Red-necked Grebes - comments - long
wormington at juno.com
Thu Mar 20 11:52:45 EST 2003
The following I sent a few days ago to Ron Tozer, but with other readers
posting some thoughts the text below may be of interest.
Some comments on your two main questions:
(1) In my "Birds of Point Pelee" I am going to introduce a term I call
"two-tier" migration, although I'm sure there is already a name for it
somewhere in the literature. Anyways, this involves species that have,
as the name implies, a two-tier migration strategy. Some examples during
spring migration include American Pipit, Golden Plover, Pectoral
Sandpiper, Baird's Sandpiper, Red-throated Loon and, of course,
Red-necked Grebe. The shorebirds, for example, fly from South America
to, for example, Texas, where they can be very common during March (some
even north to Ontario at this season). But obviously these species do
not then fly to their arctic nesting grounds, since these areas are
frozen to late May (or even later). Therefore, they spend a
considerable amount of time in arrested migration mode, fattening up and
also changing into spring plumage, before finally barrelling north once
more in late May.
In fall migration, two examples of two-tier migrants include Dunlin and
Bonaparte's Gull. With Dunlin, in the east the species migrates to
(mostly) Hudson and James Bay in July in great abundance, but does not
proceed further south until mid-September after they moult into winter
plumage (but this initial event does result in the occasional individual
being seen in southern Ontario as early as late June or early July,
including Point Pelee). With Bonaparte's, adult birds invade the Great
Lakes during August, but generally do not migrate again until December or
even mid January (well AFTER they have completed a full moult from summer
to winter plumage). NOTE: I make reference to this in my article on
Bonaparte's Gull that appeared in *Point Pelee Natural History News*.
With Red-necked Grebe, we have the same strategy. A similar pattern also
occurs with Red-throated Loon. Birds arrive EARLY on the lower Great
Lakes anytime from mid Feb through March in BASIC plumage. They then
spend a long period of time in these areas before taking on another
migration, with this second thrust putting them on the breeding grounds
(late April through May). Between the two main flights, the birds attain
breeding plumage and, presumably, take advantage of an increased food
supply (and thus gain fat). Therefore, in your question as to the
advantage of such a strategy, it is probably to take advantage of a good
food resource before attaining breeding plumage and an eventual flight to
the breeding grounds. The same strategy is also exhibited by Horned
Grebe -- often you see the odd individual in "winter" plumage that has
arrived during the period of mid Feb to late March, but it is late April
to mid May when we see the majority of birds passing through (which by
then are in breeing plumage).
(2) Re this question, you ask why birds would migrate in such cold
temperatures. Actually, they didn't! Here at Point Pelee there was a
flurry of spring migrants (blackbirds, meadowlarks, geese, ducks, gulls,
etc.) during the period of about February 15-20, because temperatures
just south of here reportedly were briefly in the 60s F. I suspect that
if you get some weather data from the East Coast around where Red-necked
Grebe winters in large numbers, you will find that there actually were
some warm periods when the birds started to appear in Ontario.
Presumably this early migration occurs every year, but in most years the
birds go mostly undetected since there is normally lots of open water for
them when they get here.
Well enough babbling . . . hopes this helps.
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Alan Wormington <wormington at juno.com>
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