Nest-guarding and intraspecific intrusions

The Decorah North Eagles have been dealing with a lot of intraspecific intrusions this year. Intrusions happen when members of other species encroach on or enter a nest in search of food, nesting materials, or a nest: think squirrels, mice, raccoons, chickadees, woodpeckers, red-tailed hawks, house sparrows, and other animals we’ve seen in and around our nests. Intraspecific intrusions happen when other eagles intrude on daily nest activities in search of food, new mates, or breeding territories. They can lead to:

  • Disturbed breeding, incubation, and brooding patterns
  • Egg failure
  • Injured or dead parents, hatchlings, nestlings, and intruders
  • Nest failure
  • New mates (in or out of season)
  • Cooperative breeding (the Fulton Trio eagles)
  • Reduced provisioning for young

While two studies found that male breeders typically guard nests about twice as much as their mates, a third study found no significant difference in the probability of intruder response by male and female breeders when both were present. Intruder response is mediated by:

  • The age of the intruder. Adults are a bigger threat than subadults or juveniles.
  • The sex of the intruder. Female breeders are more likely to respond to female intruders. Either sex will respond to male intruders, although male breeders are more likely to do so during incubatory and brooding periods.
  • The number of intruders.
  • The location and attitude of the intruders. Are they within 300 feet of the nest? Are they near the nest? Are they on the nest? Are they passing through or acting aggressively?
  • The presence of a mate. Eagles generally respond more aggressively to intruders if one partner can guard eggs and/or eaglets while the other chases intruders away.
  • The age of the eaglets. While nest guarding behavior diminishes as eaglets age, parents are free to respond more aggressively to intruders.

Intruder response might include alarm or warning vocals, chasing, or physical interaction depending on the aggressiveness of the intruder, the presence of a mate, and incubatory or brooding requirements.

Intraspecific Intruders at the North Nest

Between February 20 and March 12, we documented at least five adult and subadult intruders at the North nest on 11 out of 20 days. They followed Mr. North to the nest, perched in nearby trees, flew by the nest, attempted to steal food, and generally disturbed nesting activities. On February 23 – the day that DNF would normally have laid her second egg – she appeared to spend much of the day chasing intruders away. Prior to that, her incubatory behavior was relatively normal. After that, she stopped incubating her lone egg.

Thanks to camera operators and a boots-on-the-ground visit by Dave Kester, we know that there were multiple intruders, we know that there were simultaneous intrusions, we know that some of the intruders were adults, and we believe that at a few of them were adult females. Why were there so many eagles in the valley while DNF was laying eggs? There are a lot more eagles than there used to be, the north nest is along a migratory flyway, a warm winter resulted in early migrants and pre-migratory wandering, and the valley of the Norths provides excellent high-quality habitat for eagles waiting for the ice and snow line to move even further north. In short, it was the perfect storm. Eagles began to flood the valley just as DNF was beginning to lay eggs.

Had the intruders moved on quickly, been a less threatening combination of age and sex, or stayed away from the nest, things might have gone differently. But we believe that her response to the constant presence of adult female intruders interfered with her ability to lay eggs and produce prolactin, aka the incubation hormone. While Mr. North underwent incubation-related hormonal change, DNF guarded her nest, mate, and egg. Instead of producing prolactin, her body was producing hormones linked to aggression, territoriality, and stress.

Is it really DNF?

We briefly wondered if a female eagle had vanquished DNF and was courting Mr. North. But we’ve reviewed the footage and we believe that this is DNF based on her identifying marks and Mr. North’s responses to her vocalizations, visits, and a food gift.

Is this normal?

Absent external forces (chemicals, persecution, habitat loss, lead poisoning) bald eagle populations are generally self-regulating. Eagle populations boom until they hit stasis or carrying capacity. Once the best territories are taken and a large population of non-reproducing adults has formed, competition gets serious. How quickly does it happen? A study in Chesapeake Bay found that nearly 100% of reproductively mature birds were assimilating into the breeding population in the early 1990s. By 2013, that percentage had fallen to 17%, indicating that four out of every five birds of breeding age were floaters. That is A LOT of competition. Contests for breeding space may act as a density-dependent regulator of population growth by increasing adult mortality, and reduce reproductive rates by impairing breeding pairs’ ability to provide for or protect broods.

So in some ways, this situation is very normal. NE Iowa’s bald eagle population has expanded, there are only so many places to nest, and the north nest is along a flyway. Juveniles and subadults may try to steal food or visit the nest: adult non-breeders will attempt to become breeders by chasing away or slaying one or both resident adults and taking the territory. However, I haven’t been able to find any documentation of nest failure due to the breeding female’s intrusion response. Is this normal? We don’t know. But it falls within the spectrum of documented eagle activity. Remember, males guard more but there is no significant difference in the probability of intruder response by male and female breeders. Female response could be a previously unknown factor in establishing carrying capacity and limiting population growth. It deserves more study.

Will the egg hatch?

It will not. When I first began researching this blog, I still had some hope. But Mr. North ceased incubation around 6:00 PM on March 16 and the egg is completely covered with snow. Mr. North has cared for this egg through ice, snow, rain, and intrusions. We admire his dedication and skill and hope that he and DNF are able to reclutch this year.

References and Resources