22 January 2013

Least Tern Eggs in Nests of Piping Plover

During 1983, in the course of the annual Nebraska Game and Parks Commission survey of breeding activity of the interior Least Tern (Sterna antillarum athalassos) on the Missouri National Recreation River in northeast Nebraska, eggs of the Tern were found in nests of the belted Piping Plover (Charadrius melodus circumcintus). This survey involved weekly visits to colonies to check breeding activity and status of nest contents.

At two different midriver sandbar colony sites eggs of the Least Tern were observed in Piping Plover nests. The most obvious differences first noticed were that the eggs in two separate scrapes had an odd coloration and size and pattern of markings. The odd egg or eggs in each scrape was notably different, with markings more typical of Least Tern eggs rather than the Piping Plover. Familiarity with eggs of both species from previous field work (Nebraska Bird Review 49: 45-51, 50: 68-72) and other nests during this census (NBR 52:36-40) made the differences very apparent. A comparison with the contents of other nests at both colonies, lighter coloration, and markings that were blotched, spotted, and speckled rather than the more uniform, smaller spots and speckles of Piping Plover eggs, led to an identification of the eggs as those of the Least Tern. The marking variations can be noted in the accompanying pictures of the contents within both nests. Differences are most notable in the nest with two odd eggs. Coloration differences at both scrapes are also more apparent in color slides taken to document this occurrence.

Figure 1. Piping Plover nest with Tern egg at Ionia Bend Colony on the Missouri National Recreational River. The Tern egg is on the right side of the scrape.

The first nest was at a colony site known as Ionia Bend, at Missouri River mile 762, 8 kilometers east of Newcastle, in Cedar County, Nebraska. When the nest was located on 16 June it contained 4 Plover eggs and 1 Tern egg (Figure 1). On subsequent visits on 21 June and 29 June the contents were the same. On 6 July the Tern egg was gone but there were still 3 Plover eggs and 1 hatched Plover young. An empty scrape was recorded on the final visit on 13 July and the Plover were considered to have successfully fledged. The nearest active Tern nest during this period was 25 meters.

A second occurrence was at Hourglass Colony, at river mile 774, 1.6 kilometers east and 3.2 kilometers north of Maskell, Dixon County. The Plover nest at this site was first observed on 1 June when it contained 3 eggs. On 8 June, 4 eggs were present. On 16 June the nest had 4 Plover eggs as well as the 2 Tern eggs (Figure 2). The adult Pipling Plover was observed incubating the nest on this visit. On 22 June one of the Tern eggs was gone although the other 5 eggs were still present. The Plover eggs were gone on 29 June and these were considered to have successfully fledged young. The Tern egg was still in the scrape on this date as well as on 6 July. The nearest active tern nest during this period was 6.5 meters.

In both of these cases the odd eggs had a different fate than the other contents of the scrape. In the first nest the egg disappeared and in the second nest one egg was gone and the other remained in the nest after the Plover eggs hatched. This would indicate that these eggs were probably not laid in the scrape at the same time as the greater number of eggs.

Figure 2. Piping Plover nest with 2 Tern eggs at Hourglass Colony on the Missouri National Recreational River. The two lighter—colored eggs on the right side of the scrape are Tern eggs.

The characteristics of the vegetation and debris at both scrapes was not notably different from other Plover scrapes. Based upon the percent cover within a one meter diameter circle around the scrape, the first nest had no vegetation, with 15% debris cover of sticks, while the second site had 2.5% plant cover (a cattail stalk). The debris was dried bulrush stems with a 15% cover value. Of over 140 Piping Plover nests analyzed, 70% (99 of 142) had no vegetation present while 86% (128 of 149) had debris present of which 48 had 15% vegetative cover. Least Tern had values of 69% (80 of 116) with no vegetation and 72% (95 of 132) had debris scattered around the nest (Ducey unpublished data). These values indicate no obvious measured differences in these characteristics that would separate a Tern nest from a Plover nest.

One other feature measured, the cobble in the first scrape and bits of bark, small twigs, and dried bulrush stems in the second scrape were measurably different than the typical Tern nest. In both of these scrapes, a cover value of 37.5% was recorded. Of 133 Piping Plover nests analyzed, 105 or 79% had cobble or woody material present in varying amounts. Only 16 of 109 Least Tern nests or 15% had these fragments. And only 3 of the 16 had had a value of 37.5 percent. No Tern scrape contained fragments in an amount exceeding this value while Plover nests had values that ranged up to 100% cover of material lining the scrape. So when the Tern eggs were laid, the adult bird was placing them in a scrape with a lining not typical for the majority of Least Tern nests where this characteristic was evaluated.

Although the Piping Plover and Least Tern often breed in the same suitable habitat on portions of the Missouri. Platte, Niobrara, and Loup Rivers in Nebraska, with nests intermixed at breeding areas, these two nests were the first Piping Plover nests observed that contained eggs of the Tern. Of over 300 total nests for both species personally observed during 4 years of field research or numerous other nests observed by Game and Parks Commission personnel during their annual survey efforts, none have established a record of this occurrence previously.

The Cornell Nest Record Program also has not received any reports of similar observations (J. Collins and D.A. McCrimmon, personal communication, July 1983). Also, no mention of this occurrence can be located in the ornithological literature.

It would appear that these records are the first known observation of Least Tern eggs being found in nests of the Piping Plover.

December 1984. Least tern eggs in nests of the piping plover. Nebraska Bird Review 52(4): 72-73.