Prepared in cooperation with the U.S. Army Corps of Engineers

Demographic Responses of Least Terns 

and Piping Plovers to the 2011 Missouri 

River Flood—A Large-Scale Case Study

Open-File Report 2018–1176

U.S. Department of the Interior

U.S. Geological Survey
Cover. Background: Sandbar nesting habitat on the Garrison Reach of the Missouri River. 

Right-side photographs from top to bottom: 

Banded adult piping plover. 

Least tern adult and chick at a nest site. 

Banded piping plover chicks in a nest bowl. 

Banded piping plovers on a Missouri River sandbar. 

Photographs by U.S. Geological Survey.
Demographic Responses of Least Terns and 

Piping Plovers to the 2011 Missouri River 

Flood—A Large-Scale Case Study

By Michael J. Anteau, Mark H. Sherfy, Terry L. Shaffer, Rose J. Swift, 

Dustin L. Toy, and Colin M. Dovichin

Prepared in cooperation with the U.S. Army Corps of Engineers

Open-File Report 2018–1176

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DAVID BERNHARDT, Acting Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2019

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Suggested citation:

Anteau, M.J., Sherfy, M.H., Shaffer, T.L., Swift, R.J., Toy, D.L., and Dovichin, C.M., 2019, Demographic responses of 

least terns and piping plovers to the 2011 Missouri River flood—A large-scale case study: U.S. Geological Survey 

Open-File Report 2018–1176, 33 p., https://doi.org/10.3133/ofr20181176.

ISSN 0196-1497 (print) 

ISSN 2331-1258 (online)

ISBN 978-1-4113-4282-8
iii

Acknowledgments

This study was funded by the Integrated Science Program, Omaha District, U.S. Army Corps of 

Engineers, and the U.S. Geological Survey. We thank Casey Kruse, formerly of the Integrated 

Science Program, for posing the initial science questions and inviting Northern Prairie Wildlife 

Research Center to develop a least tern and piping plover research program on the Missouri 

River. We also thank Coral Huber, Greg Pavelka, Mike Delvaux, and Joe Bonneau for sustaining our long-term research collaboration with the Corps and for recognizing and supporting the 

scientific opportunity to evaluate flood responses of terns and plovers. We appreciate support 

and collaboration from other agencies and research entities, including the U.S. Fish and Wildlife 

Service, North Dakota Game and Fish Department, the Nature Conservancy, the Department of 

Biology at the University of North Dakota, and the Department of Fish and Wildlife Conservation 

at Virginia Tech.

The research team at Northern Prairie Wildlife Research Center contributed to this work in many 

ways that are impossible to recognize individually. Authorship of this report reflects those who 

contributed directly to producing the document, but does not identify all the individuals who 

participated in the larger effort to collect and analyze pre- and post-flood data. We especially 

appreciate the contributions of Erin Roche, Larry Strong, Jennifer Stucker, and Mark Wiltermuth 

in formulating the ideas and analyses presented here, and Max Post van der Burg in assisting 

with data analysis and generation of graphics. The hard work and dedication of over 100 seasonal field technicians who collected and archived the data is especially noteworthy. Jonathan 

Cohen and Conor McGowan provided detailed technical reviews of earlier drafts of this document.

v

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Objectives........................................................................................................................................................2

Study Areas.....................................................................................................................................................3

Garrison Reach......................................................................................................................................4

Lake Sakakawea...................................................................................................................................4

Sampling..........................................................................................................................................................4

Field Methods.................................................................................................................................................5

Nests.......................................................................................................................................................5

Chicks......................................................................................................................................................5

Adults......................................................................................................................................................6

Data Analysis..................................................................................................................................................6

A Priori Predictions...............................................................................................................................6

Nest Survival..........................................................................................................................................7

Chick Survival........................................................................................................................................8

Adult Survival.......................................................................................................................................10

Breeding Population Size ..................................................................................................................10

Results ...........................................................................................................................................................10

Nest Survival........................................................................................................................................10

Chick Survival......................................................................................................................................18

Adult Survival.......................................................................................................................................22

Breeding Population Size ..................................................................................................................23

Mark and Recapture of Least Terns ................................................................................................23

Discussion.....................................................................................................................................................26

1. Estimate Survival to Hatch of Least Tern and Piping Plover Nests and 

Compare to Pre-Flood Estimates ........................................................................................26

2. Estimate Survival to Fledging Age of Least Tern and Piping Plover Chicks 

and Compare to Pre-Flood Estimates ................................................................................27

3. Estimate Annual Adult Survival of Piping Plovers and Compare to 

Pre-Flood Estimates..............................................................................................................28

4. Estimate Annual Breeding Population of Least Terns and Piping Plovers 

and Compare to Pre-Flood Estimates ................................................................................29

5. Evaluate Feasibility of Mark-Resight Techniques to Estimate Site Fidelity 

and Annual Survival of Adult Least Terns .........................................................................31

References Cited..........................................................................................................................................31
vi

Figures

1. Map showing flood science study area during 2006–9 and 2012–14...................................3

2. Graphs showing minimum breeding population (MINBPOP) regressions used to 

extrapolate MINBPOP/breeding population (BPOP) values for estimating BPOP 

for least terns and piping plovers nesting on Lake Sakakawea and the 

Garrison Reach during 2012–14................................................................................................11

3. Graph showing cumulative estimates of piping plover nest survival (35 days) 

along the Garrison Reach by year and for the pre-flood and post-flood periods............14

4. Graph showing cumulative estimates of piping plover nest survival (35 days) 

on Lake Sakakawea by year and for the pre-flood and post-flood periods......................14

5. Graph showing daily survival estimates for piping plover nests along the 

Garrison Reach by nest age and year.....................................................................................15

6. Graph showing cumulative estimates of piping plover nest survival (35 days) 

along the Garrison Reach by nest initiation date and year .................................................15

7. Graph showing daily survival estimates for piping plover nests on 

Lake Sakakawea by nest age and year ..................................................................................16

8. Graph showing cumulative estimates of piping plover nest survival (35 days) 

on Lake Sakakawea by nest initiation date and year...........................................................16

9. Graph showing cumulative estimates of least tern nest survival (21 days) 

along the Garrison Reach by year and for the pre-flood and post-flood periods............17

10. Graph showing daily survival estimates for least tern nests along the 

Garrison Reach by nest age and year.....................................................................................18

11. Graph showing cumulative estimates of piping plover chick survival to 

fledging (25 days) along the Garrison Reach by year and for the pre-flood 

and post-flood periods...............................................................................................................20

12. Graph showing cumulative estimates of piping plover chick survival to 

fledging (25 days) on Lake Sakakawea by year and for the pre-flood and 

post-flood periods.......................................................................................................................21

13. Graph showing cumulative estimates of least tern chick survival to 

fledging (18 days) along the Garrison Reach by year and for the pre-flood 

and post-flood periods...............................................................................................................22

14. Graph showing estimates of piping plover adult annual apparent survival 

by study area and for both study areas ..................................................................................24

Tables

1. Number of sampling units and sample sizes by strata, and study areas used 

for evaluating the effects of the 2011 Missouri River flood on least terns and 

piping plovers in North Dakota...................................................................................................4

2. Explanation of variables used when building Shaffer logistic-exposure models 

to estimate daily nest survival ....................................................................................................6

3. Means and standard deviations for the “sdate” (that is, date of nest visit) 

and “sage” (that is, age of nest at visit) covariates used in nest survival 

analyses .........................................................................................................................................8
vii

4. Explanation of variables used when building Cormack-Jolly-Seber models 

to estimate daily chick survival ..................................................................................................9

5. Means and standard deviations for the “date” (that is, date a chick was 

observed) and “age” (that is, age of a chick when observed) covariates 

used in Cormack-Jolly-Seber models to estimate chick survival ........................................9

6. Sample of piping plover and least tern nests found and followed for the 

purpose of nest survival analysis by U.S. Geological Survey crews along 

the Garrison Reach and Lake Sakakawea .............................................................................12

7. Ranked set of logistic-exposure models used to estimate daily piping plover 

nest survival following the second stage of model selection: examination of 

the effect of the 2011 Missouri River flood.............................................................................13

8. Ranked set of logistic exposure models used to estimate daily least tern 

nest survival following the second stage of model selection: examination 

of the effect of the 2011 Missouri River flood ........................................................................17

9. Sample of piping plover and least tern chicks captured and banded by 

U.S. Geological Survey crews along the Garrison Reach and 

Lake Sakakawea.........................................................................................................................19

10. Daily detection probability estimates and associated 95-percent confidence 

intervals for uniquely marked pre-fledge (less than 25 days) and fledged 

(25+ days) piping plover chicks resighted/recaptured on the Garrison Reach 

and Lake Sakakawea for all years of the study.....................................................................19

11. Ranked set of Cormack-Jolly-Seber models used to estimate daily piping 

plover chick survival following the third stage of model selection: 

examination of the effect of the 2011 Missouri River flood .................................................21

12. Daily detection probability estimates and associated 95-percent confidence 

intervals for uniquely marked pre-fledge (less than 18 days) and fledged 

(18+ days) least tern chicks resighted/recaptured on the Garrison Reach 

for all years of the study ............................................................................................................22

13. Ranked set of Cormack-Jolly-Seber models used to estimate daily least 

tern chick survival to 30 days following the third stage of model selection: 

examination of the effect of the 2011 Missouri River flood .................................................23

14. Sample of piping plover and least tern adults used to estimate annual 

apparent survival ........................................................................................................................24

15. Estimates of minimum breeding population size and breeding population 

size for piping plovers and least terns nesting along the Garrison Reach 

before (2006–7) and after (2012–14) the 2011 Missouri River flood....................................25

