migration charts

[Gary Davidson]

Over the last few years, I've heard it said that migrants are arriving earlier these days. So I wondered if that was true! I selected 8 migratory species that are fairly common in the Central Kootenay region: Spotted Sandpiper, Red-naped Sapsucker, Hammond's Flycatcher, Violet-green Swallow, Red-eyed Vireo, Swainson's Thrush, Yellow Warbler and American Redstart. Then I searched eBird data for the last 10 years to find first arrival dates. I made a data chart and then a scatter plot graph for each. 

I have attached the results in an excel spreadsheet. The horizontal axis represents each of the last 10 years. The vertical access is the date of first arrival. For species that sometimes arrive in different months, the vertical axis is a bit confusing, but the actual date isn't really that important, it is trends we're looking for. 

There are a couple of cases where it appears that there might be a downward trend, Hammond's Flycatcher and the 2 warblers, for example, but is there enough data here to draw that conclusion? Maybe. Any statisticians out there who would like to weigh in?!

Something to ponder!

Gary   

[mindobarbet]

Hi all,

OK, here goes. As someone who was involved with statistics a long time ago, one of the things I would add is that many variables need to be taken into account when analyzing data. For eBird sightings, this means also taking into account the volume of bird reports. 

A report of a bird sighting is a single data entry. It exists (if eBird accepted the record), so you can suggest a trend, if that bird (data point) is recorded again, by connecting the two entries.

However, when looking at likelihood/probability - and therefore considering trends - the number of potential checklists where it was NOT seen has also to be taken into account. For example: If a person produces one checklist at the same location every day, the chances of seeing a bird are far higher (seven times) than if only one checklist is produced per week. 

In plain English - I would posit that there are more checklists submitted in warmer months than colder ones. More people go birding, and feed data to eBird, when it's nice out, and they believe that they have a greater chance of seeing birds. If that is so, then data needs to be weighted by a sighting index. Once upon a time I would have known how to do this, but I can't remember how right now. It was a long time ago....

To produce an actual trend, I believe (someone correct me if I am off course here) you have to account for all the reports which do not show the birds in question, and factor that in to create probability.

Well, you did ask, Gary....

Chris D

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From: 'Gary Davidson' via West Kootenay Birds <wkb...@googlegroups.com>
Sent: Sunday, January 25, 2026 5:29 PM
To: West Kootenay Birds <wkb...@googlegroups.com>; Lorraine Symmes <lsy...@netidea.com>
Subject: WKBirds Announcement: migration charts

 

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[mindobarbet]

Hi WKBirders:

Gary, I am basically pirating your email...but I was going to write on a related issue anyway.

Re. the Swainson's Thrush that was reported yesterday at Grohman Narrows: as it is much too early for a return from its southern migration, is it likely that this bird (and maybe the same for the bird from Silverton in December) just never left? As I know the person who saw it yesterday, I don't question its identity as it is unlikely he is mistaken (e.g. Hermit Thrush). The species isn't known for lateral migration, so it may have been able to tough it out all winter so far. Does anyone know if that has been reported before for Swainson's Thrush in the Interior?

It could be true that birds are returning earlier...but also possible that more (like the Nelson Grey Catbird) are also remaining, leading to more early season sightings, partly as they become more active, but, perhaps more tellingly, as the weather softens and more birder get out to record sightings.

Chris D

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From: 'Gary Davidson' via West Kootenay Birds <wkb...@googlegroups.com>
Sent: Sunday, January 25, 2026 5:29 PM
To: West Kootenay Birds <wkb...@googlegroups.com>; Lorraine Symmes <lsy...@netidea.com>
Subject: WKBirds Announcement: migration charts

 

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