Twigs and Trickery
Last week we talked about the wonderful Great Egrets nesting at Smith Oaks Rookery at High Island and some photographic tips. This is my fourth year to go and it can be an overwhelming place for the novice with all the NOISE and constant activity. You really don't know where to start. But, with some practice and familiarity, you start to notice more subtle details.
At High Island the ground below the nests is bare so the birds generally fly to the surrounding area to hunt for sticks. The activity is hidden from view. At a small rookery at Resoft Park we were able to see the males forage for sticks in a fence row and nearby field. They pulled sticks from trees or found dead sticks on the ground. Sometimes they grabbed sticks from the water's edge or even the open water. There definitely was a selection process going on; many sticks were picked up and discarded before the male returned to the nest with the prize.
I always wonder if she tells him what kind of stick to bring back but I have never seen her reject a stick. Maybe it is just the one he wants to bring back based on his experience. The males often start the nest alone before they attract a female, so they both know how to build nests. One structural analysis of Green Heron's nests that I found suggested they preferred rough barked y-shaped sticks to construct platform nests as it helps to stabilize the eggs. Great Egrets built the same type of nests, so that is one thought.
This is not an unusually large stick and it is definitely y-branched, but they often bring smooth sticks as well as fine twigs.
The stick transfer shows the high degree of cooperation of the bonded pair. He either helps her weave the addition or stands around for a while as she places the stick. Then he flies off on another missive.
The male has just brought is mate a stick, she is holding it in her bill. Evidently she was pleased with the offering and they are having sex. It only lasts a few seconds; she moves her tail feathers out of the way and their cloacas touch and the sperm is transferred.
This mating pair have already built a nest, it is possible she has already produced an egg. Multiple matings are the norm. The female stores the sperm and it is then used to fertilize the series of eggs she lays. For all the details see How Do Birds Lay Eggs?
The shorter story is that she will generally lay one egg a day with a clutch size of 3-5 eggs. Each egg is fertilized as it passes through her body by the stored sperm. The eggs are incubated by both parents for about 23-27 days. The chicks are not born all at the same time, but in the order they were laid and the earlier born chicks get most of the food brought by the parents. The last born is truly the runt and often does not survive.
That is the official story, but Nature is wild place.
This is a semi-wide angle view of the nesting trees at Smith Oaks. A lot of identical white birds sitting on nests, standing around and waiting for the fliers to bring more sticks. Some of the birds are mated pairs, some are still single. You can see a few males displaying from time to time.
And sometimes there is a big noisy ruckus.
Whoa. What is going on here? Did a male get too close to another? Is the bird protecting his nest, female and space from the interloper?
And this mating doesn't look consensual. Of all the normal matings I have seen, the male might flap his wings a bit for balance, but the female is still and cooperative. Here, she has raised her wings and looks as though she is not a willing participant. The male has grabbed her by the neck in his efforts. Compare this to the photos above.
This female is turned away and the male has tried to mount her from the side. She was able to thwart his advances. This is not her mate; he is off finding a stick and this interloper had been displaying in a nearby tree just before this action.
This female has a nest in one of the choice spots so we had been watching her. Her mate had flown off to get a stick and this... stranger is attempting to mate with her. She is turning her head to bite at him and is not adopting a submissive attitude.
The above are three examples I photographed over our two-day outing. I noticed a fellow Birds of Texas photographer posted a photo of a male driving away an insistent interloper from his nest and mate at Smith Oaks. So, I started Googling. (I love the internet). I found several mentions of this kind of behavior and this paper on Promiscuity in Monogamous Colonial Birds which you can read free online. It is not uncommon for extra pair couplings to take place during the breeding season. There is a lot going on in the avian world that isn't that apparent.
When I was in school in the 70s, Sociobiology was all the rage. It was a new approach of looking at social behavior as an evolutionary strategy. In rookeries, bonded pairs form a cooperative unit to raise successful chicks thus ensuring their genes are passed on to the next generation. A lone male without a mate may take advantage of an unguarded female and attempt to mate. Remember, she stores the sperm in her body to fertilize the next egg to be laid. He might be successful and has much to gain if he is. A male with a mate may also try to maximize his genetic contribution by fertilizing an egg of another male's mate. If he succeeds, then the other pair will do all the work of raising his chick.
Surveying the literature now I see a trend in the analysis toward cooperative neighborhoods; i.e if males have offspring in the colony by females other than their bonded mate, they will defend and protect all the chicks not just their own. This is thought to be advantageous for the female, who in some species often solicit extra couplings according to observations. I haven't had a chance to study this line of thought but extra pair couplings in many birds have been observed. With DNA testing it is now possible to determine if chicks are full siblings or fathered by different males.
My three examples hardly makes a large sample, but I know I will be much more observant of the mating and interactions next time I go to the rookery.
Do you think these birds are promiscuous? Where do you think the word cuckold comes from? It looks as though the female is not complicit in this trickery, but are you surprised to know some females actively solicit other males? What do you think about all of this? Let me know in the comments below.