# Determining Flight Direction of Birds Detected on Towers

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### Jessica Howell

Aug 25, 2016, 10:01:31 AM8/25/16
to Motus Wildlife Tracking System
Hello all,

I am a grad student in Christy Morrissey's lab and part of my project looks at orientation and direction of travel of Sanderling departing from a stopover site using Motus towers.  We have 4 towers in the vicinity of the site (2 north, 1 east, and 1 west).  Each have 3 antennae, one that faces either N or S, one NW or SE, and one NE or SW depending on the tower.  I'm currently attempting to determine direction of movements recorded on one or more towers by isolating data for specific birds at the time of departure (final recordings) and plotting signal strength vs. time for each antenna (see figure).  I'd seen this method in Taylor et al. 2011 and Hatch, 2015.  I have had some success with this method, but in many cases it is difficult to tell which direction, and the plots can be very messy.  Who else is working on similar issues?  Do you use different methods than these plots?  Has anyone been successful in triangulating birds picked up simultaneously at multiple towers?  I'd love to hear your thoughts.

Figure: At this tower, antenna 1 faces North, antenna 2 faces SE, and antenna 3 faces SW.  The SW antenna shows the highest signal strength at its peak, with a subsequent decrease.  The N antenna shows lower signal strength.  I cannot tell if the bird was flying straight N and physically closer to the SW tower, or if it was flying NE.
chapn127.jpeg

### Mitchell, Gregory (EC)

Aug 31, 2016, 2:37:13 PM8/31/16
to Jessica Howell, Motus Wildlife Tracking System

Hi Jessica,

Just an idea, but to aid in the interpretation of the plots, you should connect successive tag hits from a given antenna with a line.

Tara Crew may have already responded, but I think she was working on localizing birds in situations with simultaneous tower detections.

Some of the variability in your signal strengths can result from a bird flying across an antenna beam, and/or variation in the birds flight behavior (e.g., climbing or dropping in altitude).  Flights across an antennae beam tend to look fairly smooth.  Given that you have similar peaks between two separate antenna, it suggests to me that some of the variability in the plot you sent is the result of flight behavior of the bird.  My interpretation of this track is that the bird departed in a SW direction and probably departed somewhere west of the tower.

Any other thoughts?

Greg

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### Jessica Howell

Aug 31, 2016, 5:13:07 PM8/31/16
to Motus Wildlife Tracking System, jeho...@gmail.com, gregory....@canada.ca
Hi Greg,

Thank you for the great idea.  That would definitely help with visualizing the movements.  I will just have to try and figure out a way to integrate that into my R code (Suggestions on that are welcome as well, I find plotting in R rather challenging).

Yes, I've been in contact with Tara Crew as well as some others.  Right now I'm working on vector addition of signal strength of different antennae during the final moments of departure to calculate mean direction as Sissel Sjöberg and Rachel Muheim have done.  A flight across an antenna beam would theoretically look like a smooth, unimodal curve with an increase, peak, and decrease, right?  I know this bird was flying north because I picked it up on a tower farther north a few minutes later.  Otherwise, I would have come to the same conclusion.  With that information, I figure it was traveling NE and maybe it hit the N facing antenna when it was just south of the tower (Subsequent peak in the SW facing antenna as it flew over)?

### Mitchell, Gregory (EC)

Sep 1, 2016, 1:30:58 PM9/1/16
to Jessica Howell, Motus Wildlife Tracking System, jeho...@gmail.com

Hi Jessica,

Jim makes a  really good point about the lobes and the link he sent around is great.  Some of the dips in strength could be the result of the bird passing through multiple lobes.

Three quick points:

1)      the last antenna to receive a signal will *most* of the time give you an approximate vanishing bearing, i.e. N, SE, or SW in the example you provided if the bird has departed on a migratory flight.  I believe this is the approach taken by the references you originally provided.

2)      generally, when I have been tracking migratory departure flights, I am able to track the birds for an average of 15 minutes, but this varies with ground speed and the size of the antenna, and so I am curious how long the track is from the bird for which you provided the plot?

3)       based on the additional information you have provided, I am wondering if the departure you are describing is really a migratory flight?  If you picked up a bird a couple minutes later at a tower north of the tower in question, you would likely have had simultaneous detections on both towers (they sound like they are quite close together).  I would also expect that you would have picked up the bird on the north facing antenna if it ultimately began climbing in altitude during departure in a northerly direction.

Happy to follow up off-line if you want to discuss further.

Best,

Greg

From: Jim Moore [mailto:j...@marshlands.org]
Sent: September 1, 2016 10:31 AM
To: Jessica Howell; Motus Wildlife Tracking System
Cc: jeho...@gmail.com; Mitchell, Gregory (EC)
Subject: Re: [motus-wts] Determining Flight Direction of Birds Detected on Towers

One factor that needs to be considered in these sort of calculations is the beam pattern of the Yagi antenna.  It is multimodal with lobes proportional to the number of elements in the Yagi antenna.  For any given received amplitude the beam pattern can be thought of as a locus of all possible positions of the bird for that signal strength.  An article by Cisco "Antenna Patterns and their Meaning" is worth taking a look at.

I am planning on deploying two SG receivers about 0.9 Km apart this fall to get some field test data on signal vs. location that may be useful for answering questions about location vs. relative signal strength.

Cheers

-jim, W3ASA

Jim Moore
34 Moore's Rd.
Elverson, PA
19520

610.286.5187 (Home)
215.500.7525 (Cell)

----- Original Message -----

From:

Sent: Wednesday, August 31, 2016 5:13 PM

Subject: Re: [motus-wts] Determining Flight Direction of Birds Detected on Towers

### Jim Moore

Sep 2, 2016, 1:41:04 PM9/2/16
to Jessica Howell, Motus Wildlife Tracking System, jeho...@gmail.com, gregory....@canada.ca
One factor that needs to be considered in these sort of calculations is the beam pattern of the Yagi antenna.  It is multimodal with lobes proportional to the number of elements in the Yagi antenna.  For any given received amplitude the beam pattern can be thought of as a locus of all possible positions of the bird for that signal strength.  An article by Cisco "Antenna Patterns and their Meaning" is worth taking a look at.

I am planning on deploying two SG receivers about 0.9 Km apart this fall to get some field test data on signal vs. location that may be useful for answering questions about location vs. relative signal strength.

Cheers

-jim, W3ASA

Jim Moore
34 Moore's Rd.
Elverson, PA
19520

----- Original Message -----
Sent: Wednesday, August 31, 2016 5:13 PM