16. Estimates of minimum breeding population size and breeding population 

size for piping plovers nesting along Lake Sakakawea before (2007–8) and 

after (2012–14) the 2011 Missouri River flood ........................................................................25

17. Number of individual least terns banded between 2012 and 2014 that were 

missing color bands or alphanumeric bands based on the number of years 

between recapture.....................................................................................................................26
viii

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square meter (m2) 0.0002471 acre 

hectare (ha) 2.471 acre

square hectometer (hm2) 2.471 acre

square kilometer (km2) 247.1 acre

square centimeter (cm2) 0.001076 square foot (ft2)

square meter (m2) 10.76 square foot (ft2) 

square centimeter (cm2) 0.1550 square inch (ft2) 

square hectometer (hm2) 0.003861 section (640 acres or 1 square mile)

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.
Demographic Responses of Least Terns and Piping Plovers 

to the 2011 Missouri River Flood—A Large-Scale 

Case Study

By Michael J. Anteau, Mark H. Sherfy, Terry L. Shaffer, Rose J. Swift, Dustin L. Toy, and Colin M. Dovichin

Abstract

A catastrophic flood event on the Missouri River system 

in 2011 led to substantial changes in abundance and distribution of unvegetated sand habitat. This river system is a major 

component of the breeding range for interior Least terns 

(Sternula antillarum; “terns”) and piping plovers (Charadrius melodus; “plovers”), both of which are Federally listed 

ground-nesting birds that prefer open, unvegetated sand and 

gravel nesting substrates on sandbars and shorelines. The 2011 

flood inundated essentially all tern and plover nesting habitat 

during 2011, but it had potential to generate post-flood habitat 

conditions that favored use by terns and plovers in subsequent 

years. We compared several tern and plover demographic 

parameters during the pre-flood and post-flood periods on 

the Garrison Reach and Lake Sakakawea, North Dakota, to 

determine how this event influenced these species (both species on the Garrison Reach, and plovers only on Lake Sakakawea). The principal parameters we measured (nest survival, 

chick survival, and breeding population) showed spatial and 

temporal variation typical of opportunistic species occupying 

highly variable habitats. There was little evidence that nest 

survival of least terns differed between pre- and post-flood. 

During 2012 when habitat was most abundant on the Garrison 

Reach and Lake Sakakawea, piping plover nest survival was 

higher than in any other year in the study, but returned to rates 

comparable to pre-flood years in 2013. Chick survival for 

terns on the Garrison Reach and plovers on Lake Sakakawea 

showed a similar pattern to plover nest survival, with the 2012 

rate exceeding all other years of the study, and the remaining pre-flood and post-flood years being generally similar but 

slightly higher in post-flood years. However, plover chick 

survival on the Garrison Reach in 2012 was similar to preflood years, and increased annually thereafter to its highest 

rate in 2014. Although wide confidence intervals precluded 

firm conclusions about flood effects on breeding populations, 

the general pattern suggested lower populations of plovers but 

higher populations of least terns immediately after the flood. 

Despite near total absence of breeding habitat on either study 

area during the flood of 2011, populations of both species persisted after the flood due to their propensity to disperse and/or 

forgo breeding for at least a year. Tern and plover populations 

have similarly persisted and recovered after the flood, but their 

mechanisms for persistence likely differ. Data on tern dispersal 

is generally lacking, but they are thought to show little fidelity 

to their natal grounds, have a propensity to disperse potentially 

long distances, and routinely forgo breeding until their second 

year, thus a lost opportunity to breed in a given area may be 

easily overcome. Plovers exhibit stronger demographic ties 

to the general area in which they previously nested, yet they 

occupy much broader nesting habitat features than terns and 

exploit three major landforms in the Dakotas (free-flowing 

rivers, reservoir shorelines, and wetland shorelines). Consequently, dispersal to and from non-Missouri River habitats and 

potential to exploit non-traditional habitats likely sustained 

the Northern Great Plains population through the flood event. 

Terns and plovers normally occupy similar habitats on the 

Missouri River and both species experienced similar loss of a 

breeding season due to the flood. Persistence of these populations after the flood underscores the importance of understanding their unique demographic characteristics and the context 

within which the Missouri River operates. 

Introduction

Least terns (Sternula antillarum) and piping plovers 

(Charadrius melodus) nest in spatially and temporally variable habitats in the North American midcontinent. It has been 

proposed that periodically high reproductive rates, long life 

spans, adaptation to the use of variably available nesting 

habitat, and high dispersal capabilities of these species make 

them especially well-suited to colonizing newly established 

habitats (Catlin and others, 2010; Anteau and others, 2014a,b). 

The Federal listing status of interior least terns (endangered; 

hereafter referred to as “least terns” or “terns”) and northern 

Great Plains piping plovers (threatened; hereafter referred to 

as “plovers”) has motivated substantial work to understand 

ecological relationships between the birds and their nesting 

habitats. The Missouri River system supports nesting populations and is an important component of recovery efforts for 

both species. Rapid colonization of novel habitats, in the form 
2 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

of constructed sandbars or dynamic reservoir shorelines, has 

been documented through directed research on the Missouri 

River (Catlin, 2009; Sherfy and others, 2012a; Anteau and 

others, 2014a). Pulses of productivity lasting several years 

have occurred after colonization events, facilitating persistence of populations at larger regional scales (Catlin, 2009; 

Cohen and others, 2009). 

In a naturally functioning river system, sediment is 

eroded, transported, and deposited by seasonally variable 

flows, creating and maintaining emergent sandbars. Spring 

pulses are particularly important ecological events because 

they can transport substantial sediment loads and can scour 

vegetation from previously created sandbars. Construction and 

operation of dams on the Missouri River has attenuated peak 

spring flows, resulting in declines in abundance and quality of 

unvegetated sandbar habitats favored by nesting terns and plovers. The Missouri River flood of 2011 (hereafter also referred 

to as “the flood”) was a historically and ecologically significant event in which spring and summer flows far exceeded all 

historical records for the post-dam era; all main-stem dams 

were releasing flood flows simultaneously and reservoirs were 

at their highest recorded elevations. The magnitude of this 

event surpassed the 1997 flood, which was previously considered the modern-era benchmark for unprecedented high spring 

flows (Dixon and others, 2011). Consequently, some natural 

hydrologic processes were mimicked during the flood in a way 

that is unexpected on a regulated river, and the resulting flow 

induced substantial habitat alteration and markedly increased 

sandbar habitat available for use by terns and plovers. 

Outcomes of major flood events are visually striking in 

flowing river systems, where marked increases in flow and 

stage can have catastrophic effects on people, infrastructure, 

and ecological communities. Dams on a regulated river system 

control downstream flow and stage as well as upstream water 

level. Thus, catastrophic events such as the Missouri River 

flood of 2011 can also have profound effects on the impounded river reaches, particularly when the event’s magnitude exceeds historical norms for water level. Impounded and flowing 

segments of regulated rivers are often viewed as discrete and 

independent units, although they may share some biological 

attributes for which flood effects should be evaluated simultaneously. Nesting terns and plovers are two such attributes, and 

this study seeks to provide context for flood effects on terns 

and plovers across the continuum of Missouri River nesting 

habitats (riverine sandbars and reservoir shorelines).

Prior to the flood, several studies generated estimates of 

tern and plover demographic parameters when the birds occupied either engineered or degraded riverine habitats, periodically at densities sufficiently high enough to impair productivity (Catlin, 2009; Sherfy and others, 2012b; Shaffer and 

others, 2013; Anteau and others, 2014c). Habitat distribution 

and abundance on the post-flood river more closely represent 

natural conditions than any other time in the post-dam era. 

The availability of nesting and foraging habitat is the principal factor affecting productivity and abundance of terns and 

plovers (Anteau and others, 2014a,c), and managing habitat 

is the principal tool available to managers for recovery of tern 

and plover populations (Sherfy and others, 2008). Consequently, understanding how the birds responded numerically 

and demographically to this major habitat alteration is critical 

to future river management. The immediate post-flood years 

were also an unprecedented opportunity to document demography and vital rates of the birds in nearly natural nesting 

habitat conditions, providing previously unavailable context 

for understanding their responses to a spatially and temporally variable system. Directed studies also have examined 

causative mechanisms driving demographics, such as food 

limitation and predation, on natural and constructed sandbars 

on the pre-flood river, but it is not known whether these same 

mechanisms remain important on the post-flood river. Also, 

several studies developed predictive and explanatory models 

for plover habitat, distribution, and nest and chick survival 

in response to reservoir water-level fluctuations (Anteau and 

others, 2012a,b; 2014a,b,c). However, those studies followed 

a long successive drawdown when lake levels were low. It is 

unclear if those models would be appropriate following reservoir refilling and when the reservoir is at levels above that 

observed in previous studies.

We also sought to evaluate the viability of some new 

research techniques. Substantial effort has gone into marking 

and resighting adult piping plovers on the Missouri River, resulting in enhanced knowledge of survival rates and fidelity to 

breeding and wintering areas (Catlin, 2009; Roche and others, 

2010). Although substantial effort also has gone into banding 

adult least terns to address local questions about habitat use 

and movements (Sherfy and others, 2012b; Toy and others, 

2017), there has been comparatively little effort into resighting 

adult terns. Knowledge of annual survival rates and fidelity 

is equally important for management of terns as for plovers, 

but there is uncertainty about whether reasonable resighting 

rates of marked adult terns can be readily obtained (Toy and 

others, 2017).

