This may be the last dollop of photos for a while, depending on how much time I have on the road. But today we have a great series from Bruce Lyon, continuing his story of the Northern Harrier nest I posted about several weeks ago. Here are his pictures and notes; the photos are stunning, and be sure to read the text:
The male provided much of the family food, particularly during incubation and the early chick stage. Male harriers almost never go to the nest but transfer prey items to the female in the air. The next two photos show a prey transfer. The female (brown bird) chased the male (gray bird), who has a mouse (you can see it under his fanned tail). In prey transfers, the male drops the prey item when the female is close enough, and she then quickly snatches the prey item. In the particular transfer shown below, the male seemed to drop the mouse a bit early and the female had to go into a nose dive to get the mouse (second photo). It took me a lot of attempts to finally get a decent set of images of transfers because it is difficult to predict where the transfer will occur and they are not always very close. The two photos of the prey transfer were taken with my Canon 500mm F4 lens and are heavily cropped.
Female returns to the nest with the mouse. I was able to watch the female calmly coming and going to the nest simply by throwing some camo netting over myself and camera. As far as both the male and female were concerned, I disappeared once the camo netting was over me:
Female coming in to the nest with a lizard (a Western Fence Lizard [Sceloporus occentalis] I believe):
The harriers often had what I interpret as a ritualized copulation interaction. Often when the male returned to the nest area and the female was not on the nest but perched on a bush, he would land on her back for a second, crouch down and then take off. I saw this happen about a dozen times and watched carefully and never saw anything resembling a real copulation. I have no idea whether this serves any function, but in the old days this behavior have been interpreted in context of ‘maintaining the pairbond’. The next two photos show one of these pseudo-copulations:
The harrier nest had a fairly high rate of ‘brood reduction’ (chick death). Of the five chicks that hatched, only two survived. Often extreme brood reduction like this is associated with asynchronous hatching, where different eggs hatch on different days. David Lack, the influential English ornithologist, proposed that brood reduction is a mechanism that allows birds to adjust their family size to an unpredictable food supply (they have to choose the number of eggs to lay before they know precisely how much food will be available for the kids). He also suggested that asynchronous hatching provides the parents an efficient mechanism to trim the brood size to match food supply because the smaller, later-hatched chicks are the first to go if there is not enough food for all. The harriers seem to fit this pattern—two chicks hatched on the first day, two chicks hatched on the second day and the last chick hatched later (presumably the next day but I did not check the nest for a couple of days). Since the chicks were not tagged I don’t know which chicks perished but there was a very clear size hierarchy early on and I am pretty sure that the two survivors were the chicks that hatched on the first day. It is well known that female harriers, who do the actual chick feeding at the nest, do not preferentially feed the smallest chicks—the largest chicks would have a competitive advantage in grabbing food due to their larger size. Here is the first day of hatching:
When I next, checked the nest a week after hatch there were four chicks left. The chick on the right had recently swallowed a large mouse intact and the tip of the mouse’s tail is sticking out the chick’s beak:
When I next checked the nest, three weeks after the first eggs hatched, there were only two chicks left:
One month after hatch the chicks were developing the gorgeous rufous coloration of juvenile harriers:
Almost 50 days after hatching the two survivors are full grown and could fly very well:
After the chicks fledged I made a discovery that could explain the extreme brood reduction, and it also provides a an example of an idea I have been thinking about for awhile. I observed the male fly by the nesting area with a mouse. The female flew up to him and I expected a prey transfer, but it did not happen. Instead, the male kept flying. The photo below, of the male with a pocket gopher, illustrates what I saw but is not the actual event. Roughly a mile further south a second female flew up and got the prey item from the male—he had a second family! I eventually found the second nest and it contained four large chicks ready to fledge. This nest was about three weeks behind the first one but since it had double the family size I suspect the male was bring most of his prey items to this nest.Polygyny, where one male has several mates, is fairly common in harriers and individual males can have ‘harems’ of 2 to 5 females. Back to the issue of brood reduction, I now wonder if the first female laid a clutch size that would have been ideal had her mate invested only in her nest. David Lack proposed brood reduction to deal with ecological uncertainty but I have been wondering for some time whether social uncertainty—specifically whether feeding by the male is predictably or not—might play a role in some species. Then the harriers provided a possible example. Natural history at its best!