Spring/Summer 2020
The Congress colony returned on March 13th and first big flight was Saturday March 14th. Stuart Bat Cave in Kickapoo Canyon State Park is spectacular and early nowadays. Bats have also moved back to Frio and Fern Caves out west and most have made their way back to Central Texas and points north. We have entered the normal spring pattern.
Early May: Bats are flying later nowadays, around dark, as is typical at this time of year. Flights had been well before dark up until the third week of April.
Late May: Bats are being more cautious as maternity season approaches and weather patterns become more unstable (especially around Memorial Day). They are flying later and emergences are more spread out than earlier in the month.
Fall/Winter 2019
Heavy rains at the end of October brought our annual rainfall back closer to normal, but fall totals still were nowhere near the huge rainfall surplus (6.42″) of earlier in the year, and we ended the year with a big deficit (-4.01″ ABIA/Mabry). Jan 2020 started out about -0.35″ ABIA/Mabry. Feb yearly totals are starting out around -0.05″ ABIA/Mabry.
Fall migration started 06 Nov 2019 and another surge left 22 Nov 2019.
Up to 400k on warm nights through Nov 2019. They left in stages on 06 Nov, 09 Nov, and the last group 06 Dec.
Some bats had left 06Nov2019, and another big cold front (22Nov2019) likely pushed some more bats south in migration, but we think there were still ~100k ~200k bats at the bridge until 06Dec2019, with additional surges as migrants pass through from the north. Another wave (~100k) left Congress 09Dec2019. We’ve seen a series of migratory moves from Devil’s Sinkhole, Stuart Bat Cave, Frio, Huber Limestone Mine, Old Tunnel (06Nov), and at least three from Congress (06Nov, 09Nov, 06Dec).
Flights had mostly been before dark early last December, but not in big numbers. No flights Dec 16th through Dec 24th, even Christmas Eve with 58 F temps and gentle south winds. 38 F dew point.
Up to 60 overwintering at Congress Bridge in Jan 2020, with another 80k at UT and other Austin roosts.
Flight Time Conditions of Temperature in mid 60s F, Dew Point at least in the 50s F, and RH 50%-80% with Winds < 8 mph could trigger flights around dark, sometimes as early as sunset.
80 F temps (> 10 Cooling Degree Days) triggers big flights.15 Jan 2020 ~64k flew from Congress
There appeared to be about 140k bats overwintering in various locations in Greater Austin. 120 k between Congress and UT combined. Many seemingly came to the bridge when temperatures rose, then left in colder weather for locations where buildings were leaking warm air (Such as the UT tower courtyard). Temperature fluctuations caused others to go into, and come out of, torpor, while remaining at the bridge.
Spring/Summer 2019
Our radar study was put on hold from Dec 22, 2018 to Jan 28, 2019 as the government shutdown took the weather radar viewer offline. We lament not being able to monitor the Congress bats that were over-wintering.
Early Feb showed bats in big numbers at Fern and Stuart caves in West Texas; they appeared to be staging for early migration eastward.
They appeared to be moving east toward Frio before the subsequent cold snaps held them up.
Without continuity of observation we don’t know if they were there all winter or if they were early migrants.
An increase in numbers to ~230k occurred at Congress 09 March 2019.
Could be migrants pulsing through or some of our resident bats returning.
Intakes surged at the same time as we got 6 bats brought to our facility that weekend.
Spring return migration started around 09 Mar 2019, but cold weather suppressed flights until 19 Mar 2019.
The colony seemed to be at full strength around 20 Mar 2019 after surges of bats pulsed through the area.
Bats entered “slacker season,” when they sleep late and emerge at dark, on May 16, 2019.
Good viewing from the bridge, after maternity season, did not start until late in 2019 (29 July) and started getting a little sketchy around 18 September 2019.
Fall 2018
Congress migration appeared to have started 07 Nov 2018 and continued through 09 Nov 2018.
