We've been getting questions about egg timing. Why did Mom and Dad lay eggs later this year? Overall, avian egg-timing in the temperate zone is heavily regulated by the light cycle. As the days begin to lengthen, birds' gonads swell and produce sex hormones. Around the end of January, our bald eagle pairs switch from infrequent copulation to frequent, highly enthusiastic copulation, putting us on notice for egg watch! Females chase males, males entice females, copulation takes place on a regular basis, and our eagles start laying eggs in mid-February. We have a general egg outline, but what factors correlate to the onset of egg-laying in any given year?
New mates can equal later eggs
While it doesn't always happen, a new mate (male or female) often pushes egg-laying a little later in the eagles and falcons we watch. Sometimes it happens for very obvious reasons, like mid-season mate replacement. Let's say that female falcon A and male falcon B are nesting together on a cliff in mid-March. Falcon A has begun the process of egg production: her egg follicles are swelling, she's gaining weight, and she's copulating like crazy! Unfortunately, she's killed or displaced by female falcon C, who is producing plenty of sex hormones, but no eggs. B doesn't waste any time courting C. His bowing, gifts of food, and copulation will trigger her to begin egg production, but their egg-laying schedule will be pushed back by as much as or more than ten days. Absent other factors, it should float earlier in the years to come.
Later egg-laying can also happen in new couples that don't experience mid-season mate replacement. Both Mom Decorah and Ma FSV laid the first eggs of their lives in early March (March 2, 2009 for Mom Decorah and March 3, 2007 for Ma FSV) but moved earlier as the years progressed. Mom had just achieved sexual maturity when she bonded with Dad, but we don't know if the same is true of Ma FSV. Is initial egg production - an eagle's first egg - a factor in egg timing? How about receptivity in a new pair versus an already bonded pair? We don't know, but it's something we'll be looking at.
Cold dry weather? Predict eggs on the later side!
Having said that, we didn't have a new eagle in Decorah or at Fort St. Vrain this year. So what happened? We decided to look at weather. Using the nearest weather stations we could find to each nest (especially important in Colorado with its altitude-based weather patterns), we obtained historical figures for temperature and dewpoint from January 30 to February 10. There isn't a long time period between the onset of egg-laying and egg-laying itself, so we started around the time that both sets of eagles usually ramp up copulation and ranked our results from latest to earliest laying date.
FSV: January 30 to February 10
| 1st Egg Date | High Temp | Mean Temp | Low Temp | Dewpoint |
| 2/21/14 | 31 F | 19 F | 6 F | 12 F |
| 2/17/13 | 48 F | 32 F | 17 F | 15 F |
| 2/16/16 | 46 F | 31 F | 17 F | 17 F |
| 2/14/15 | 60 F | 44 F | 28 F | 21 F |
| 2/14/17 | 56 F | 45 F | 33 F | 21 F |
| 2/12/18 | 51 F | 37 F | 22 F | 22 F |
Decorah Eagles: January 30 to February 10 |
| 1st Egg Date | High Temp | Mean Temp | Low Temp | Dewpoint |
| 2/23/14 | 12 F | 4 F | -3 F | -3 F |
| 2/21/18 | 37 F | 20 F | 8 F | 4 F |
| 2/20/17 | 30 F | 21 F | 12 F | 15 F |
| 2/18/16 | 29 F | 22 F | 16 F | 19 F |
| 2/18/15 | 24 F | 16 F | 9 F | 12 F |
| 2/17/12 | 34 F | 28 F | 22 F | 23 F |
In general, both sets of eagles laid eggs slightly earlier during periods of unusual warmth and slightly later during periods of unusual cold. But much to my surprise, humidity (as measured by dewpoint) turned out to be a slightly better predictor of egg-timing than temperature! What was going on with that - and with the one interesting exception in Decorah in 2015? Time to do a little reading!
It is widely believed that breeding female raptors gain weight by building fat and storing minerals to provide the energy reserves needed for egg-laying and incubation. But a study of female barn owls found that almost all of their pre-laying weight gain was due to water accumulation! From the study's discussion:
"To conclude, the gain in body mass observed in breeding barn owls is not due to an accumulation of energy reserves but of an accumulation of water." Incubating female barn owls undergo a more complete molt than incubating bald eagles, but egg production mechanisms are very similar and it seems reasonable to believe that water retention would be similar in both species. The nutrients required for egg formation can be obtained by routine food intake during egg formation, but if the weather is extremely dry, it might take longer for female bald eagles to accumulate the water reserves they need for egg production, especially give that drier weather results in higher levels of evaporative and cutaneous water loss. It wouldn't make a few weeks difference in any given year, but it might make a few days!
I couldn't find an explanation for the anomaly in Decorah in 2015, but this is a relatively small dataset. As we keep records longer, we may learn more about trends in egg-laying. It also might be worth getting weather stations down below our nests to collect local data about temperature, humidity, and wind at each nest. We still have a lot to learn about the ways in which weather and climate affect the birds we watch.
Did you know?
The barn owl study also had some interesting information about fat/lipid dispersal in a breeding female barn owl's body. Although breeding females had
fewer body lipids than non-breeding females, they had more subcutaneous fat in the area of their brood patch. The accumulated fat caused their brood patches to increase in size! So Mom's more visible brood patch and slightly larger appearance aren't just our imaginations. She has gained water weight and shifted fat to her brood patch in preparation for egg-laying and incubation. I'm sure that more than a few of us can relate to that!
While we aren't sure why Midwest peregrine falcons are laying eggs earlier, they are laying significantly earlier than they did at the beginning of falcon recovery in our area. Interested in the topic? Check out
Average Nesting Trends!
Why didn't you use data from the North pair?
This was a tough one to leave out, especially given that many watchers think Mr. North was a new mate in 2016 - a year of later egg production. However, we only have three years of production here, the eagles have not established a regular laying pattern (2018 was also late), and we have no weather history anywhere near the nest, so I'm not comfortable including their data. We'll see what happens in the years to come.
Things that helped me learn about this topic
