Greetings all,
As promised in a post I made last night, being in Arizona offered a preview or
sorts of the impending arrival of neotropical migrants in western Oregon. I'll
start by pointing out that my experience with Oregon's spring migration
phenology is heavily biased towards western Oregon because I have never lived
east of the Cascades, and I have rarely visited the eastern two-thirds of the
state before the last week of May.
Veteran birders across the state can often tell you almost down to the day and
certainly down to the week when the first sightings of various neotropical
migrants can be expected in their local area. One thing many of us have been
curious about is how fast these species come north. Is the northbound migration
of songbirds somewhat incremental or are these birds moving quickly up the west
coast from the south and arriving here within a few days of when the first
arrivals hit areas far to our south? Based on my combined experiences of living
for many years in Eugene and in the Portland area, I have generally been
inclined to think that most neotropical migrants are moving somewhat
incrementally, in large measure because of the 3-4 day lag (if not longer with
some species) between the Eugene and Portland area arrival dates of many
species. Given some of my assumptions, I expected to see more neotropical
migrants arriving last week in Arizona than we did.
During our trip we did most of our birding in Santa Cruz County, Arizona, so I
pulled up the eBird bar charts for that county to see when the initial spike of
reports occurs for many of our shared neotropical migrants. I was particularly
interested in those species that don't routinely winter in Arizona and those
species that arrive in western Oregon between about the 15 April and 10 May,
which is when the bulk of our neotropical migrants appear. I also looked at
some of the earlier-arriving birds like Nashville Warbler and Pacific-slope
Flycatcher. What I found is that there is roughly a two to three-week
differential between the first waves of each species reaching SE Arizona and
the first waves of that same species arriving in Oregon and this seemed pretty
consistent across the board. Of course, there are all sorts of variables that
make perfect apples to apples comparisons difficult, if not impossible, despite
what appears to be a somewhat consistent spacing of arrival dates across many
species. For example, can we assume that a substantial percentage of the
populations of these neotropical birds migrating into Arizona are continuing on
towards Oregon? How much of a factor is so-called 'leapfrog' migration? This is
where the earliest-arriving birds are more southerly breeders that stop well
short of Oregon and then are leapfrogged by those populations that nest in
Oregon and farther north. This would involve a second wave of later migrants
passing through Arizona and other points south that may be difficult to detect
if that species is common breeder to the south.
We are within about 20 days of the first migrant Nashville Warblers and
Pacific-slope Flycatchers being reported in western Oregon, thus I expected to
seem some of both in Arizona. I was surprised that we saw neither of these
species during our trip despite spending considerable time birding in a variety
of woodland habitats where other migrants were encountered. Looking at the bar
charts for Santa Cruz County, it appears that Pacific-slope Flycatcher reports
start to build during the third week of March (last week) and Nashville
Warblers spike during the fourth week of March (now). It has been a delayed
spring in southern Arizona, with snow cover in many places just a couple of
weeks ago. Pacific-slope Flycatcher is definitely a leapfrog migrant in western
Oregon and it may be that the first birds that arrive in Oregon are coming on
heading that does not pass through Arizona. The first birds normally start
arriving in western Oregon around 12-15 April, and by late April breeding
habitats in the Coast and Cascade Ranges and the foothills of western Oregon
valleys are well populated with territorial singers. Then in early May there is
usually a small but noticeable second wave of Pac-slopes that can often be
heard giving their high-pitched "tink" call (not singing) as they move through
the westside lowlands, usually in habitats where they don't breed. These are
presumed to be birds that will summer and nest far to the north of us, where
the leaf out and early insect hatches occur much later in Spring. We also saw
no Warbling Vireos, MacGillivray's Warblers, Western Tanagers, or
Black-throated Grosbeaks, all of which typically arrive in western Oregon
between about16 and 25 April. Most of these species start appearing in numbers
in Santa Cruz County plus or minus the first week of April, again roughly 2-3
weeks earlier than they arrive here.
As noted in the subject line, there was one conspicuous outlier that did not
align with the timing differential that seems to show up with essentially all
of the other neotropical migrants...and that was Yellow Warbler. In western
Oregon, Yellow Warbler is the latest arriving spring warbler, rarely showing up
before the last day or two of April and generally not being conspicuous in
western Oregon until after the first of May. And yet, during our trip to
Arizona we saw several Yellows, all males (many singing) in lowland riparian
corridors where they were presumably establishing breeding territories. Looking
at the Santa Cruz bar chart it is apparent that Yellow Warblers don't routinely
winter in the county and in fact appear to be quite rare there from
December-February. According to the bar charts, early arrivers show up during
the first week of March and then build to peak numbers locally by the third
week of March, fully 6-8 weeks earlier than they arrive in Oregon. There are
all sorts of unanswered questions that can be raised. Where do these
early-arriving Arizona birds winter? Is that wintering grounds a source for
birds that summer much farther north? If not, where are the more northerly
breeders wintering and does their migration route to Oregon and beyond pass
through Arizona? Given how early Yellow Warblers flood the southern Arizona
riparian corridors, how difficult is it to perceive the presence of birds that
might still be moving north through Arizona in April? I wish I knew the
answers. With advancements in tracking technologies, the ever-expanding network
of Motus tracking towers, and the continuing spring migration monitoring going
on at Bear Divide in California, many of these questions could eventually be
answered, and we may learn why Yellow Warbler migration timing in southern
Arizona looks so outwardly different than the patterns shown by other
neotropical migrants that summer in Oregon and migrate to and through Arizona
each spring.
Just some things to ponder as we patiently await the height of spring migration.
Dave Irons
Beaverton, OR
