June 01, 2005
Brood Dynamics
As we mentioned on the cam page, assuming the osprey eggs follow the average incubation period of 38-39 days and assuming the eggs are all fertile, we expect hatching around the following days:
1st egg - June 5 or 6
2nd egg - June 8 or 9
3rd egg - June 11 or 12
Also, today we saw a little cat and mouse game played by the couple. Seems one wanted the sod-like material near the eggs and the other didn't. So back and forth they went. It made for an interesting series of photos.
Now that we're close to hatching time, a couple of our cam watchers wanted to know if we would see any chick aggression in the osprey nest or if the chicks would coexist like the eaglets did.
In the past four years that we've run our live Osprey Cam, we haven't had a three-chick brood for a sustained period of time. In 2001 and 2002, we had one chick fledge each year, and in 2003, we had two chicks fledge. Then in 2004, we had three eggs hatch, but the youngest chick only lived about three days and seemed to die of natural causes. The two remaining chicks then fledged successfully.
The topic of chick survival (or brood reduction) in osprey nests is an interesting one. Back in 1978, biologist R.J. O'Conner reported his theory that brood reduction in osprey nests might occur in three ways: fratricide (chick kills chick), infanticide (parent kills chick by selectively feeding individuals instead of all the chicks) or suicide (chick voluntarily stops trying to eat).
When most people think of fratricide, they imagine one chick pecking another chick to death or pushing a chick out of the nest. But fratricide can take more subtle forms, such as the oldest chick preventing the youngest from getting adequate nourishment.
The firstborn in an osprey nest has the advantage because raptor eggs hatch asynchronously, so the first chick is bigger. When that size advantage is coupled with aggression (which is likely in areas with inadequate food), the oldest chick can often keep the younger chicks from eating by pecking them and intimidating them into submission. Often the mother will feed those that are closest to her and most persistent; this is especially true if food is not plentiful. Basically it's survival of the biggest and the most aggressive when food is at a premium.
Last year on our Osprey Cam, we did not see a blatant act of aggression from the older chicks toward the younger chick that died, and we did not notice clear acts of aggression between chicks in our other cam nests from past years. We've been told by our biologist that the raptor nests at and around Blackwater Refuge do not seem to exhibit a great deal of fratricide. While sibling rivalry is common, it does not seem to lead to death or starvation. This follows the other popular theory that when fratricide occurs it is often the result of a food shortage -- something that is not a problem at Blackwater and around other parts of the Chesapeake Bay.
But this was not always true. Two Chesapeake Bay osprey studies -- one done in 1975 and one done in 1985 -- showed that aggression in ospreys nests had increased as the health of the Bay decreased. In the mid-80s, the Chesapeake Bay was going through a health emergency, and its fish and shellfish stocks were dropping dramatically. Below is a chart that shows how chick aggression and family dynamics changed as the health of the Bay changed:
Chick aggression became prevalent when the Bay's resources began to decline. The two brood deaths were attributed to malnutrition, as the osprey females had gone from feeding all their young sequentially, to feeding the bigger and most aggressive of the chicks.
As for the males, during the 1985 study they were having to spend more time away from the nest to find food, and the food they delivered was smaller in size. Even when some males delivered a large amount of food, it appeared that not all the chicks were getting fed equally as the female and the dominant young consumed most of the food and fish remains were left uneaten in the nest.
The Chesapeake Bay situation seemed to indicate a clear correlation between food supply and chick aggression, and other notable studies have backed this theory up. But as is often the case in nature, there always seems to be a few exceptions to every theory, and several studies have found chick aggression appearing even in areas with an adequate food supply.
One such exception appeared in a 1991 study of New Jersey ospreys, where the biologists concluded that while the local food supply was sufficient, a contributing factor to brood reduction was the hunting ability of the male ospreys. More experienced males would forage more successfully, and with greater vigor, than less experienced males. Thus the biologists concluded that even though there was plenty of fish in local waters, the weight differences among the chicks and the witnessed chick aggression was likely affected by the skill and experience of the males in providing for their families. In particular, the younger fathers had a difficult time struggling to meet the needs of a three-chick nest.
Our osprey pair this year is a new couple to us and are not the same ospreys that we've had in previous years on the cam nest. This year's pair laid their eggs rather late, which made us speculate that they might be an inexperienced couple. If their eggs hatch, it will be interesting to observe how they handle their brood and the coming parental challenges.
Until next time,
Lisa - webmaster
[Referenced Studies:
McLean, P.K. 1986. The feeding ecology of the Chesapeake Bay Ospreys and the growth and behavior of their young. The Wilson Bulletin 103(1): 105-111, 1991.
O'Conner, R.J. 1978. Brood reduction in birds: selection for fratricide, infanticide and suicide. Anim. Behav. 26:79-96.
Steidl, Robert J. 1991. Growth and brood reduction of Mid-Atlantic Coast Ospreys. The Auk 108:363-370.
Stinson, C.H. 1977. Growth and behavior of young Ospreys Pandion haliaetus. Oikos 28:299-303.]
Posted by Webmaster at June 1, 2005 07:09 PM