Identifying Communal Crow Roost Dynamics and Transitions through Citizen Science

Abstract

American Crows (Corvus brachyrhynchos) form large, nightly communal roosts of 100s to 100,000s of individuals throughout the non-breeding season. Obtaining comparable estimates of their numbers facilitates monitoring and understanding of roost dynamics. We describe a simple counting method that utilizes citizen scientists to produce annual estimates of crows at a large communal roost in Bothell, WA. Over the last ten years, volunteers have counted migrating crows during their pre-roost approach from multiple flyover locations. Observed changes in the number of crows led to the discovery of a spontaneous roost dissolution, ephemeral roosting sites, and two newly formed, longer-term communal roosts. Similar citizen scientist-based approaches can be applied at other sites to better assess the general dynamics of crow roosting behavior.

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Figure 1. American Crows (Corvus brachyrhynchos) pre-roosting near a communal roost in Redmond, WA. Photograph by Douglas W. Wacker, 11-14-2024.

Introduction

American Crows (Corvus brachyrhynchos) form communal roosts, primarily during non-breeding, numbering in the 100s to 100,000s (Rhoads, 1886; Caffrey et al., 2025). Crows move daily between their nightly roost and diurnal foraging areas (Stouffer and Caccamise, 1991). They follow similar routes when returning to the roost, and form temporally and spatially predictable groups (Kalmbach, 1916; Moore and Switzer, 1998). Crows remain in cacophonous pre-roost aggregations before moving en masse to their roost where they remain until morning (Abadi et al., 2019).

Estimating roosting crow numbers is challenging due to the number of individuals, circling flights over the roosting area, potential inaccessibility, and darkness. Kalmbach (1916) outlined two approaches for counting: sampling perching crows during roosting and counting flying birds as they arrive. Granger and Provorse (2025) made a Herculean effort to sample perching crows in an urban Portland, OR roost, but this was only possible because their site was accessible. All roosts identified in western Washington state are on wetlands or forested restorations, difficult to traverse in darkness without disrupting birds.

Here, we describe a citizen scientist-based effort, aligned with the annual Audubon Christmas Bird Count (CBC), to estimate the number of crows using a communal roost in Bothell, WA, by counting approaching individuals at multiple locations, consistent with flightline observations of Red-winged Blackbirds (Agelaius phoeniceus) Meanley (1965). Over a 3-year period, we identified unusual increases and decreases in our counts, allowing us to detect a spontaneous roost dissolution, ephemeral and potentially transitional roosting sites, and two emergent, longer-term communal roosts.

Methods and Materials

American Crow counts across a 30-year Christmas Bird Count (CBC) dataset from the Pilchuck Audubon Society varied widely, even when corrected for effort (Figure 1). We hypothesized that the more extreme year-to-year vacillations reflected methodological issues. Crows form large, moving aggregations as they fly towards their communal roosts (Kalmbach, 1916; Moore and Switzer, 1998), and in some years volunteers may have counted such groups, while in others they did not. We therefore created a protocol to standardize counts across years which limited counting flying crows to within the CBC circle during their afternoon approach to a communal roost in Bothell, WA.

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Figure 2. American Crow counts by year (1984-2013) from the Pilchuck Audubon Society’s Edmonds Christmas Bird Count (CBC), whose counting circle centers near Martha Lake, WA (47.85296, -122.24848) (National Audubon Society). Counts include all crows seen by volunteers during a 24-hour period. Yearly vacillations were likely due, at least in part, to methodological inconsistencies. The dotted line represents a linear fit and the shaded bounding area the 95% confidence interval.

Figure 2The number of American Crows counted during the Pilchuck Audubon Society’s Edmonds Christmas Bird Count (CBC) by year from 1984-2013 trended upwards, but varied considerably from year-to-year. Alt Text:

Volunteers were assigned to locations ineach of the cardinal directions around the Bothell roost from 2023-2025. Crow counts and directions of travel from these observations were used to identify four predictable, pre-roost flyover locations along four waterways (Figure 3). From 2016 to2025, during Pilchuck Audubon Society’s Edmonds CBC, two to three volunteers were stationed at each location and independently conducted per-minute counts of crows flying over from 90 minutes prior sunset until 30 minutes after sunset. When numbers swelled, volunteers block counted groups of birds (Bibby, 2000). Total counts were averaged across observers.

Results

Between 2016 and 2021an annual average of 13110 +/- 1860 crows were counted flying towards the Bothell roost (Table 1). During this time, large pre-roosting aggregations were consistently observed near the roost. In 2022, total counts swelled to 20,774, and pre-roosting behavior did not appear unusual. Based on this increase, we resurveyed crow approach behavior. In November 2023, we observed a newly-established roost in Redmond, WA (Figure 3). This site was visited in 2019, and noroosting behavior was observed. Anecdotal reports from nearby businesses suggested that some crows had used this site for 1-2 years.