Surges of migrants moved through periodically through November
Last big surge—23 Nov—471k in the bridge
Warm, calmer weather brought out only 45k on 29 Nov, 22k on 17 Dec, and 25k on 20 Dec
20 Nov: 360k in the bridge
15 Nov: Warmer southern breeze brings 230k bats out from Congress
01 Dec: First calm evening in weeks brings surge from Congress—147k
02 Dec: Another 197k flies S with N wind at 9 mph—Dittmar also strong
Return of radar after government shutdown shows 100k at Congress on 28 Jan 2019
Spring/Summer 2018
24 Jan 2018: First big emergence at Bracken (300k) since early Jan (100k) and early Dec (33k)
Congress bats returned from Mexico on 06 Mar 2018 last year and gave birth in the first week of June.
Last year’s pups started flying 14 July 2018, we started noticing them at the bridge around 22 Jul 2018.
Winter 2017
Last emergences from Congress before New Years, ~57k 20 Dec 2017, and ~20k 21 Dec 2017
~500k bats had returned to both Frio & Huber during the warm spell, but left 23 Dec 2017
We think they were returning to the sun-warmed bridges and parking garages after spending one cold night in the caves
Late Dec 2017 there seemed to be ~500k still in North San Antonio returning to Bracken during warm spells
We started seeing fallout on Jan 2nd, the second night temps dropped to the 20’sF
After the cold snap of early Jan 2018, there seem to be ~95K returning to Bracken from Schertz / Cibolo during warm spells
06 Jan 2018: First flight in two weeks, just as we started to see fallout from roosts
08 Jan 2018: Warm evening, ~875k at McNeil ~187k at Bracken ~40k Congress
11 Jan 2018: Last big flight at Congress—nothing much since then
Fall 2017
The long, drawn-out Tadarida migration of Fall 2017.
The first round of Hill Country migration appears to have been triggered by a cold front on 24 Oct 2017 (UTC) and started on 26 Oct 2017 (UTC).
At first we wondered if this was normal foraging, taking advantage of the NW wind at only 5 mph.
But after monitoring over the next few weeks, we think many bats did indeed start leaving on 26 Oct 2017 (UTC).
Much colder weather seemed to stop southward movement from 28 Oct 2017 to 31 Oct 2017 (too cold to fly?)
Weather warmed quite a bit after that, showing lots of bats in the east. Roosts in the western Hill Country (Frio, Devil’s Sinkhole, and Fern) had smaller emergences over few days, but seemed to bounce back periodically (newly arriving migrants?).
Huber, Old Tunnel, and Bracken stayed large for a few weeks, either from retained summer residents or from newly arriving northern migrants.
The Hill Country (Huber Old Tunnel and Devil’s Sinkhole) mostly had cleared out by 10 Nov 20217.
Did Bracken bats move to Frio 12 Nov 2017 UTC as start of their migration? Storm may have suppressed Bracken numbers; we’ll see.
Many colonies seem to split during the early part of migration, with some heading in a migratory direction, others in a foraging direction. More so than during the summer.
This year’s migration seems to be playing out over a 3-1/2 -week period.
The next night, 26Oct2017, 1/5 as many bats at Devil’s Sinkhole, 1/2 as many at Huber and Old Tunnel, but surges occurred periodically in most roosts over the next 3 weeks.
Here’s more migration activity. Again, half (or more) of Huber bats, and what looks like all remaining Devil’s Sinkhole bats head SSW in definite migration pattern. Very few from Old Tunnel, but what few emerged headed in a migratory direction.
And here’s the same night, 28OCT2017 UTC showing remaining Devil’s Sinkhole bats heading south for the winter.
As temps at emergence warm a little, more bats emerge. More than expected at Huber. Are these the remaining summer residents or newly arrived migrants from north?
Next warm night shows activity at Huber, Old Tunnel, McNeil, Congress, and Bracken, but Devil’s Sinkhole had very few bats fly.
After no flights recently, the last ~200k of the Frio Cave bats head south to migrate through Eagle Pass.
The Frio Cave flight starts just north of Uvalde and heads southwest.
