Northern Prairie Wildlife Research Center
Nesting chronology of piping plovers on alkali lakes is summarized in Appendix A. A 7-day laying period and 28-day incubation period (35 days total from initiation to hatching) (Wilcox 1959, Cairns 1982, Prindiville Gaines and Ryan 1988) should be considered standards for projecting initiation or fledging dates or for calculating nest survival rates (Mayfield 1975, Johnson 1979). When scheduling monitoring visits, estimated dates of hatching or fledging based on observation of nests during laying (i.e., partial clutches) should be considered to be accurate within ±2 days, because laying and incubation periods vary among individual pairs and can be influenced by general weather trends.
A 25-day standard has been used on Great Plains river systems (Missouri River, Lake Diefenbaker) (Pavelka and Kruse, Goossen et al., this proceedings). However, plover chicks occasionally are observed taking short flights at earlier ages (to 20-22 d) in some of these habitats. Young observed at age 22-24 days but missing when next checked after 25 days of age are considered fledged if no contradictory evidence exists (G. Pavelka and C. Kruse, unpubl rep).
On Great Plains alkali lakes young plovers fledge at 21-28 days (Whyte 1985, Prindiville Gaines and Ryan 1988). As they near this age, some tend to wander far (≥400-600 m) from their nests and become difficult to find (Whyte 1985; B. Root and R. Murphy, pers obs).
Not only are some ±3-wk-old juveniles likely overlooked because they wander or fledge earlier than 25 days, but it generally is impractical for investigators to visit alkali lake sites frequently enough to count young just as they are fledging and they may disperse soon after. Juveniles marked with radio transmitter moved 30-45 km within 2-3 days of fledging in North Dakota (J. Knetter, unpubl data).
Thus, counts of fledged young, which are thought to represent recruitment of new individuals into the fall population, may underestimate numbers produced on alkali lakes if based on a 25-day standard. Cairns (1982) noted a similar problem on the north Atlantic Coast.
To avoid such bias, 16 days have been used to represent fledging age on Great Plains alkali lakes (Mayer and Ryan 1991), because no mortality occurred from this age through fledging in an early North Dakota study (Prindiville Gaines and Ryan 1988). More recent evidence of mortality after 16 days, albeit apparently minor (B. Root, unpubl data), has prompted some investigators to raise this fledging age criterion slightly, to 18-20 days. The authors recommend this range as the standard for estimating fledging rates of juvenile plovers on Great Plains alkali lakes as a compromise between what likely are biased (high and low) measures.
Estimates of reproductive success for some similarly problematic groups of bird species are based on such compromises: 80% of the age of first flight is a criterion used for raptor productivity (Steenhof and Kochert 1982).
When comparing or pooling data within and among regional populations (Ryan et al. 1993), knowledge of chick survival and movements during the 16-25-day pre-fledging/fledging period may help correct for differences in fledging age criteria.
For example, the Atlantic Coast standard for calculating fledging rates is "25 days of age or flying (whichever comes first)" (USFWS 1996).
Until such survival data are available for alkali lakes, investigators should try to monitor young plovers through 25 days of age, in addition to routine reporting based on the 18-10-day criterion recommended here.
Initiation dates can be determined for nests found with incomplete clutches by backdating. Four eggs nearly always represent a complete clutch for all but late nests (those initiated in June), which may include only three eggs. Egg laying is assumed to occur on days before nest discovery (e.g., if one egg is present when discovered on 5 June, the initiation date is 4 June).
Often a clutch is already completed when a nest is discovered. If such nests are successful, their chronology might be estimated from observation of recently hatched chicks. Experienced investigators can visually estimate age of chicks within 3-4 days based on behavior, size relative to adults, and plumage development. A photographic guide is available as an aid (Kruse, in press).* Hatching date can then be estimated by backdating.
Nests discovered with complete clutches may be destroyed before hatching occurs, particularly if not protected with predator exclosure fences (Melvin et al. 1992). Initiation dates cannot be assigned to such nests unless they are known to be incubated >25 d, in which cases initiation can be projected within ± 2 d.
Incubation stage, and thus initiation date may be estimated (± 3-4 days) by briefly floating eggs (Westerkov 1950, Hays and Lecroy 1971) without affecting hatchability (Alberico 1995). In the U.S., handling of viable eggs normally is not allowed under federal and, in some cases, state permits (see Legal Concerns).
Eggs within 3-5 days of hatching also appear subtly bi-colored, with the "cap" more lightly colored than the rest of the eggshell; within 2 days of hatching, initial pipping is evident as faint "starring" on the eggshell (B. Root, pers obs).
* Available from Casey D. Kruse at COE-Omaha District, Box 710, Yankton, SD 57078