Objectives

The primary goal was to document demographic responses of terns and plovers to the habitat alterations induced by the 

2011 Missouri River flood by comparing vital rates measured 

in pre-flood and post-flood systems for the Garrison Reach and 

Lake Sakakawea, including the following:

1. Estimate survival to hatch of least tern and piping plover 

nests and compare to pre-flood estimates.

2. Estimate survival to fledging age of least tern and piping 

plover chicks and compare to pre-flood estimates.

3. Estimate annual adult survival of piping plovers and 

compare to pre-flood estimates.

4. Estimate annual breeding population of least terns and 

piping plovers and compare to pre-flood estimates.
Study Areas 3

The secondary goal was to support future research and 

decision making for implementation of management actions 

by providing preliminary information on bird responses to 

management actions under the river’s new conditions, which 

includes the following:

5. Evaluate feasibility of mark-resight techniques to estimate site fidelity and annual survival of adult least terns.

Study Areas

We selected two study areas for field work during 

2012–14—one river reach (Garrison Reach) and one reservoir (Lake Sakakawea) (fig. 1). When available, we compiled 

previously collected data for the same study areas during 

pre-flood years (2006–9). Both study areas have been extensively described previously (Sherfy and others, 2008; Shaffer 

and others, 2013), and both were affected markedly by the 

Missouri River flood of 2011. The flood event had a temporally discrete effect on riverine and reservoir habitat, although 

effects on habitat could vary over a series of years. In both 

areas, the immediate effect of a catastrophic flood is that 

nesting habitat is eliminated due to inundation. Subsequent 

decreases in water level would expose new bare-substrate 

habitat that could be exploited by plovers and terns. In the case 

of reservoirs, this would consist of shoreline where vegetation 

had been removed by inundation (Anteau and others, 2012a; 

Anteau and others, 2014b), and on riverine habitat, it would 

consist of newly deposited sandbars or existing sandbars that 

had been scoured of vegetation (Catlin and others, 2010). In 

both cases, the flood-created habitat could potentially be available to nesting birds for several years, although degradation 

processes such as subsequent inundation, vegetation regrowth, 

and erosion would vary spatially.

Figure 1. Flood science study area during 2006–9 and 2012–14.

^

MONTANA

WYOMING

Bismarck

108° 105° 102° 99° 96°

48°

46°

44°

42°

NORTH DAKOTA 

Lake

Sakakawea

SOUTH DAKOTA

MINNESOTA

NEBRASKA

CANADA

IOWA

Garrison Reach

Lake Oahe

GARRISON DAM

White Tail Bay
4 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Garrison Reach

This study area was defined as the 86 river miles (RM) 

of sandbar and shoreline habitat extending from the Garrison 

Dam, North Dakota (RM 1390) to the headwaters of Lake 

Oahe, N. Dak. (RM 1304) (fig. 1). U.S. Geological Survey 

(USGS) crews had done field work on plovers and terns previously on this river reach in 2006 and 2007 (Shaffer and others, 

2013). This study area is used for nesting by plovers and terns.

Lake Sakakawea

This study area was defined as the 178 RM of reservoir 

habitat extending northwest from the Garrison Dam, N. Dak., 

to White Tail Bay, N. Dak. (fig. 1). USGS crews had done field 

work on plovers and terns previously in this study area during 2006–9 (Anteau and others, 2012a,b; Shaffer and others, 

2013). This study area is predominantly used for nesting by 

plovers with only occasional use by terns.

Sampling

A complete census of terns and plovers on all available nesting habitat in both study areas was impractical. We 

employed a probability-based sampling effort on GRR and 

SAK during 2012–14 to obtain unbiased population and demographic estimates while remaining consistent with pre-flood 

sampling methodology. Under a probability-based sampling 

effort, data are collected on a sample of units that represents a 

larger whole. In this report, these sampling units were spatially defined and included both study areas without overlap 

(Cochran, 1977; Shaffer and others, 2013).

Missouri River shoreline and sandbar habitats are highly 

dynamic, varying extensively in distribution and abundance 

within and among years. Because of their ephemeral nature, 

defining individual shoreline and sandbar habitats as “sampling units” is inappropriate (Sherfy and others, 2008). 

Beginning in 2006, we addressed this challenge by dividing 

each study area into sampling units that are spatially defined 

and invariant (Shaffer and others, 2013). Unit lengths differed by study area because of differences in habitat structure 

and bird density; the intent was that a crew would be able to 

completely search two units of habitat for nests in a single day. 

We divided the river reach (GRR) into 4-RM sampling units 

(N = 22) defined by the river bank as the outer edge during 

2006–7. For 2012–14, we divided GRR into 3-RM sampling 

units (N = 28; table 1). We divided the reservoir (SAK) shoreline into sampling units (N = 545) of approximately 2 kilometers (km) using an intermediate lake level at an elevation 

of 554 meters (m) (more information in Shaffer and others, 

2013); our available population of Lake Sakakawea sampling 

units varied annually due to water-level fluctuations inundating island units and refinement of the definition of suitable 

habitat (table 1; Anteau and others, 2014a). 

We further refined our probability-based sampling effort 

by classifying our sampling units into one of several strata 

(that is, groups sharing similar properties). If the differences between the stratified groups are large, then dividing a 

sample into strata and sampling within a stratum can increase 

the overall precision of estimates (Shaffer and others, 2013; 

Table 1. Number of sampling units and sample sizes by strata, and study areas used for evaluating the effects of the 2011 Missouri 

River flood on least terns and piping plovers in North Dakota.

[N, number of sampling units; n, sample sizes; GRR, Garrison Reach; SAK, Lake Sakakawea; --, no data]

Study area Year

Low strata Medium strata High strata Total

N n N n N n N n

GRR 2006 6 3 5 4 10 10 21 17

2007 6 3 5 4 10 10 21 17

2012 9 5 11 6 8 3 28 14

2013 13 4 8 7 8 5 26 16

2014 13 5 8 8 5 3 26 16

SAK 2006 403 7 88 5 53 5 544 17

2007 403 5 88 7 53 18 544 30

2008 403 5 88 7 53 18 544 30

2009 403 24 88 6 53 5 544 35

2012 150 13 79 12 23 15 254 40

2013 150 6 79 16 20 11 254 33

2014 92 13 80 18 20 9 192 40
Field Methods 5

Cochran, 1977). During pre-flood years (2006–9), we classified our sampling units into one of three strata based on the 

number of plover and tern nests found in each unit according 

to the U.S. Army Corps of Engineers tern and plover monitoring data (Shaffer and others, 2013). Beginning in 2012, we 

used a USGS-created predictive habitat model informed by 

water levels, vegetation colonization rates, and topography 

similar to that of Anteau and others (2014a,b) to stratify our 

sampling units. For all years, we excluded RM 1388–1391 

from our GRR sample because these RMs had no history of 

emergent sandbar habitat; this brought our GRR study area 

to a total of N = 21 4-RM sampling units during 2006–7 and 

N = 28 3-RM sampling units during 2012. During 2013–14 

we excluded an additional two 3-RM sampling units on 

GRR because they did not contain suitable breeding habitat, 

bringing our total to N = 26. Similarly, not all of the original 

545 sampling units on SAK were considered viable nesting habitat for plovers or terns. During the course of our 

work on this reservoir we were able to further refine the total 

sampling units through proximity to bluffs (that is, 25-m rise 

within 250 m of shoreline; Anteau and others, 2014a), an 

opportunistic habitat survey of more than 100 sampling units 

during 2013, historical nest-monitoring data, and USGS nesting data (table 1). We used Neyman allocation to optimally 

distribute our annual sampling effort among the three strata 

(Cochran, 1977).

Field Methods

During all years we intensively (2–3 surveys per sample 

unit per week) surveyed for plover and tern nests and marked 

chicks along a subset of sampling units in both study areas. 

Beginning in 2012, we expanded our surveys to include 

marked adults of both species. We recorded observations 

of uniquely marked adults when visiting intensively surveyed sampling units (hereafter referred to as “on-segment” 

sampling units) as well as when visiting sampling units not 

included in our sample of intensively surveyed units (hereafter 

referred to as “off-segment” sampling units). Although we collected data on least terns on Lake Sakakawea throughout the 

study and include these data in this report, our sample sizes 

were not sufficient for statistical analysis.

Nests

We defined a nest as a scrape or depression containing at 

least one egg. Crews of 2–4 individuals searched for nests on 

all available nesting habitat at 2–3-day return intervals from 

mid-April (the arrival of piping plovers to the study areas) 

through mid-August (the end of least tern nest initiation). 

Crews searched for nests by walking grid patterns through 

available nesting habitat (both GRR and SAK) or using behavioral cues of adults (predominantly on SAK). During all subsequent visits to a sample unit, crews revisited all previously 

found nests to record status and searched all available habitat 

including any new habitat that had become available. Crews 

only searched during suitable weather conditions, as identified 

in our U.S. Fish and Wildlife Service Threatened and Endangered Species Permit.

When a nest was found, we estimated the incubation 

stage by floating one or two eggs (Mabee and others, 2006). 

We used incubation stage to estimate nest initiation date by 

backdating from the discovery date and, excluding the first egg 

laid, assuming one egg was laid per day for terns and every 

other day for plovers. We estimated hatch dates by assuming a 

25-day incubation period for plovers and a 20-day incubation 

period for terns (Shaffer and others, 2013). Nests were visited 

more frequently near the estimated hatch date to decrease the 

uncertainty associated with nest fating and increase the probability of banding fully hatched chicks in the bowl (Grant and 

others, 2005; Shaffer and others, 2013). 

