7/18/16 – At the end of June we saw some things on the Cathedral of Learning Peregrine falcon cam that dismayed some viewers. ‘I thought falcons mated for life?’ was the recurring question. What was going on? Both Hope and Magnum were seen in the scrape courting Terzo. Poor guy, he will take whichever female is there, it does not matter to him, but why are two females tolerating each others presence?
Magnum was there on the 21st through the 23rd, the archives are down for the 24th, and by the 25th Hope was back in the scrape and seen daily up to this point, except on 7/15/16, the day C1 and Terzo were in. It appears Magnum has returned to her bridge for now.
Is there room for more than one pair in the vicinity?
Maybe…studies have shown that in dense populations Peregrines space themselves about 4 km apart1. For a frame of reference, the CoL is 3.54 km from the Gulf Tower, so it is not likely that another falcon will move in and create room between these two prime nesting locations. The Gulf Tower site is 5.93 km from the McKees Rocks Bridge, and the Neville Island, Tarentum, and Westinghouse Bridges are much farther away. So, it is possible that there is room for more, but there are other considerations as well, for example, where all the Bald eagles are making home.
Peregrine falcon nesting biology
In growing populations, Peregrine falcon pairs fledge an average of 1.15-1.66 chicks per year, varying widely from year to year2,3,4. This means that failure of eggs to hatch and death prior to fledging is quite common, as clutch size is usually 3—4 eggs2. When a clutch is lost early pairs sometimes are able to produce a second clutch1.
Scientists report lifetime productivity highs include a male that fledged 20 of his own offspring and a female that fledged 22 of her own3. In Downtown Pittsburgh, Louie fledged 52, Tasha fledged 44, Dorothy fledged 43, Boris fledged 39, Dori fledged 28, Erie and E2 each had 22 fledge (including this season’ fledges). One thing this tells me is that these nesting locations are prime sites and the birds that held them for so long were exceptional birds. It is hardly fair to compare other Peregrines to them, that would be like comparing our nation’s children’s intelligence to Albert Einstein’s, hardly a level playing field. When you include the success of the birds in the surrounding areas to get an idea of what is happening in the population, the numbers are closer to what scientists have reported in the past.
Why might reproductive output vary, and which nests have the greatest success?
Seasons with heavy or long periods of rains result in fewer chicks fledged in the population and catastrophic losses for some pairs2,5. Nest locations that offered protection via an overhang were more successful than those left exposed 2,6,5. With no choice of alternate nest sites, during wet years some nest sites remained flooded and falcon pairs did not breed that season5,7. This might be one reason we see Dori nesting in more than one location, it is advantageous for her to have an alternate location in case of some type of disturbance or weather problem.
The height of the nesting location was also important, with higher nests being more successful because falcons could perch nearby and keep an eye on the nest and hunt at the same time, stooping on prey from their perch; these nests fledged more chicks than those in lower locations8,2,6,9,10,7. Nest locations near abundant food sources and cover for hunting were preferred6,10,9,7,11. The buildings near downtown and the buildings and trees near the CoL would offer much more protection than the bridges of Pittsburgh, allowing the falcons to surprise attack their prey.
As falcons aged they became more successful2. Pairs of mixed age (adult and juvenile) were significantly less successful than fully adult pairs12. Birds, as well as other animals, including humans, learn from their experiences. Older birds are able to apply this wisdom to raise more successful young, even if just by giving them a size advantage over their competitors as they start out in life.
How much fidelity to nest site or mates do we see?
In one study, breeders occupied the same territory for only several years, on average, 3.4 years for males and 3.7 years for females12. Some non-breeders are presumably present among the breeding population because often a new falcon would appear within hours of the previous territory-holder disappearing (these ‘floater’ birds are at risk of attack by territory-holders)3. Studies have showed that roughly 5% of the breeding birds in a population were new that year, sometimes after the loss of a mate or an unsuccessful breeding season, less often to move to a better location, and only 30% of those relocations involved a fight—1.7% of all nesting attempts3,13. Again, consider that the Pittsburgh falcons named above held their territories for 5—13 years and still counting, this seems unusual when compared to the entire population, but it is not a representative sample. Also, there might not be as much pressure from un-mated ‘floaters’ in this area.
One two-year old female paired with males in adjacent territories one year but failed to produce eggs3. One male held one territory, then moved to another location and took over a territory when the resident male was killed in a collision with an airplane. The following year he returned late and displaced a newcomer, only to raise the other male’s chicks. Finally, the following year, he raised his own chicks3. Territorial males are willing to mate with any female who allows it, and courtship feeding and copulations occur in the absence of a long-term pair bond3. Un-mated males remain in their territory and court females, while un-mated females roam widely14. Peregrines have also been reported to come together in groups of as many as seven in the fall in playful exuberance14. Therefore, in Peregrine falcons, fidelity is stronger towards the territory than the mate3. And, after the nesting season is over, the rules of territoriality can be relaxed a bit.
Do the young return to the nest?
Fledged young return to the nest to beg for food, one remaining until November, and one female returned a year after fledging and was seen with her parents12. Some fledglings traveled far during their first month, but some remained with their parents for several months, or returned the following year12. Once a one-year old female returned to her hacking place and mated with the resident male3. So, the fact that C1 has been seen and heard near her scrape five weeks after fledging is not unusual.
Females tend to disperse farther from their natal territory for breeding than males and dispersal distance was farther for Midwestern Peregrines than reported for falcons from Alaska (232 miles for females vs. 108 miles for males in the Midwest; 68 miles for females vs. 40 miles for males in Alaska). This may be due to scarcity of acceptable nesting sites, as the falcons seem to be drawn to large bodies of water3,12. Females that disperse farther from their natal grounds breed at a younger age12. It is hard to say where C1 might end up, or where her mate will be from, but it is important for Peregrines to disperse and for populations to have new emigrants to maintain genetic diversity.
Migrating birds from the arctic possibly follow the same route as they have in the past, as one Peregrine was captured in the same locality as two years previously, some of these birds winter in the coastal US8. While the population in Pittsburgh does not migrate today, in the past the wild Peregrines of Pennsylvania did migrate south and return around March to claim their territories again14. It is possible that this lack of migration has contributed to the longer territory-holding I mentioned.
So, will Magnum return to challenge Hope for the CoL? Maybe, but she has to weigh the risks of getting injured over the benefit of the location, and consider the alternative, which is to remain at Neville Island, or find a new site altogether.
Images courtesy of The National Aviary and Wild Earth.
1 Olsen & Olsen 1988 The Emu 88:195—201
2Mearns & Newton 1988 J. An Ecol 57:903—916
3Tordoff & Redig 1997 J Raptor Res 31:339—346
4Craig et al. 2004 J Wildlife Mgmt 68:1032—1038
5Olsen & Olsen 1989 The Emu 89:1—5
6Mendelsohn 1988 Peregrine Falcon Populations, The Peregrine Fund
7Brambilla et al. 2010 Ibis 152:310—322
8Henderson 1965 Wilson Bulletin 77:327—339
9Sergio et al. 2004 Ecography 27:818—826
10Jenkins 2000 Ibis 142:235—246
11Bruggeman et al. 2016 Ibis 158:61—74
12Zuberogoitia et al. 2009 J Ornithol 150:95—101
13Nesje et al. 2000 Molecular Ecology 9:53—60
14Bent 1961 Life Histories of North American Birds of Prey, part 2. Dover