First warm night brings out more bats than previous chilly nights> Still another 200k from Frio that could have been hunkered down.
Migration on hold with recurrence of warm weather
Still on hold with possible exception of half Old Tunnel bats
Cold weather Oct 28-30 UTC
Warm weather Nov 3-5 UTC
Below, a cold front pushes through just before emergence at Bracken.
No bats appear to have emerged early to ride it south.
Here’s the night’s emergence soon after frontal passage.
No migratory movement noted even after storms passed a few hours after normal flight time
Second night of cold weather triggers Frio bats to resume migrating. Seems like 450k headed southward. Will they return?
Some Bracken bats also appear to be leaving. Stuart makes no obvious move. Nothing from Devil’s Sinkhole, Old Tunnel, or Huber, or anywhere else north of Wimberley.
Again Frio bats headed SW in migratory direction, likewise Stuart, but far fewer. Still lots of bats at Bracken. Nothing from Devil’s Sinkhole or Huber.
Tiny bunch at Old Tunnel.
Frio bats appear to head SW past (hopefully around or above) the Anacacho Wind Farm
Most of the Hill Country seems done. Nothing except at Fern and a few at Old Tunnel. Still a million or so at Bracken, but none at McNeil or Huber. Congress, surprisingly, still has ~90k.
Frio much larger than expected and Bracken diminished; possibly from storm but also possible some moved to Frio as part of migration.
Still lots of bats at Bracken and Congress. Half of Frio again heading SW in migratory direction.
Nothing from Congress on this warm evening. Did they leave last night? Radar does not show them leaving.
McNeil and Huber have had no emergences since 08 Nov 2017 UTC.
Devil’s Sinkhole appeared to be done at that same time but had a surge from 13 Nov 2017 to 15 Nov 2017, now next to nothing.
Still lots of bats at Bracken Frio and Fern.
The bats seem to be moving in stages with parts of the colonies moving in separate increments to more SW roosts.
Big surprise from Devil’s Sinkhole!
Surge from Fern, fewer at Foster St and Devil’s Sinkhole
Way fewer at Devil’s Sinkhole, surge at D’Hanis – did they move there?
Surely this cold front pushed the last of the Mexican free-tailed bats southward on their annual migration.
This video shows Fern Cave, two waves of bats from Devil’s Sinkhole, Frio, and Huber all showing massive numbers of bats moving in a migratory direction.
Sure enough Bracken is a day behind, as usual. The majority of all other colonies seem to have left.
We’ll see on the next warm night, if there is one.
Just when we thought it was all over (nothing last night), this happened!
It was the first warm night after the first really cold night, with a North wind at 12 mph and they used it to head on south.
Seasons past – Congress Avenue Bridge info
- Spring return migration started around 09 Mar 2019, but cold weather suppressed flights until 19 Mar 2019. Typically we start to see spring migrants in early March, with Congress reaching full size by the second or third week of March.
- The colony seemed to be at full strength around 20 Mar 2019 after surges of bats pulsed through the area.
- Bats entered “slacker season,” when they sleep late and emerge at dark, on May 16, 2019.
- Good viewing from the bridge, after maternity season, did not start until late in 2019 (29 July) and started getting a little sketchy around 18 September 2019.
- Fall migration started 06 Nov 2019 and another surge left 22 Nov 2019. The bats are generally with us through Halloween, then head south on the first strong cold front.
- Up to 400k on warm nights through Nov 2019. They left in stages on 06 Nov, 09 Nov, and the last group 06 Dec.
- Up to 60 overwintering at Congress Bridge in Jan 2020, with another 80k at UT and other Austin roosts. They seemed to come and go from the bridge to seek shelter in heated, poorly insulated buildings during cold spells. If you have bats roosting in your roof, then you probably have heat leaking from your building envelope into eaves and attics. No charge for the free energy-auditing services—just the Congress bats’ contribution to combat climate change!
- We expect the return migration of the main Congress colony in the first week of March 2020.