Nests were assigned one of four fates (successful, probable successful, failed, unknown) based on evidence in and 

around the nest bowl (Mabee, 1997). A nest was only fated 

as “successful” if at least one live chick was observed in the 

nest bowl. If other evidence of hatching (Page and others, 

1985; Dirks, 1990) was found at the nest bowl (for example, 

eggshells, pipping fragments, chick droppings, chick tracks, or 

chicks near the nest bowl) we fated the nest as “probable successful.” Nests were classified as “failed” if eggs were found 

destroyed or missing but could not have hatched based on the 

estimated incubation stage at last visit. If circumstances were 

unclear, we fated the nest as “unknown.” We recorded the date 

that evidence for the nest fate was collected and considered 

this the “nest fating date.” For our later analyses, we combined 

“successful” and “probable successful” into one category; 

nests of “unknown” fate were excluded from analyses. 

Chicks

The USGS crews visited nests daily near the estimated 

hatch date to band chicks in the nest bowl. Crews captured 

chicks by hand or using butterfly nets. Because of permitting 

restrictions, crews were not always able to band all chicks in a 

brood, and in these situations we adopted a sampling strategy 

in which 3 chicks were banded in broods of 4–2 chicks from 

broods of 3, and 1 chick from a brood of 2 chicks. If there was 

only one chick present, we banded that chick. 

Chicks of both species were marked using a metal band 

and a combination consisting of (1) several individual color 

bands (plovers: 2006–9, 2012; terns: 2006–7, 2012), (2) a 

single yellow alphanumeric band (terns: 2013–14), or (3) a 

yellow alphanumeric flag (plovers: 2013–14). Piping plovers 

were banded with size 1A aluminum metal bands placed 

above the tibiotarsal joint and combinations of as many as four 

plastic color bands (below the tibiotarsal joint) and a plastic 

flag or alphanumeric flag (above the tibiotarsal joint opposite 

from the metal band). Least tern chicks were banded with a 

size 1A stainless steel band placed below the tibiotarsal joint 
6 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

along with as many as three color bands (two placed below 

the tibiotarsal joint and one placed above) or a single alphanumeric band (below the tibiotarsal joint opposite from the 

metal band).

Following banding, crews visited sites with marked 

chicks at 2–3-day intervals and resighted uniquely marked 

individuals by reading band combinations through spottingscopes, via digital cameras, or by recapturing chicks by 

hand and reading their bands. Crews continued to visit sites 

at 2–3-day intervals until there were no additional plovers 

or terns available to be resighted or until the end of the 

field season.

Adults

The banding of adult terns and plovers was predominantly concentrated to “on-segment” sampling units where 

nests and chicks were already being followed at 2–3-day 

intervals; however, because adults can easily move among 

miles of river and reservoir shoreline habitat, adults were 

often banded when nests were found at nearby “off-segment” 

locations. We captured adults using a combination of remotely 

operated bow-net traps (used mostly to capture adult least 

terns) and remotely operated walk-in traps (used exclusively to 

capture adult piping plovers). We only trapped under permitted weather conditions and after at least 2 days of incubation. 

During trapping events, live eggs were replaced with artificial 

eggs to minimize the potential for egg damage.

Adult piping plovers were banded with size 1A aluminum metal bands placed above the tibiotarsal joint and 

combinations of as many as four plastic color bands (below 

the tibiotarsal joint) and a plastic flag or alphanumeric flag 

(above the tibiotarsal joint opposite from the metal band). 

Adult least terns were banded with size 1A stainless steel 

bands placed below the tibiotarsal joint along with as many 

as three color bands (below the tibiotarsal joint) or a single 

alphanumeric band (below the tibiotarsal joint opposite from 

the metal band).

When visiting a site, the crews recorded the band combinations of all birds observed, the degree of completion with 

which they read the band combination, and the method used 

to resight the individual. If no adults were observed during a 

site visit, crews simply recorded the date, time, location, and 

purpose of the site visit.

Data Analysis

A Priori Predictions

Prior to analyses to estimate nest and chick survival in 

the periods before (2006–9) and after the flood (2012–14), we 

compiled a set of predictions as to how survival could have 

responded to the flood. For each prediction we describe possible effects of the flood and how this effect would be modeled. We then built and evaluated models that reflected these 

predictions; we present examples of what these models would 

look like in the context of a nest survival analysis. We used 

general terminology so that predictions could be applied to 

nest survival to hatch and chick survival to fledge. A description of each variable used when building nest survival models 

is shown in table 2. Data analyzed for this study are available 

as a USGS data release (Sherfy and others, 2018). 

Table 2. Explanation of variables used when building Shaffer logistic-exposure models to estimate 

daily nest survival.

[GRR, Garrison Reach; SAK, Lake Sakakawea]

Variable Description

constant Single-level, categorical; daily survival is constant across date and age.

firstyear Single-level, categorical; first year following the 2011 flood (2012) 

different than other years.

floodtrend Continuous; years following the flood follow a log-linear trend.

otheryear Single-level, categorical; second and third years following the 2011 flood (2013–14) 

different than other years.

postflood Single-level, categorical; years following the 2011 flood (2012–14).

preflood Single-level, categorical; years prior to the 2011 flood (2006–09).

sage Continuous; age of the nest (days) at a nest visit.

sdate Continuous; date the nest was visited.

studyarea Two-level, categorical; location of the nest (GRR, SAK).

year Multilevel, categorical; year the nest was initiated (2006–09, 2012–14).
Data Analysis 7

Prediction: There is no detectable effect of the flood on survival estimates.

1. Survival varies by study area but not by year, for 

example S(studyarea)

Prediction: There is sufficient annual variation in survival 

estimates to mask an effect specific to the flood. 

2. Survival varies by year differently for each study area, 

for example S(studyarea*year)

3. Survival varies by year in the same way for each study 

area, for example S(studyarea + year)

Prediction: There is a constant effect of the flood on survival 

in all post-flood years.

4. Survival in the pre-flood years is different than survival 

in the post-flood years; the magnitude of the flood effect 

differs by study area, for example S(studyarea*preflood 

+ studyarea*postflood)

5. Survival in the pre-flood years is different than survival 

in the post-flood years; there is no difference in the effect 

of the flood by study area, for example S(preflood + 

postflood)

Prediction: There is an immediate effect of the flood on survival, but in the years following the flood the response follows 

a log-linear trend.

6. Survival in pre-flood years varies by study area and is 

different than survival in post-flood years (which also 

vary by study area); survival follows study area dependent log-linear trends in post-flood year, for example 

S(studyarea*preflood + studyarea*postflood*floodtrend)

7. Survival in pre-flood years is different than postflood years and survival follows a log-linear trend 

in post-flood year, for example, S(preflood + 

postflood*floodtrend)

Prediction: There is an effect of the flood on survival only in 

the first year following the flood (that is, survival during all 

other “post-flood” years resembles the pre-flood period).

8. Survival in the first year after the flood is different than 

all other years; survival varies by study area in the first 

year after the flood and following a separate pattern 

in all other years, for example S(studyarea*firstyear + 

studyarea*otheryear)

9. Survival in the first year after the flood is different than 

all other years, for example S(firstyear + otheryear)

Prediction: There is an effect of the flood on survival in all 

post-flood years, but the effect is most pronounced for the year 

immediately following the flood.

10. Survival in pre-flood years varies by study area, survival 

in the first year post-flood is different from pre-flood 

years and varies by study area, survival in all other 

post-flood years is also different than previous years and 

varies by study area, for example, S(studyarea*preflood 

+ studyarea*firstyear + studyarea*otheryear)

11. Survival in pre-flood years is different than in the first 

year post-flood and survival in all other post-flood years 

is different than in all previous years, for example, 

S(preflood + firstyear + otheryear) 

Nest Survival

We built logistic-exposure models in program R (Shaffer, 

2004; R Core Team, 2014) to estimate the daily probability of 

survival (S) for plover and tern nests on GRR and SAK. We 

analyzed nest-visit data for each species separately and followed a three-step model-selection approach. 

Prior to investigating our predictions of how nest survival 

may have changed following the flood, we sought to account 

for variation in nest survival due to nest age, seasonal date, 

and study area (see table 3 for the means and standard deviations [SDs] for these estimates). For the first model-selection 

step, we built models that estimated survival in relation to 

study area only (note that nest survival for LETE was only 

analyzed for a single study area: GRR), nest age at visit 

(“sage”) only, nest visit date (“sdate”) only, both sage and

sdate, and all combinations of sage and sdate by study area. 

A description of variables used in nest survival analysis is 

shown in table 3. We ranked models using Akaike’s information criterion corrected for small sample size (AICc) and 

selected the model with the lowest AICc for the next step of 

model selection.

Using the top-supported model from our first modelselection step, we next built models that reflected our 

11 a priori predictions describing how nest survival might 

have changed following the flood for plovers and terns. In 

this second model-selection step, we again built and ranked 

models separately for plovers and terns. 

We reasoned that annual changes in the effect of nest age 

and date on nest survival could underlie the response patterns 

of nest survival following the flood. Thus, for our third modelselection step, we built a series of post-hoc models investigating whether there was support for changes in annual patterns 

of both covariates. We constructed a similar set of models 

to the models built in the first step, but this time included an 

interaction with year as well as study area for both sage and 

sdate. We ranked models using AICc and selected the model 

with the lowest AICc for the next step of model selection.