Table 1. Annual total crow counts from 2016-2025 Pilchuck Audubon Society’s Edmonds CBCs. Although counts at LW, LBC, and NC decreased precipitously after 2022, estimates at LW and NC remained in the 1,000s, while counts at LBC dwindled into the low 100s. It is likely that crows altered their flight paths as they abandoned the Bothell roost in favor of roosts in Redmond and North Bothell (Figure 2).

Table 1 Total annual crow counts from the 2016-2025 Pilchuck Audubon Society’s Edmonds CBCs. Numbers at original counting sites were stable from 2016-2021, nearly doubled in 2022, then decreased precipitously from 2023-2025 (Figure 2) Alt Text:

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In 2023, to account for crows moving towards this new roost, we established a new counting location along the Sammamish River (SR), 3.4 km SSE of the Bothell roost (Figure 3). 2023 total counts at MC, LW, and LBC decreased to 8,969, while counts at SR were 9,823. In 2024 and 2025, counts at MC, LW, and LBC continued to decline to 4,043 and 3,634, respectively. These reductions were concomitant with informal observations of fewer crows using the Bothell, WA, roost. Since January 2025, the Bothell roost has been abandoned.

Numbers at SR, representing birds moving towards the Redmond roost, decreased in 2024 and 2025 to 8,603 and 7,397, respectively. Due to these changes, approach behavior was resurveyed. In December 2024, a roost of thousands of crows was discovered in Mountlake Terrace, WA (47.782571, -122.329949). It is unclear how long crows had used this site;it had not previously been recognized as a roost and by January 2025 it was abandoned. In December 2025, we observed a roost of approximately 200 birds in north Bothell, WA. Anecdotal reports from surrounding businesses suggested it had been present for 1-2 months. By January 2026, this site was abandoned. However, in March 2026, observations of nearby pre-roosting behavior led us to discover another, larger roost in North Bothell (47.807556, -122.210194) very close to this ephemeral roosting site (Figure 2). It is unclear when this roost was established, but it was not previously detected and consists of thousands of individuals, suggesting it is also new. This roost is 5.5 km NNE of the former, now abandoned Bothell, WA roost and is also adjacent to North Creek. Habitat Descriptions Needed

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Figure 3 . Crow counting locations for assessing roost usage appear as binoculars on the map: Swamp Creek (SC), Lynnwood, WA (47.852659, -122.260422), North Creek (NC), Mill Creek, WA (47.85203, -122.21981), Lake Washington (LW), Kenmore, WA (47.75636, -122.26382), and Little Bear Creek (LBC), Woodinville, WA (47.79298, -122.14161). In 2023 an additional counting location was established at Sammamish River (SR), Woodinville, WA (47.739647, -122.154530). Arrows show the general direction of daily movement of crows towards their roosts. The Bothell communal roost (now defunct) appears in the center of the map as a crow icon in a gold circle (47.76096, -122.18621). The newly identified roosts in Redmond (47.70682, -122.15139) and North Bothell (47.807405, -122.207482), south and north of this former roost, appear as crow icons in white circles. Ephemeral roosting sites appear as crow icons in orange circles. Map created in ArcGIS Online.

Figure 3 The five crow counting locations, three communal crow roosts, two ephemeral roosting sites, and waterways surrounding Lake Washington in western Washington state. Alt Text:

Discussion

Over ten years, we used a citizen scientist-based approach to count crows moving towards a communal roost in Bothell, WA. This roost was gradually abandoned without intentional human harassment, and two new roosts were established along waterways connected to the previous roost. Roost shifting is much more likely to be reported in the scientific literature if induced by human harassment (Gorenzel et al., 2000; Avery et al., 2008). The reasons for spontaneous roost shifts remain unknown at this timeand warrant further study. Because these shifts are unpredictable, it is important to monitor roosts to document changes as they occur. While our count estimates are likely conservative, they are comparable because they are a consistent protocol, and their precision and accuracy could be assessed using accessible roosts where ground-truthing is possible (Granger and Provorse, 2025).

Consistent monitoring using our approach can help address multiple open questions about crow roosting behavior. Roost numbers are greatest during winter, but roosts are also used during autumn and late summer, and sometimes even during the breeding season by non-breeding individuals (Caffrey et al., 2025). Better monitoring could assess seasonal and geographical variation in roosting behavior and allow timely assessment of roost dissolution or new roost establishment if numbers change. As crows abandoned the Bothell roost, multiple smaller roosts appeared and disappeared. Better monitoring of roost dynamics could help determine whether such ephemeral roosts only form during roost dissolution or are more common than currently understood. de

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