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Thursday, November 14, 2013

FIELD NOTES: WHY HUMMING BIRDS MOST FEED LATE IN THE TWILIGHT OF THE DAY



Most people familiar with hummingbirds know, that they are the smallest birds on earth and that their flight is unique in the avian world. One of the most amazing things about these birds is the fact, that by the time we say ONE, they have flapped their wings between 20 and 80 times, thus enabling them, to fly up to 45 feet in one second. However, this ability has a high energetic cost, that imposes a high energy intake on these little creatures. This forces hummingbirds to eat their own weight, every day, in order to survive.


Therefore, the last hours of the day are of vital importance to hummingbirds, as they need to gorge themselfs with food, at the end of the day, so they may survive the night without feeding.  The tropics have an even distribution of 12 hours of day light and 12 hours of darkness, so hummers most adopt complex physiological strategies to live trough the night. The loss of body heat is great for little animals with high metabolism, and to warm blooded hummingbirds, this is a huge problem, as they cannot replenish their caloric requirements at night, and ambient temperature can drop very fast.  In many of the habitats, where these little birds make their home, such as the high Andes, where mountains can reach an altitud of 18,000 feet, seasonal changes occur in a daily bases. High tropical mountains have a summer and a winter every day. Their temperature changes can go from high summer temperatures during the day,  to freezing nights in 12 hours. So, how do these little birds cope with these extreme conditions and survive the night hours? Well, by feeding really well at the end of the day, and making sure, they can reach a good food sourse, early in the morning. But this is not enough. Twelve hours without eating is a live or die problem. Hummingbirds have the hability to go into TORPOR or NOCTIVATION, a hibernation like state, where the birds lower their body temperature, heart beats, and breathing and get into a self induced hypothermia. This allows them to cut back their metabolism up to 95% and the requiered energy use up to 50%. Torpor differs from hibernation, because birds can do this at will. To come out of torpor, hummingbirds vibrate their wings and shiver to warm up to readiness. They can do this, up to two hours before day break. Some researchers feel that a circadian clock triggers the behavior and once the hummers come out of torpor, they can go into sleep for a few hours to get the rest they need. Apparently the torpor stages are not real sleep.





Knowing this, is a key element, for those of us who keep feeders for the hummers. Therefore, in areas, where hummers may finish the contents of feeders, by the end of the day, refilling the bottles at that time, with enough sugar water, enables the birds to have a late afternoon and early morning supply to feed on. Some may argue, that this is interfering with nature's ways, and they are right. Others, may support, that its better to supply a wide array of flowering plants, that may produce enough nectar up to the late afternoon hours as a solution, however, it still is human intervention. I have noted, that feeders have help to increase the numbers of hummers in all the areas of Topotepuy, the botanical garden that I work. Never the less, I cannot be sure, that this is due to a better breeding success due to food availability near their nesting areas and living territories, or, that more birds have learned to feed off the bottles, and they stay around throughout the day at the feeding stations. One thing is for sure, agresive interactions at the feeders increase as the light diminishes at dusk, this may be induced, by the feeding urgency, that these little birds have to fill up with enough caloric intake, that would allow them, to make it through the 12 hours of night time without ingesting any nectar or sugar water.


Threatening display between Copper Rumped
Hummingbirds. Note their body language.

Recording these events is important to study the bird's
behavior, and the use of our flashes and a high speed shutter
allows the photonaturalist to record postures that may be too fast to see with the naked eye.

Pointing your bill, blowing wind and confronting your adversary
stablishes who will remain at the feeder, this is the so called pecking order behavior where dominant birds are the first to eat.

The bird at the feeder maintains its position and turning sideways to feed
may tell its adversary, I am here to stay. This act of self confidence sends a clear mesage to the other hummer. Note how the opposer's bill and flapping wings are directed to the bird at the feeder. Using wing generated wind and sounds may be part of a humming bird's repertoir in agonistic behavior. 

It is a known fact that hummingbirds can go into TORPOR or NOCTIVATION at night. This is a lethargic state assumed by the birds at night, where they lower their metabolic rates up to 95% by slowing their heart, lung activity and lowering their body temperature to a pseudo hypothermia state where this activities are hardly percievable by the human eye. The literature cites, that hummers are not sleeping while noctivating, and they will actually go into sleep, once they get out of the lethargic stage by shivering and flapping their wings. Only then they may sleep for a few hours before dawn! Hummers most have a great memory to map in their territory which plants are in bloom, how rich in nectar are their flowers, and how many flower patches are near their resting sites, as this is a mater of life and death.


All the images in this articles were taken between 17:00 and 18:30 making imperative the use of flashes. The flash mode was set at multiple, the ISO at 800 and the focus of the camera was preset, to enable shooting in near darkness possible. The technique does guarantee 100% results, but as soon as a hummer gets into pre-focused framing, one triggers the camera and one out of 5 images can be right on the money.  The photos were taken in RAW, and then, processed in Adobe Camera Raw, which helped me correct the under exposed images, and make the best out of my files.


There are two species of hummingbirds visiting my feeder The Copper Rumped Hummingbird (Amazilia tobaci) and the Glittering Throated Emerald (Amazilia fimbriata). The Emeralds are the first to go during the late afternoon, however, the Copper Rumped hummers remain past dusk and their agressive displays increases into a free for all, and they they disapear until dawn. Learning the most about the animals you are about to photograph is important, but if we are to alter behaviours by making feeding stations, blinds or visiting vulnerable areas, it is imperative to learn to draw a line between our goals and the well being of your subjects.

Hope you enjoy the info and we will be seen you soon.

Best regards

Leopoldo "Leo" García Berrizbeitia
Photonaturalist.


BEST PLACE TO LEARN AND PHOTOGRAPH HUMMERS IN CARACAS



















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