We computed cumulative nest survival by taking the 

product of daily nest survival estimates out to 21 days for least 

tern nests and out to 35 days for piping plover nests. We estimated associated standard errors (SE) using the Delta method 

and package “msm” (Jackson, 2011). We used a log-odds 

transformation of the daily survival probability to compute 

associated 95-percent confidence intervals to ensure estimates 

were bounded between 0 and 1 (Armstrong and others, 2002). 
8 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Table 3. Means and standard deviations for the “sdate” (that 

is, date of nest visit) and “sage” (that is, age of nest at visit) 

covariates used in nest survival analyses. Also reported are the 

means and standard deviations for estimated initiation date and 

nest age at discovery for piping plover (PIPL) and least tern (LETE) 

nests on the Garrison Reach of the Missouri River (LETE: 2006–7, 

2012–14; PIPL: 2006–7, 2012–14) and Lake Sakakawea (LETE: 

2007–09, 2012–14; PIPL: 2006–9, 2012–14).

[sdate, date of nest visit; sage, age of nest at visit]

Species Variable Mean

Standard 

deviation

(days)

Piping plover sdate June 14 17.2 

initiation date May 30 15.1 

sage 16 days 9.0 

nest age at discovery 5.5 days 5.0 

Least tern sdate June 26 12.7 

initiation date June 16 11.4 

sage 11 days 6.0 

nest age at discovery 3.6 days 3.8 

Chick Survival

We built Cormack-Jolly-Seber (CJS) models using package “RMark” in program R and MARK (Laake, 2013; White 

and Burnham, 1999) to estimate the daily probability of apparent survival (φ) and detection (p) for tern and plover chicks 

marked and followed on GRR and SAK. The probability of 

true survival is generally confounded with the probability 

of emigration from a site when estimating apparent survival 

using CJS. However, for unfledged chicks, the probability of 

emigration from a site is essentially 0, and thus we can interpret all apparent survival estimates made prior to fledging age 

as true survival estimates. 

We analyzed tern and plover mark-recapture data separately following a four-step model-selection approach. We 

used program RELEASE to calculate estimates of over-dispersion (ĉ) separately for the tern and plover capture histories.

We created age-structured capture histories for both 

species that consisted of 30 occasions (that is, hatch day–age 

29 days). If a chick was observed by USGS crews, this occasion was coded as “1.” If USGS crews visited the site at which 

a chick had been banded prior to an age at which we would 

have considered that chick fledged (terns: 18 days, plovers: 

25 days) and the chick was not observed, then this occasion 

was coded as “0” (for example, if crews visited the site where 

tern chick A was banded on occasion 17 and the chick was not 

seen, the occasion was coded as “0”). After chicks fledge (that 

is, can fly) they are no longer restricted to the site at which 

they were banded and could reasonably be seen anywhere on 

a several RM stretch. Thus, if USGS crews visited any site in 

the study area after an age at which a chick was considered 

fledged and the chick was not observed, this occasion was also 

coded as “0” (for example, if crews were on GRR on occasion 

25 for tern chick A and the chick was not seen, the occasion 

was coded as a “0”). If USGS crews did not visit a site of an 

unfledged chick, this occasion was coded as “.” and censored 

from the analysis (for example, if crews did not visit the site at 

which tern chick A was banded on occasion 10, then the chick 

was not seen because nobody was there to see it and the occasion was coded as “.”). 

We were primarily interested in estimating daily apparent 

survival probabilities for both species. Thus, our first modelselection step focused on parameterizing our model of detection probability (Lebreton and others, 1992). In all models of 

detection, we accounted for differences in detection based on 

whether or not a chick was older than the fledge age for the 

species in question (that is, “postF” compared to “d2toF”). 

We built models in which detection varied by all combinations 

of chick age (“age”), the year of the study (“year”), and the 

study area (“studyarea”). In all cases, we modeled studyarea, 

year, and age effects separately for pre-fledge and post-fledge 

stages. We ranked models using Akaike’s information criterion 

corrected for small sample size (AICc) (or small sample size 

and overdispersion (QAICc) when appropriate), and selected 

the model with the lowest AICc or QAICc as our parameterization for detection probability when building subsequent 

models for apparent survival. Under all parameterizations 

for detection probability, apparent survival was modeled as 

(φ[postF*year*studyarea + age*year*studyarea*d1toF]), 

where “*” indicates an interaction and “+” indicates an additive relationship (note that GRR was the only study area for 

least terns). A description of variables used when building 

models for chick survival is shown in table 4.

Prior to investigating our predictions of how daily chick 

survival may have changed following the flood, we sought 

to account for variation in chick survival due to chick age, 

seasonal date, and study area (see table 5 for mean and SDs 

for these covariates). Additionally, in all models of apparent 

survival, we accounted for differences in survival based on 

whether or not a chick was older than the fledge age for the 

species in question (that is, “postF” compared to “d1toF”). 

For our second model-selection step, we built models that 

estimated survival in relation to study area only, chick age at 

visit (“age”) only, chick visit date (“date”) only, both age and 

date, and all combinations of age and date by study area. In all 

cases, we modeled studyarea, date, and age effects separately 

for pre-fledge and post-fledge stages. We ranked models using 

QAICc, and selected the model with the lowest QAICc as 

our parameterization of apparent survival, which included 

parameters for detection probability that we identified during 

the first step. 
Data Analysis 9

Table 4. Explanation of variables used when building Cormack-Jolly-Seber models to estimate daily 

chick survival.

[GRR, Garrison Reach; SAK, Lake Sakakawea]

Variable Description

age Continuous, occasion-specific; age of the chick (days) when observed.

date Continuous, occasion-specific; date the chick was observed.

constant Single-level, categorical; daily survival is constant across date and age.

postF Single-level, categorical; daily survival or detection is different following fledging age.

d1toF Single-level, categorical; daily survival is different prior to fledging age.

d2toF Single-level, categorical; daily detection is different prior to fledging age.

studyarea Two-level, categorical; location where the chick was banded (GRR, SAK).

year Multilevel, categorical; year the chick hatched (2006–09, 2012–14).

preflood Single-level, categorical; years prior to the 2011 flood (2006–09).

postflood Single-level, categorical; years following the 2011 flood (2012–14).

floodtrend Continuous; years following the flood follow a log-linear trend.

firstyear Single-level, categorical; first year following the 2011 flood (2012) different than 

other years.

otheryear Single-level, categorical; second and third years following the 2011 flood (2013–14) 

different than other years.

Table 5. Means and standard deviations for the “date” (that 

is, date a chick was observed) and “age” (that is, age of a 

chick when observed) covariates used in Cormack-JollySeber models to estimate chick survival. Also reported are the 

means and standard deviations for the number of visits in the 

encounter history, estimated hatch date, and age at banding 

for piping plovers (PIPL) and least terns (LETE) captured and 

banded along the Garrison Reach of the Missouri River (LETE: 

2006–7, 2012–14; PIPL: 2007, 2012–14) and Lake Sakakawea 

(PIPL: 2006–9, 2012–14).

Species Variable Mean

Standard 

deviation

(days)

Piping plover Visits 14.9 4.3

Date July 6 15.1

Hatch date June 28 14.8 

Age 9 days 5.5 

Age at banding 2.5 days 3.5 

Least tern Visits 19.7 4.3 

Date July 12 10.2

Hatch date July 4 8.7 

Age 8 days 5.5

Age at banding 1.8 days 2.7 

Using the top-supported model from our second 

model-selection step, we next built models that reflected our 

11 a priori predictions describing how chick survival might 

have changed following the flood for plovers and terns. In this 

third model-selection step, we again built and ranked models 

separately for plovers and terns. 

We reasoned that annual variation in the effect of chick 

age and date on chick survival could underlie the response 

patterns of chick survival following the flood. Thus, for our 

fourth model-selection step, we built a series of post-hoc 

models investigating whether there was support for changes in 

annual patterns of both covariates. We constructed a similar set 

of models to the models built in the second step, but this time 

included an interaction with year as well as study area for both 

age and date. We again built and ranked models separately for 

plovers and terns.

We computed cumulative chick survival to fledging by 

taking the product of daily chick survival estimates out to 

18 days for least tern chicks and out to 25 days for piping 

plover chicks. We estimated associated standard errors using 

the Delta method and package “msm” (Jackson, 2011). We 

used a log-odds transformation of the daily survival probability (Armstrong and others, 2002) to compute associated 

95-percent confidence intervals to ensure estimates were 

bounded between 0 and 1. 
10 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Adult Survival

We built CJS models using package “RMark” in program 

R and MARK (Laake, 2013; White and Burnham, 1999) to 

estimate the annual probability of apparent survival (φ) and 

detection (p) for terns and plovers marked or resighted as 

adults on GRR and SAK during 2012–15. As previously mentioned, the probability of true survival is generally confounded 

with the probability of emigration from a site when estimating 

apparent survival using CJS. Even though USGS crews were 

present in an additional two study areas in 2014, we restricted 

our mark-recapture analysis to individuals resighted on GRR 

and SAK. Based on sightings outside this study area by USGS 

crews of plovers originally banded on GRR and SAK, we 

knew that our estimates of apparent survival would reflect 

permanent emigration out of the study area.

We completed two separate analyses to estimate annual 

apparent survival and detection for piping plovers. 

• In the first analysis, we built four-occasion markrecapture histories without regard to the study area in 

which a plover was seen (that is, as long as it was seen 

in either GRR or SAK in a year it was coded as “1”). 

The results of this analysis would provide an overall 

estimate of apparent survival and detection for the 

entire study area. 

• In the second analysis, we built separate four-occasion 

mark-recapture histories for plovers observed on SAK 

and GRR. In this analysis, there were two groups to 

reflect the two study areas; however, unlike the first 

analysis, where each marked individual only has one 

capture history, an individual could have two capture 

histories if it was observed in multiple study areas. 

The results of this analysis would give us study-areaspecific estimates of apparent survival and detection. 

In both analyses, we included resights/recaptures of 

adult plovers only.

