One way to estimate the number of birds that arrived with the storm is to use eBird reports. Although the number of observers present and the amount of birding time will clearly have an effect on the number of birds observed, many if not most species had no presence in the province prior to Dorian making landfall, so it’s easy to identify those species. Several species, such as some aerial insectivores, were present in small numbers prior to Dorian, but we know from in-person accounts that a large number were also seen afterward, so it’s likely safe to attribute a spike in new observations of these species to recent arrivals. Although there were many more observers and birding hours after the storm, I did not correct for this increased effort in data reported here since I saw little difference in the overall shape of plots with a correction applied. In the end, I decided it was more informative to include raw individual counts instead of corrected counts.
Data analysis
Observations from one week before and one week after the storm were summarized by grouping data into localities. To combine personal locations and hotspots, all locations Black Skimmer stretching its long moulting wings at Lower East Chezzetcook, Hfx,
Sep 8. Photo by Alix d’Entremont
were binned by latitude and longitude, pooling any sites within 500 m of each other.
The number of observer hours was calculated by multiplying the number of observers by the number of survey hours for each checklist.
Calculating the number of individual birds present is inherently difficult, especially with the eBird dataset, since duplicated observations are common between days and locations but there is no direct way of accounting for them. I treated individuals from each location to be different, regardless of which day they were seen, and I lumped together all locations within 500 m of each other in order to remove duplicated locations (i.e., hotspots and personal locations—this is why eBird users should always use hotspots when they can!). Individuals of a given species seen on different days but at the same location were treated as the same so that the maximum number of individuals was assumed to be the total number of individuals present at any given time. Using this method to count individuals does not account for birds moving between locations and may result in an overestimate. However, in all probability there were so many birds that went unreported that the true number of arrivals is still likely to be much larger than what is reported here.
To calculate the number of new arrivals, I used the difference in the total number of individuals observed 7 days before and 7 days after the storm for each species and location. I chose to look at week-long periods to account for observer bias from weekend birding.
Species that do not typically occur in Nova Scotia as part of their annual cycle were
considered vagrants, while species that have been reported to eBird in no more than 3 of the 10 past years were considered very rare.
Seabirds include any species that can rest at sea (either on the wing or on water), and aerial insectivores include swallows, flycatchers, swifts, and nightjars.
Geographical distribution of observations The largest number of new arrivals was
observed between Halifax and Sheet Harbour (Fig. 3, left), to the east of where the eye struck land. Here, the winds would have blown onshore. The highest concentration was around Martinique Beach and West Jeddore, but there was a marked increase in the number of observations throughout the province, even where there was more observer effort the week prior to the storm.
Figure 3. Left: Difference in the number of individual birds observed after Hurricane Dorian made landfall in Nova Scotia on Sep 7, 2019. Black dots represent areas only visited after the storm. Right: Change in observation hours before and after Hurricane
Dorian made landfall in Nova Scotia on Sep 7, 2019. Black dots represent areas only visited after the storm.
American Avocet at Hartlen Pt., Hfx, Sep 27. Photo by Lynn Fergusson
The highest number of new arrivals was closely associated with where observers spent most of their time, especially around Halifax (Fig. 3, right), with Hartlen Point having the highest number of observer hours. There was also an increase of individuals observed across the province, even in areas where there were more birding hours before the storm.
Observations before and after the storm
A total of 70,376 individuals of 237 species was observed in the first week following the storm, contrasting with 45,504 individuals of 203 species during the week prior to the storm. A total of 5,675 individual vagrants of 56 species were observed after Dorian, of which 5,092 of 14 species were seabirds.
Aerial insectivores accounted for 6,020 individuals of 18 species, and vagrant passerines accounted for 457 individuals of 23 species.
When looking at observations over time, there is a sharp increase in species and individuals observed the day after the storm, as well as the following weekend, which can be attributed at least in part to the number of observers present (Fig. 4). However, when comparing species to one another, we can see a clear difference between them. Observations of Laughing Gulls spiked the day after Dorian but slowly tapered off during the rest of the week, while Black Skimmers peaked a week later, perhaps after birders spread their effort across more areas and because of the large numbers of skimmers congregating (Fig. 4, top). The remainder of vagrant seabirds consisted mostly of terns and Lesser Black-backed Gulls and followed a trend similar to that of Laughing Gulls.
Glossy Ibis at West Chezzetcook, Hfx, Sep 22.
Photo by JoAnn Yhard
Figure 4. Total number of individual birds observed per day by group. The X-axis shows the number of days since Dorian’s landfall in Nova Scotia (indicated with a black line), and the Y-axis shows the count of individuals for that group. Shaded grey areas are weekends. Top plot does not include Laughing Gulls and Black Skimmers in the count of vagrant seabirds. The count of vagrant passerines does not include aerial insectivores.
0 200 400 600 800 1000 1200 1400 1600
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Vagrant seabirds Laughing Gull Black Skimmer
0 2000 4000 6000 8000 10000 12000 14000
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Aerial insectivores Other passerines Other birds
0 5 10 15 20 25 30 35
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Vagrant shorebirds Vagrant passerines
Table 1. Summary of vagrant species observed one week before and one week after Dorian’s landfall in Nova Scotia: “Change”
is the difference. Totals are summed maximums over all days for each locality within the province. Population trends over 40-50 years are from the North American Breeding Bird Survey (Sauer et al. 2017). Population sizes are from the Avian Conservation Assessment Database (Partners in Flight 2019). Numbers in square brackets are for global populations (when NA data is un-available).
Species Before Change Population Trend N.A. Population
Northern Shoveler 3 +10 Stable [5.9 mil.]
Ruddy Duck 0 +2 Stable 1.2 mil.
