Unusually high tides can cause birds to feed in places they normally do not frequent. Like this Snowy Egret pictured here.
If you doubt the profound effect of tidal movements on coastal birds, consider this: The chief source of predation on Pacific Coast Black Rails comes in the form of herons chomping them during extremely high “spring tides”, when coastal marshes are flooded and the rails have no place to hide. Northern Harriers will sometimes prey on rails as well. Also, I’ve read that flooding caused by spring tides sometimes make it possible to see Black Rails flushed by the water; perhaps the only chance a birder has to see this species without using a tape, a drag rope, or a ten lifetimes’ worth of luck.
Tides create situations that can work to the advantage or disadvantage of both birds and birders. However, before a birder can take advantage of tides, a few fundamentals must be understood:
How many high and low tides are there in a day?
There are about 2 high and 2 low tides per day; the period between each high and low is about 6 hours. This is not exact because the moon takes 24 hours and 50 minutes to circle the earth, not exactly 24. The moon’s gravitational pull has the strongest influence on the creation of tides, though the sun also plays a role.
What time is high tide, or low tide?
The answer to this changes every day, and varies site to site. If high tide at Cape May Point is at 5:00 p.m. this evening, tomorrow evening’s high tide will occur about 50 minutes later. Because the sun and the earth’s rotation also influence tides, 50 minutes is only an approximation.
Generally speaking, the tide is high later the farther inland or “upstream” you go. So, if it is high tide at the mouth of a river or estuary at 5:00 p.m., high tide could come an hour or more later farther inland, depending on the shape of the estuary and how far inland the site in question.
How can I find out when high or low tide will be for a particular site?
My favorite web site for this is:
http://tbone.biol.sc.edu/tide/index.html
Local fishing tackle shops will often provide tide charts for free or stop in at the CMBO Northwood Center and pick one up.
Because tide timing varies dramatically from location to location, it is important to choose the tidal station closest to where you will be birding.
When are there especially high high tides or low low tides?
During the time of the new moon and the full moon, the sun and the moon act together on the tide (because they are in a line with the earth), and create what are called “spring tides.” Spring tides can be several feet higher than their counterparts during the two quarter moons of each month. During a spring tide, not only are the high tides higher, but the low tides are lower.
Tide height can also be heavily influenced by wind. For example, a strong onshore wind can create a higher than usual tide.
What is “slack tide?”
The tide is said to be slack when it is changing between incoming and outgoing or vice versa. Slack tides feature little water movement (and often little bird movement, too.)
How do I know what tide is best for birding?
That varies from site to site, and the best way to find out is to try different sites at different tide heights! However, there are some rules of thumb to guide you, and these will be the subjects of future columns. Stay Tuned!