We completed a single analysis to estimate annual apparent survival and detection for least terns. We built four-occasion mark-recapture histories without regard to the study area 

in which a tern was seen or captured (that is, as long as it was 

seen in either GRR or SAK in a year it was coded as “1”). We 

included resights/recaptures of adult terns only.

We analyzed tern and plover mark-recapture data separately following a two-step model-selection approach. We 

calculated estimates of over-dispersion separately for the tern 

and plover capture histories using the median ĉ test in program 

MARK.

For all analyses we built simple models in which detection and apparent survival were either constant or varied by 

study area (where appropriate). We ranked models using AICc 

and QAICc, and selected the model with the lowest QAICc as 

our parameterization. 

Breeding Population Size

We used the minimum breeding population (MINBPOP) 

estimator developed and described by Shaffer and others 

(2013) to estimate the breeding population (BPOP) of adult 

piping plovers (GRR: 2006–7, 2012–14; SAK: 2007–8, 

2012–14) and least terns (GRR: 2006–7, 2012–14). We 

defined the BPOP for a study area as the number of adults in 

the study area that attempted to initiate at least one nest. The 

BPOP is an unbiased estimate of breeding population size (that 

is, corrected for detection). 

To generate a BPOP estimate for each study area, we had 

to first define a metric that represented the absolute MINBPOP 

in a study area based on our periodic nest visits. This metric 

(MINBPOP) was based on the sum of the number of active 

nests, recently failed nests that had advanced to incubation, 

and previously hatched nests for each day of the nesting 

season. The MINBPOP was then computed for each day of the 

nesting season and then we used the maximum for each study 

area and year. Shaffer and others (2013) simulated MINBPOP 

and associated BPOP values for conditions in each of the two 

study areas to determine the study-area-specific detection ratio 

(MINBPOP/BPOP) necessary to compute the BPOP estimate. 

See Shaffer and others (2013) for a more detailed explanation 

including model assumptions.

The BPOP can be calculated as the MINBPOP of the 

study area divided by the estimated detection ratio for that 

study area based on the level of nest success observed in that 

study area in the given year; this assumes daily nest detection 

rates are approximately 0.5 and nest visits occur semi-weekly 

(Shaffer and others, 2013). We used previously published estimates of BPOP for pre-flood years on GRR and SAK (Shaffer 

and others, 2013) and used the regression presented in figure 

30 of Shaffer and others (2013) to extrapolate the BPOP for 

GRR and SAK in post-flood years using study-area-specific 

estimates of annual nest success (fig. 2).

Results

Nest Survival

We included 810 piping plover nests and 219 least tern 

nests initiated during pre-flood years (2006–9) in our analysis 

of nest survival (table 6). During post-flood years (2012–14), 

an additional 770 piping plover nests and 241 least tern 

nests were included in our nest survival analysis (table 6). 

An additional 68 least tern nests were tracked and fated on 

Lake Sakakawea during this time period (2006–14) but were 

not included in our nest survival analysis due to low annual 

sample sizes. On average, plover nests were discovered at a 

mean age of 5.5 days (plus or minus [±] 5.0 days SD) and tern 

nests were found at a mean age of 3.6 days (± 3.8 days SD) 

(table 3). The mean initiation date for plover nests was May 30 

(± 15 day SD) and for tern nests was June 16 (± 11.4 day SD) 

(table 3). 
Results 11

1.0

0.8

0.6

0.4

0.2

1.0

0.8

0.6

0.4

0.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Minimm breeding population/breeding population

findprob = 0.5, return = 3 findprob = 0.5, return = 7

findprob = 0.9, return = 3 findprob = 0.9, return = 7

Garrison River Reach, Gavins Point River Reach, and Lake Sakakawea

Nest success

EXPLANATION

High renesting probability

Low renesting probability

Daily nest detection probability

Return interval, in days

findprob

return

Figure 2. Minimum breeding population (MINBPOP) regressions used to extrapolate MINBPOP/breeding population (BPOP) 

values for estimating BPOP for least terns and piping plovers nesting on Lake Sakakawea and the Garrison Reach during 

2012–14. Figure and legend from Shaffer and others (2013).
12 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Table 6. Sample of piping plover and least tern nests found and followed for the purpose of nest survival analysis by 

U.S. Geological Survey crews along the Garrison Reach and Lake Sakakawea.

[Pre-flood refers to years prior to the 2011 Missouri River flood; Post-flood refers to years following the 2011 Missouri River flood; GRR, Garrison Reach; 

--, data were not collected in a given year; SAK, Lake Sakakawea]

Species Study area

Pre-flood Post-flood

Total

2006 2007 2008 2009 2012 2013 2014

Piping plover GRR 235 201 -- -- 57 138 225 856

SAK 57 109 93 115 90 122 138 724

Least tern GRR 114 105 -- -- 61 75 105 460

SAK1 -- 9 17 6 8 14 14 68

1

Although least tern nests on Lake Sakakawea were found and followed in 6 of the years, we did not include these nests in our nest survival analysis due 

to small annual sample sizes.

Piping plover nest survival varied by date and nest 

age as well as study area. Nest survival was lower on 

Lake Sakakawea than on the Garrison Reach in all years. 

Daily nest survival (DSR) in both study areas was associated with date but not in the same manner. The DSR on 

the Garrison Reach increased as the season progressed 

( β

ˆ

PIPL-GRR-sdate = 0.017 ± 0.004 SE), whereas the DSR of 

nests initiated on Lake Sakakawea decreased as the season progressed ( β

ˆ

PIPL-SAK-sdate = −0.024 ± 0.004 SE) (table 7, 

model 1). On the Garrison Reach, DSR increased with nest 

age ( β

ˆ

PIPL-GRR-sage = 0.031 ± 0.007 SE) but we observed no such 

relation for nests initiated on Lake Sakakawea.

After controlling for the influence of study area, seasonal 

date, and nest age, we found strong support for annual variation in piping plover nest survival for both study areas (figs. 3 

and 4; table 7, model 1). We additionally found weak support 

for an increase in cumulative nest survival in both study areas 

in post-flood years relative to pre-flood years (table 7, model 3 

compared to model 11). However, examination of the individual annual estimates of cumulative nest survival to 35 days 

indicates that this effect was largely driven by high nest 

survival for both study areas during 2012 (figs. 3 and 4). We 

derived these estimates by assuming “sdate” was equal to the 

mean nest initiation date (May 30) and the associated “sage” 

was equal to 1. For both study areas, we generated annual 

estimates using model S(studyarea*year + sdate*studyarea 

+ sage*studyarea) (Garrison Reach: Scumulative ± SE: S2006 = 

0.245 ± 0.019, S2007 = 0.515 ± 0.022, S2012 = 0.729 ± 0.023, 

S2013 = 0.465 ± 0.025, S2014 = 0.458 ± 0.022; Lake Sakakawea: 

Scumulative ± SE: S2006 = 0.126 ± 0.019, S2007 = 0.155 ± 0.019, S2008

= 0.226 ± 0.022, S2009 = 0.002 ± 0.001, S2012 = 0.554 ± 0.027, 

S2013 = 0.203 ± 0.021, S2014 = 0.063 ± 0.010; table 7, model 1), 

(figs. 3 and 4). We estimated cumulative nest survival during the pre-flood (2006–7 for the Garrison Reach and 2006–9 

for Lake Sakakawea) and post-flood (2012–14) periods using 

model S(flood*studyarea + sdate*studyarea + sage*studyarea) 

(table 7, model 10). The estimates (Garrison Reach: Scumulative

± SE: Spre-flood = 0.366 ± 0.020, Spost-flood = 0.499 ± 0.020; Lake 

Sakakawea: Scumulative ± SE: Spre-flood = 0.112 ± 0.012, Spost-flood = 

0.208 ± 0.017) are plotted in figures 3 and 4.

There was strong support for annual trends in the 

influence of nest age on piping plover DSR on the Garrison Reach during pre-flood years in our post-hoc analysis. 

We estimated daily nest survival rates for both study areas 

using the top-supported model from the first round of modelselection, model S(sdate*studyarea + sage*studyarea), and 

annual estimates using post-hoc model S(studyarea*year + 

sdate*studyarea*year + sage*studyarea) with the “sdate” 

value associated with “sage” = 1 solved for May 30. Daily 

survival rate increased with nest age on the Garrison Reach 

during both pre-flood years, and the age-related trends were 

strongly supported for both 2006 ( β

ˆ = 0.029 ± 0.013 SE) 

and 2007 ( β

ˆ = 0.072 ± 0.017 SE) (fig. 5); date-related trends 

were also strongly supported for Garrison piping plovers 

for 2006 ( β

ˆ = 0.031 ± 0.006) and 2007 ( βˆ = 0.017 ± 0.007) 

(fig. 6). However, there was no support for annual effects of 

age (fig. 5) or seasonal date (fig. 6) during 2012–14. On Lake 

Sakakawea, age-related trends were only supported during 

2006 ( β

ˆ = −0.059 ± 0.027; fig. 7). Seasonal date-related trends 

indicating declining nest survival with increasing seasonal 

date were supported for 2007 ( β

ˆ = −0.018 ± 0.007), 2008 

( β

ˆ = −0.041 ± 0.009), 2009 ( βˆ = −0.016 ± 0.009), and 2013 

( β

ˆ = −0.055 ± 0.009) (fig. 8). 

