The Grube Magnitude Index

Tuesday, March 27 was one of March’s lions, rather than lambs.  After several days of north winds, last night brought an abrupt shift of winds, with gusts to 30mph.  These winds would continue at dawn for one of the windiest mornings on the old tower site since the first days of the count season. Temperatures were also similar to early March, with a morning start of 37 degrees.

The day’s flight would finish much like the flight before the front.  Slow.  Only 1,564 birds.  The day was more reminiscent of a north wind morning, with few passerines moving, and more longshore lake birds moving.  The gulls would provide a nice diversity, with a season high 36 Herring Gulls, and both Glaucous and Great Black-backed Gulls being logged.  The day screamed for a passing Thayer’s or Iceland, but none flew past.

Other highlights included 2 Snow Geese, White-winged Scoter, Great Egret, Lapland Longspur, and Purple Finch.  The biggest highlight was an amazingly early Chimney Swift.  According to eBird, there have been no sightings north of Nashville TN.  However, the funneling along Lake Michigan continues to provide new surprises every day.

After a single day of birding, how do you best quantify the the magnitude of flight witnessed?  Enter the Grube Magnitude Index (sounds like a Big Bang Theory episode).  Developed by Dr. Ken Brock, and later refined through many permutations, it essentially produces a magnitude score based on the total diversity of birds seen in a morning, while also taking into account the total individuals seen.  For total individuals seen, each species is compared to the mean average of previous counts, creating an index point for the 60 most common green tower migrants.

The species chosen for the Grube Magnitude are, for the most part, well-recognized longshore migrants.  An effort was made to avoid species that breed near the Dunes State Park green tower site (e.g., Brown Thrasher, Prairie Warbler, Towhee, and Field Sparrow), as there is always uncertainty about whether these are local birds or migrants

Standards were calculated from each species mean using the below equation.  The advantage of this approach is that indexes of the less common species more closely match their flight numbers (i.e., they are not overpowered by the standard), thereby emphasizing flight diversity.

  Std = loge(4^mean)

Obviously there are other parameters not mentioned.  One such limits a single species score to 10, thus a single record flight from one species will not skew the total index score.  

Grube Index baselines for 60 common migrants.

The computer program uses the given date and location to search database files for each of the above species.  The number of each species is divided by its corresponding standard; these quotients are referred to as indexes.  The sum of all indexes involved in the flight is the Grube Magnitude.

Consider the following printout of the 25 April 2009 count at the Dunes S.P. green tower site (Table II).  The left number gives the species order in the list of 60 selected species.  Missing numbers reflect species that were not recorded on this flight. Note below that the number #1 (KILL ) was absent in this day’s count.  For the Solitary Sandpiper the count of 8 was divided by the standard (5.55) to yield the index.  The Grube Magnitude for this flight (sum of all the indexes) is 41.58.

The 20 best flights, based on Grube Magnitude, are listed in Table III.

The longshore flight magnitude is named for Brendan Grube’s leadership in guiding us all to the lakefront’s most superb watch site.  The index and it’s permutations have been developed by Ken Brock.  If you have ideas on making it better or questions, let us know in the comments box.  We can also provide a full synopsis of the Grube Magnitude Index IIIa in it’s current form.  Thanks to Ken Brock for allowing us to share this unique way to measure the longshore flight.  We’ll also try to include Index numbers for this year’s flight in various posts too.  You can see above that three of our flights this year have already made it into the top 20.


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