter ends) and because most come from the greater Seattle area. Many records come from Grant Country because Rohwer and his students studied blackbirds there in the 1970s and 1980s. Finally, a few professional researchers have filed cards from their studies. Thus, we have many records for Red-winged Blackbirds, Canada Geese, Golden Eagles, and Chukars.

Canada Goose and gosling
Canada Goose and gosling (Branta canadensis).
Photo by Stuart MacKay

All of these observations point to the fact that we badly need a renewed commitment to finding nests and visiting them repeatedly to track their outcome. Indeed, so few of the records currently available are characterized by repeated visits that we have made no effort to summarize nest survival in this first set of accounts. Yet nesting success is fundamental to estimates of the viability of populations and is just one of the many ways that contributing records to this program could contribute effectively to conservation of the birds of Washington.

Format of the Accounts

We provide statistical summaries for species for which we had at least 8 breeding records; for those with fewer, we describe each record. For species whose records are summarized statistically we use the following standardized format:


Eggs of Sharp-tailed Grouse
Eggs of Sharp-tailed Grouse (Tympanuchus phasianellus).
Photo by Sharon Birks

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: list 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 day 40 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 preparator measured sizes of all enlarging pre-and post-ovulatory follicles. (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. Graphs showing these changes are presented only when the decline (or increase) in clutch size was statistically significant. For these cases 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.

Eggs of Brewer's Blackbird
Eggs of Brewer's Blackbird (Euphagus cyanocephalus), with egg of parasitic cowbird (Molothrus ater).
Photo by Scott Wilsom