Yellow-billed Cuckoo 0 +1 -52% 9 mil.
Black-necked Stilt 0 +17 Stable [750 k]
American Avocet 0 +6 Stable 450 k
Upland Sandpiper 1 -1 Stable 750 k
Marbled Godwit 0 +25 Stable 160-200 k
Stilt Sandpiper 5 -1 Declining? ~1.2 mil.
Baird's Sandpiper 4 -1 Unknown 300 k
Buff-breasted Sandpiper 1 +1 Declining? 56,000
Western Sandpiper 0 +1 Unknown 3.5 mil.
Long-billed Dowitcher 0 +2 Unknown 500 k
Wilson's Phalarope 0 +1 Declining? ~1.5 mil.
Great Skua 0 +2 ? 32,000
South Polar Skua 3 -1 ? 11,000
Little Gull 0 +2 Unknown 100-200
Laughing Gull 0 +3,143 Increasing 528-538 k
Lesser Black-backed Gull 10 +122 Unknown Unknown
Bridled Tern 0 +4 Increasing ~14,000
Gull-billed Tern 0 +32 Stable? 6-8,000
Caspian Tern 15 +163 Stable? 66-70,000
Forster's Tern 0 +254 Stable? 98,000
Royal Tern 0 +33 Increasing? 100-150 k
Sandwich Tern 0 +13 Increasing 75-100 k
Black Skimmer 0 +1,294 -87% 65-70 k
White-faced Storm-Petrel 0 +2 Decreasing [2.7 mil.]
Brown Pelican 7 -4 Increasing 191-194 k
Great Egret 0 +3 Increasing 180 k
Snowy Egret 2 +6 Increasing 143 k
Little Blue Heron 2 +4 -55% Unknown
Cattle Egret 0 +1 -50% ~1.16 mil
Green Heron 0 +8 -68% Unknown
Yellow-crowned Night-Heron 7 +1 Stable? [75,000]
Glossy Ibis 0 +15 680% 13-15 k
Red-bellied Woodpecker 1 -1 Increasing 10 mil.
Hammond's Flycatcher 0 +1 Increasing 20 mil.
Dusky Flycatcher 0 +1 Stable ~8.8 mil.
Western Kingbird 0 +2 Stable ~20 mil.
Yellow-throated Vireo 1 +3 64% [4.4 mil.]
Northern Rough-winged Swallow 0 +1 [-18%] [18 mil.]
Purple Martin 0 +379 -37% 7 mil.
Blue-grey Gnatcatcher 1 +2 Increasing ~118 mil.
Northern Wheatear 5 -5 Unknown [125 mil.]
Clay-coloured Sparrow 0 +1 -51% ~48 mil.
Lark Sparrow 3 +3 -32% [11 mil.]
Yellow-breasted Chat 0 +1 -37% ~12 mil.
Yellow-headed Blackbird 0 +1 Declining? ~9 mil.
Worm-eating Warbler 0 +10 Stable 830 k
Louisiana Waterthrush 1 -1 34% [450 k]
Golden-winged Warbler 1 -1 -68% 410 k
Blue-winged Warbler 0 +1 -22% 710 k
Prothonotary Warbler 0 +4 -42% 1.6 mil.
Swainson's Warbler 0 +1 Increasing? 160 k
Kentucky Warbler 0 +3 -36 2.8 mil.
Hooded Warbler 0 +10 96% 5.2 mil
Cerulean Warbler 0 +3 -74% 570 k
Yellow-throated Warbler 0 +7 63% 2 mil.
Prairie Warbler 4 +1 -66% 3.5 mil.
Summer Tanager 0 +5 Stable ~10 mil.
Blue Grosbeak 0 +2 Increased? ~18 mil.
Indigo Bunting 1 -1 -31% ~75 mil.
Dickcissel 3 +3 -14% 28 mil.
Total Individuals 81 +5,596
Total Species 22 +12
Aerial insectivores (Fig. 4, middle) only peaked on the day after the storm and quickly tapered off to near zero over the next three days, while other passerine species remained.
It’s possible that aerial insectivores made a rapid migration southward, as they were observed to be very active and relatively few fatalities were reported. We can also see that aerial insectivores peaked a day earlier than all other passerines. It’s unclear why this would be the case, but it’s possible that effort was
focused on coastal areas where both aerial insectivores and non-passerines were most easily found, whereas other passerines were in more forested areas where relatively few people were present. Perhaps if birders had spent more time in forested areas on Sunday other passerines would have peaked on that day as well.
As observed by birdwatchers, the greatest change in numbers was among aerial insectivores, vagrant seabirds, and vagrant
passerines, with close to zero observed prior to the storm (Fig. 4, bottom). As we’d expect with vagrant twitchery, there are two obvious peaks, which correspond to the weekends.
Vagrant passerines and vagrant shorebirds appeared in similar numbers, but again we see that vagrant passerines peaked on the Monday.
Species of concern
Several species that were brought here with the storm have experienced steep population declines in the past few decades.
Black Skimmers are on the Partners in Flight Yellow List, with declines of 87% from 1966-2015 (Partners in Flight 2019). The North American breeding population currently stands at 65,000-70,000 individuals, meaning that after Dorian, birdwatchers in Nova Scotia observed approximately 2% of this continent’s breeding population. While the main cause of these declines is attributed to habitat loss, this storm was certainly no help (NABCI 2016).
There were also huge numbers of aerial insectivores seen after the storm (about 6,000 in the first week), most of which were Barn Swallows and Bank Swallows. Ten of the sixteen species in this group have seen significant declines in the past 40-50 years, including Barn Swallows (a 46% decline), Chimney Swifts (a 72% decline), and Bank Swallows (an 89% decline; Sauer et al. 2017).