[vernalpool] Re: rain before, not during: Big Night uncertainty

Hi Laura,

In my experience, I have seen substantial migrations on nights when it has
not been raining. My own sense over the years is that the threshold for
triggering migrations by the early spring-breeding amphibians drops over the
course of the potential breeding season.

So, for example here in NE Mass., in the past 23 years I think that the
earliest date on which I have seen a fairly large scale migration event by
Ambystoma spp., wood frogs, and spring peepers was on March 9. That early
in the season, the conditions have to be REALLY good - warm, lots of rain,
etc.

Much earlier in the season (in say late February, around here) there will be
essentially no migration no matter how good the conditions are - heavy rain,
temps in the 50's, fully thawed ground etc. - I *think* that is because the
amphibians have what I refer to as a gated response - they are, I believe,
inhibited from migrating before a certain time of year, based on their
internal photoperiod, no matter how good the conditions.

Alternatively, in springs in which good conditions don't materialize until
late in the season, i.e. there are no good rainy nights until early April,
the threshold for migration is much lower and you will often see large
migrations when the ground is just wet even if not raining at night.

Anyway, that's my sense based on my experiences.

One more pet soapbox - I always cringe at the use of "big night" as a term.
Having conducted drift fence and minnow trap surveys at dozens of pools
over many years, I have learned that there are, in most years, a number of
nights on which large proportions of a given population of Ambystoma or wood
frogs immigrate to their pools. More often than not there is no single "big
night".

Best of wishes with your work,
Bryan

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