Least tern nest survival was explained by date and nest 

age (table 8, model 1). The DSR for tern nests initiated on 

the Garrison Reach increased with nest age ( β

ˆ

LETE-GRR-sage = 

0.553 ± 0.225 SE). Although there was no significant relation between seasonal date and nest survival, we did find 

support for an interaction between nest age and seasonal date 

( β

ˆ

LETE-GRR-sage*sdate = −0.003 ± 0.001 SE); as the nesting 

season progressed, the higher survival associated with older 

nests attenuated.
Results 13

Table 7. Ranked set of logistic-exposure models used to estimate daily piping plover nest survival following the second stage of model 

selection: examination of the effect of the 2011 Missouri River flood. Models ranked according to lowest Akaike’s Information Criterion 

(AICc) value corrected for small sample size.

[No., the model rank; Pred., the prediction the model was intended to represent; ΔAICc, the change in AICc relative to the top-ranked model; k, the number of 

parameters in the model; wi, the model weight; *, a multiplicative relationship; +, an additive relationship; <, less than]

No. Pred. Model ΔAICc k wi Deviance

1

1 2 studyarea*year + sdate*studyarea + sage*studyarea 0.00 16 0.99 5,233.31

2 3 year + sdate*studyarea + sage*studyarea 9.79 12 <0.00 5,251.12

3 6 preflood*studyarea + postflood*studyarea*floodtrend + 

sdate*studyarea + sage*studyarea

117.91 10 <0.00 5,363.25

4 11 firstyear+ otheryear + sdate*studyarea + sage*studyarea 122.27 8 <0.00 5,371.63

5 7 preflood + postflood*floodtrend + sdate*studyarea + 

sage*studyarea

122.82 8 <0.00 5,372.18

6 10 firstyear*studyarea + otheryear*studyarea + sdate*studyarea + 

sage*studyarea

124.18 10 <0.00 5,369.52

7 9 firstyear + sdate*studyarea + sage*studyarea 126.43 7 <0.00 5,377.79

8 8 firstyear*studyarea + sdate*studyarea + sage*studyarea 127.83 8 <0.00 5,377.19

9 5 preflood + postflood + sdate*studyarea + sage*studyarea 172.23 7 <0.00 5,423.59

10 4 studyarea*preflood + studyarea*postflood + sdate*studyarea + 

sage*studyarea

174.18 8 <0.00 5,423.54

11 1 sdate*studyarea + sage*studyarea 196.76 6 <0.00 5,450.12

1

AICc of top-ranked model was 5,265.37.

We generated annual nest survival estimates for least 

terns on the Garrison Reach using model S(year + sdate*sage) 

(table 8, model 1) and cumulative survival with 95-percent 

confidence intervals using the Delta method. For these and 

all subsequent least tern nest survival estimates, we derived 

these estimates by assuming “sdate” was equal to the mean 

nest initiation date (June 16) and the associated “sage” was 

equal to 1. After controlling for the influence of study area, 

date, and nest age, we found support for annual variation in 

least tern nest survival (fig. 9; Scumulative ± SE: S2006 = 0.607 

± 0.031, S2007 = 0.803 ± 0.020, S2012 = 0.748 ± 0.027, S2013 = 

0.818 ± 0.021, S2014 = 0.729 ± 0.025); however, we found no 

support to indicate this annual variation was related to any of 

our predicted effects of the flood (table 8, model 2 compared 

to models 3–6). Estimates of cumulative nest survival during 

the pre-flood (2006–7) and post-flood (2012–14) period were 

generated using model S(flood + sdate*sage) (table 8, model 4; 

Scumulative ± SE: Spre-flood = 0.708 ± 0.023, Spost-flood = 0.760 ± 

0.020). Examination of the individual annual estimates of 

cumulative nest survival to 21 days indicates that although 

cumulative nest survival was higher during 2012–14 (postflood) than in 2006 (pre-flood), it was similar to nest survival 

during 2007 (pre-flood) (fig. 10). 

We generated overall trend estimates of least tern daily 

nest survival by age using the top-supported model from 

the first round of model-selection, model S(sage + sdate + 

sage*sdate), and annual estimates using post-hoc model S(year 

+ sdate + sage*year + sdate*sage) with the “sdate” value associated with “sage” = 1 solved for June 16. Age-related trends 

were strongly supported for 2006 ( β

ˆ = 0.647 ± 0.236 SE), 

2007 ( β

ˆ = 0.680 ± 0.245 SE), 2012 ( βˆ = 0.602 ± 0.245 SE), 

2013 ( β

ˆ = 0.501 ± 0.245 SE), and 2014 ( βˆ = 0.704 ± 0.249 

SE) (fig. 10).
14 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Figure 3. Cumulative estimates of piping plover nest survival (35 days) along the Garrison Reach 

by year and for the pre-flood and post-flood periods. Pre- and post-flood period indicated with 

horizontal line.

2006 2008 2010 2012 2014

0

0.2

0.4

0.6

0.8

1.0

Garrison piping plovers

Year

Cumulative survival

Pre-flood Post-flood

Horizontal extent represents the

years of data collection

Vertical position represents the mean

across years of data collection EXPLANATION

95-percent confidence interval

Figure 4. Cumulative estimates of piping plover nest survival (35 days) on Lake Sakakawea by year and 

for the pre-flood and post-flood periods. Pre- and post-flood period indicated with horizontal line.

2006 2008 2010 2012 2014

0

0.2

0.4

0.6

0.8

1.0

Sakakawea piping plovers

Year

Cumulative survival

Pre-flood Post-flood

Horizontal extent represents the

years of data collection

Vertical position represents the mean

across years of data collection

EXPLANATION

95-percent confidence interval
Results 15

0 5 10 15 20 25 30 35

0.90

0.92

0.94

0.96

0.98

1.00

Garrison piping plovers

Nest age (days)

Daily survival

2006

2007

2012

2013

2014

EXPLANATION

Overall trend of daily survival 

by age

95-percent confidence envelope

Figure 5. Daily survival 

estimates for piping plover 

nests along the Garrison 

Reach by nest age and 

year. The solid black line 

represents an overall trend 

of daily nest survival by age, 

and the solid gray shading 

represents a 95-percent 

confidence envelope.

0

0.2

0.4

0.6

0.8

Garrison piping plovers

Initiation date

Cumulative survival

May 15 May 20 May 25 May 30 June 04 June 09 June 14

2006

2007

2012

2013

2014

EXPLANATION

Overall trend of daily survival 

by nest initiation date

95-percent confidence envelope

Figure 6. Cumulative 

estimates of piping plover 

nest survival (35 days) along 

the Garrison Reach by nest 

initiation date and year. The 

solid black line represents 

an overall trend of daily nest 

survival by nest initiation date, 

and the solid gray shading 

represents a 95-percent 

confidence envelope.
16 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

0 5 10 15 20 25 30 35

0.80

0.85

0.90

0.95

1.00

Sakakawea piping plovers

Nest age (days)

Daily survival

2006

2007

2008

2009

2012

2013

2014

EXPLANATION

Overall trend of daily 

survival by age

95-percent confidence 

envelope

Figure 7. Daily survival 

estimates for piping plover 

nests on Lake Sakakawea 

by nest age and year. The 

solid black line represents 

an overall trend of daily nest 

survival by age, and the solid 

gray shading represents 

a 95-percent confidence 

envelope.

0

0.2

0.4

0.6

0.8

Sakakawea piping plovers

Initiation date

Cumulative survival

May 15 May 20 May 25 May 30 June 04 June 09 June 14

2006

2007

2008

2009

2012

2013

2014

EXPLANATION

Overall trend of daily 

survival by nest 

initiation date

95-percent confidence 

envelope

Figure 8. Cumulative 

estimates of piping plover 

nest survival (35 days) on Lake 

Sakakawea by nest initiation 

date and year. The solid black 

line represents an overall 

trend of daily nest survival by 

nest initiation date, and the 

solid gray shading represents 

a 95-percent confidence 

envelope.
Results 17

Table 8. Ranked set of logistic exposure models used to estimate daily least tern nest survival 

following the second stage of model selection: examination of the effect of the 2011 Missouri 

River flood. Models ranked according to lowest Akaike’s Information Criterion (AICc) value 

corrected for small sample size.

[No., the model rank; Pred., the prediction the model was intended to represent; ΔAICc, the change in AICc relative to the top-ranked model; k, the number of parameters in the model; wi

, the model weight; *, a multiplicative 

relationship; +, an additive relationship]

No. Pred. Model ΔAICc k wi Deviance

1

1 3 year + sdate*sage 0.00 8 0.62 772.65

2 1 sdate*sage 3.44 4 0.11 784.13

3 9 firstyear + sdate*sage 3.80 4 0.09 784.49

4 5 preflood + postflood + sdate*sage 4.17 5 0.08 782.86

5 11 firstyear + otheryear + sdate*sage 4.58 5 0.07 783.27

6 7 preflood + postflood*floodtrend + sdate*sage 5.96 6 0.03 782.64

1

AICc of top-ranked model was 788.72.

Figure 9. Cumulative estimates of least tern nest survival (21 days) along the Garrison Reach by year 

and for the pre-flood and post-flood periods. Pre- and post-flood period indicated with horizontal line.

2006 2008 2010 2012 2014

0

0.2

0.4

0.6

0.8

1.0

Year

Cumulative survival

Pre-flood Post-flood

Garrison least terns

Horizontal extent represents the

years of data collection Vertical position represents the mean

across years of data collection

EXPLANATION

95-percent confidence interval
18 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Figure 10. Daily survival 

estimates for least tern nests 

along the Garrison Reach by 

nest age and year.

