2018 Osprey Research and Production Summary

Here are all the 2018 results, for those of you who like the numbers/ details!

2018 Osprey Research and Production Summary
by Vanessa Greene

Spring came quite late in 2018, with winter hanging on well into April. The first Ospreys were not reported until ‪April 10-11‬! A heavy snowfall of over 12 inches blanketed the Twin Cities metro area ‪on April 15.‬ I observed my first Ospreys ‪on April 12‬ and successfully read the first bands that day. Ospreys arrived slowly and the first signs of incubation were not documented ‪until April 26.‬ In comparison, we have had eggs laid as early as ‪April 12‬ in past years.

This year can be characterized once again as a productive one for the Osprey population in this 35 year study with the population continuing to grow and expand, although at a slower rate than past years. Overall, productivity numbers remain very similar to last year. There were 136 nests which were occupied* by a pair of adult ospreys. (132 in 2017. There may be more nests we do not know about.) Two additional nests were frustration nests and therefore not counted as separate territories. Eggs were laid in 125 nests (122 in 2017. There were also two additional nests that were discovered later in the breeding season and although no chicks were present, it was not known if eggs were laid or not) and 96 of these nests had at least one chick that was confirmed to have fledged successfully or survived to fledging age (95 in 2017). We documented 40 nests which failed (37 in 2017). We separate failed nests into two distinct subcategories; nests where a pair was present but no eggs were laid (9) and nests where eggs were laid but they failed to successfully fledge a single chick or the cause of failure was unknown (31). (Not laying eggs is considered to be a kind of nest failure by other scientists.)

There were 205 chicks that were known to have fledged successfully or survived to fledging age! (214 in 2017). Most successful nests had two to three chicks, 51 nests with two chicks, 29 nests with three chicks, and 16 nests that produced a single chick. The mortality rate this year was higher than last year with 19 chicks which were known to have died or disappeared before fledging, (19 last year also) and with three adults which disappeared mid-season and were presumed to have died. There were 15 nests that failed after signs of hatching were observed but before we were able to see or accurately count chicks (five last year), seven nests where no signs of hatching were ever observed, nine nests where the cause of failure was unknown, in addition to the nine nests were no eggs were laid. There were only 87 adult Ospreys identified by their bands, as the number of banded birds continues to drop. Three of these were from Iowa. We located 12 new nesting territories, including four nests that were newly discovered this year although reports indicated that they had been there for one or more years. Only two of these new nests successfully fledged chicks. In addition, there were six nests which had been unoccupied for 3-20 years, which were reoccupied this year. There were also 14 nests which had been occupied last year that were not occupied by Ospreys this year. One of these was taken over by geese and two were taken over by Great Horned Owls. There were eight banded Ospreys which were believed to have bred successfully for the first time and their average age was 4.75 years old. (Average age of first successful breeding for males this year was 4.71 years and for females it was five years). It is interesting to note that of the 136 occupied territories this year, 75 were on osprey nesting platforms, 23 were on cell or radio towers, 18 were on ballfield lights, 15 were on a power pole or transmission tower, and three were on other manmade structures. Two nests were built in a dead tree late in the season, with no eggs laid.

The overall productivity of occupied nests which were successful this year was 71%, (72% in 2017, 76% in 2016, 68% in 2015, 70% in 2014, 67% in 2013, and 77% in 2012). The mean number of young fledged per successful nest was 2.13% (2.25 in 2017, 2.24 in 2016, 2.43 in 2015, 1.77 in 2014). The mean number of young fledged per active nest was 1.64%, (1.75 in 2017,1.84 in 2016, 1.88 in 2015, 1.41 in 2014) and the mean number of young fledged per occupied nest was 1.51%, (1.62 in 2017, 1.70 in 2016, 1.65 in 2015, 1.25 in 2014). These numbers reflect a slight decrease in overall productivity per nest, although the number of chicks fledged per successful nests remained similar to last year. There were two nests which have failed for four years in a row.

There was a baffling and disturbing outcome on one nest, where the single chick was discovered alone on the nest. Three monitors took turns watching over this nest closely for nearly a week, and no adults came to care for this chick. Other adult Ospreys showed up occasionally, but provided no parental care. In fact, they seemed to cause the chick distress. It was through careful behavioral observations that we concluded that these visiting ospreys were not this chick’s parents, and it appeared that both parents had died somehow, since adults typically do not abandon their offspring. The nest was in a very inaccessible spot, on a large transmission tower in a lake, so mounting a rescue involved considerable legal and physical hurdles and we were not able to arrange for this before the chick died / disappeared. Those involved with this situation were deeply saddened and suspected that these adults may have been killed. It is very unusual for two adults to die at the same time.

The three oldest males this year, dropped to 15 years of age. Our oldest male last year at 23 years, did not return. Two additional older males, 18 and 17 years of age last year, also did not return. Our oldest females are younger with four that were 11 years old, which all bred successfully. It is interesting to note that we have recorded 2,270 chicks which survived to fledging age on monitored nests in the metro area since the inception of this project in 1984. 

(*Successful nests are those that were known to have fledged at least one chick successfully, active nests are those where eggs are laid and occupied nests are those where pairs are present at a nest site for a period of time, regardless of the time of year or whether or not they lay.)