0 5 10 15 20

0.90

0.92

0.94

0.96

0.98

1.00

Garrison least terns

Nest age (days)

Daily survival

2006

2007

2012

2013

2014

EXPLANATION

Overall trend of daily survival by age

95-percent confidence envelope

Chick Survival

We included 586 uniquely marked piping plover chicks 

and 338 uniquely marked least tern chicks resighted/recaptured during pre-flood years (2006–9) in our analysis of 

chick survival to fledging (table 9). During post-flood years 

(2012–14), we included an additional 828 piping plover chicks 

and 346 least tern chicks in our analysis of chick survival 

to fledging (table 9). An additional 60 least tern chicks were 

uniquely marked and resighted/recaptured on Lake Sakakawea 

during this time period (2006–14), but were not included in 

our chick survival analysis due to low annual sample sizes. On 

average, plover chicks were banded at a mean age of 2.5 days 

(± 3.5 days SD) and tern chicks were banded at a mean age of 

1.8 days (± 2.7 days SD) (table 5). The mean hatch date for 

plover chicks was June 28 (± 14.8 day SD) and for tern chicks 

was July 4 (± 8.7 day SD) (table 5). 

Daily piping plover chick detection was determined by 

whether or not a chick had fledged, the study area where the 

chick hatched, and the year of the study. Detection probabilities were higher for pre-fledge chicks and were generally higher on Lake Sakakawea than on the Garrison Reach 

(table 10).

Piping plover chick survival was correlated with 

chick age but not with date. The DSR increased with 

chick age for plover chicks hatched on the Garrison Reach 

( β

ˆ

PIPL-GRR-age = 0.042 ± 0.021 SE) and on Lake Sakakawea 

( β

ˆ PIPL-SAK-age = 0.037 ± 0.001 SE). After controlling for 

the influence of chick age on survival, we found support 

for flood-related variation in piping plover chick survival 

(table 11, model 1). We additionally found weak support 

for an increase in cumulative chick survival in both study 

areas in post-flood years relative to pre-flood years (table 11, 

model 2 compared to model 11). We generated annual estimates using model [φ(postF*studyarea + studyarea*d1toF 

+ studyarea*year*d1toF + studyarea*d1toF*age), 

p(postF*year*studyarea + d2toF*year*studyarea)] (table 11, 

model 8), and cumulative survival and associated 95-percent 

confidence intervals (vertical bars) using the Delta method. 

Examination of the individual annual estimates of cumulative 

chick survival to fledging (25 days) indicates the two study 

areas followed separate patterns. In the first year following the 

flood (2012), chick survival on the Garrison Reach was similar 

to that estimated for 2007, but it progressively increased 

through 2014 (φcumulative ± SE: φ2007 = 0.398 ± 0.048, φ2012 = 

0.435 ± 0.079, φ2013 = 0.519 ± 0.070, φ2014 = 0.714 ± 0.072) 

(fig. 11). Chick survival steadily increased following the flood 

(fig. 11). On Lake Sakakawea, chick survival was demonstrably higher in 2012 than any of the other years (φcumulative ± SE: 

φ2007 = 0.318 ± 0.081, φ2008 = 0.177 ± 0.060, φ2012 = 0.594 ± 

0.109, φ2013 = 0.442 ± 0.097, φ2014 = 0.4694 ± 0.206) (fig. 12). 

We do not present annual estimates for 2006 and 2009 on 

Lake Sakakawea because low sample sizes precluded accurate 

estimates of annual survival in these years. We did not find 

any strong support for annual trends in the effect of chick age 

or seasonal date on piping plover chick DSR in our post-hoc 
Results 19

Table 9. Sample of piping plover and least tern chicks captured and banded by U.S. Geological Survey crews along 

the Garrison Reach and Lake Sakakawea.

[Pre-flood refers to years prior to the 2011 Missouri River flood; Post-flood refers to years following the 2011 Missouri River flood; 

GRR, Garrison Reach; --, data was not collected in a given year; SAK, Lake Sakakawea]

Species Study area

Pre-flood Post-flood

Total

2006 2007 2008 2009 2012 2013 2014

Piping plover GRR -- 355 -- -- 141 201 232 929

SAK 12 98 123 8 142 93 19 485

Least tern GRR 171 167 -- -- 109 105 132 684

SAK1 -- 9 15 6 10 17 5 60

1

Although least tern chicks on Lake Sakakawea were captured and banded in 6 of the years, we did not include these nests in our analysis 

of chick survival due to small annual sample sizes.

Table 10. Daily detection probability estimates (p) and associated 95-percent confidence intervals for uniquely 

marked pre-fledge (less than 25 days) and fledged (25+ days) piping plover chicks resighted/recaptured on the 

Garrison Reach and Lake Sakakawea for all years of the study. All estimates were generated using model [φ(postF + 

studyarea*d1toF*preflood + studyarea*d1toF*firstyear + studyarea*d1toF*otheryear + studyarea*d1toF*age), 

p(postF*year*studyarea + d2toF*year*studyarea)], (tables 10 and 11, model 1).

[GRR, Garrison Reach; SAK, Lake Sakakawea]

Study area Stage Year Estimate 95% LCL 95% UCL

GRR Pre-fledge 2007 0.522 0.481 0.562

2012 0.443 0.386 0.502

2013 0.389 0.348 0.431

2014 0.319 0.281 0.360

Fledged 2007 0.242 0.176 0.322

2012 0.275 0.182 0.392

2013 0.248 0.168 0.349

2014 0.206 0.146 0.283

SAK Pre-fledge 2006 0.660 0.592 0.721

2007 0.737 0.657 0.804

2008 0.378 0.308 0.452

2009 0.615 0.531 0.693

2012 0.675 0.492 0.817

2013 0.565 0.481 0.645

2014 0.611 0.515 0.699

Fledged 2007 0.321 0.192 0.485

2008 0.352 0.201 0.541

2012 0.111 0.062 0.191

2013 0.189 0.101 0.326

2014 0.231 0.076 0.522
20 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

analysis. Estimates of cumulative chick survival during the 

pre-flood (2007–8) and post-flood (2012–14) period were 

generated using model [φ(postF + studyarea*preflood*d1toF + 

studyarea*postflood*d1toF + studyarea*d1toF*age)] (table 11, 

model 4) for both the Garrison Reach (φcumulative ± SE: φpre-flood = 

0.398 ± 0.048, φpost-flood = 0.561 ± 0.043) and Lake Sakakawea 

(φcumulative ± SE: φpre-flood = 0.253 ± 0.051, φpost-flood = 0.508 ± 

0.068). All estimates were generated assuming “date” associated with “age” = 1 equal to the mean hatch date (June 28).

Daily least tern chick detection was determined by 

whether or not a chick had fledged, the year of the study, 

and the age of the chick. Detection probabilities were generally higher for pre-fledge chicks and declined with chick age 

(table 12).

Least tern chick survival was explained by chick age 

but not by seasonal date. Daily survival of tern chicks on 

the Garrison Reach increased with chick age ( β

ˆ

LETE-GRR-age = 

0.041 ± 0.001 SE). After controlling for chick age, we found 

strong support for annual variation in least tern chick survival (fig. 1; fig. 13, model 1). We generated annual estimates 

using model [φ(postF + d1toF + year*d1toF + d1toF*age), 

p(postF*year*age + age*year*d2toF)] (fig. 13, model 1) and 

cumulative survival and associated 95-percent confidence 

intervals using the Delta method. Specifically, the results 

indicate chick survival was markedly higher in the year immediately following the flood than in any other year (fig. 13, 

model 2 compared to models 3–6) (φcumulative ± SE: φ2006 = 

0.688 ± 0.053, φ2007 = 0.446 ± 0.055, φ2012 = 0.931 ± 0.053, 

φ2013 = 0.513 ± 0.078, φ2014 = 0.461 ± 0.113). We estimated 

cumulative chick survival during the pre-flood (2006–7) 

and post-flood (2012–14) period using model [φ(postF + 

d1toF*preflood + d1toF*postflood + d1toF*floodtrend + 

d1toF*age), p(postF*year*age + age*year*d2toF)] and found 

that post-flood survival was higher than pre-flood survival) 

(φcumulative ± SE: φpre-flood = 0.570 ± 0.040, φpost-flood = 0.752 ± 

0.063). All estimates were generated assuming “date” associated with “age” = 1 equal to the mean hatch date (July 4). 

We did not find any strong support for annual trends in the 

effect of chick age and date on least tern chick DSR in our 

post-hoc analysis. 

Figure 11. Cumulative estimates of piping plover chick survival to fledging (25 days) along the 

Garrison Reach by year and for the pre-flood and post-flood periods. Pre- and post-flood period 

indicated with horizontal line.

2006 2008 2010 2012 2014

Garrison piping plovers

Year

Cumulative survival

0.8

0

0.2

0.4

0.6

1.0

Pre-flood Post-flood

Horizontal extent represents the

years of data collection

Vertical position represents the mean

across years of data collection

EXPLANATION

95-percent confidence interval
Results 21

Figure 12. Cumulative estimates of piping plover chick survival to fledging (25 days) on Lake 

Sakakawea by year and for the pre-flood and post-flood periods. Pre- and post-flood period indicated 

with horizontal line.

2006 2008 2010 2012 2014

0

0.2

0.4

0.6

0.8

1.0

Sakakawea piping plovers

Year

Cumulative survival

Pre-flood Post-flood

Horizontal extent

represents the

years of

data collection

Vertical position represents the mean

across years of data collection

EXPLANATION

95-percent confidence interval

Table 11. Ranked set of Cormack-Jolly-Seber models used to estimate daily piping plover chick survival following the third stage 

of model selection: examination of the effect of the 2011 Missouri River flood. Models ranked according to lowest Quasi-Akaike’s 

Information Criterion (QAICc) value corrected for small sample size and overdispersion (ĉ = 2.91). For all models, detection (p) was 

parameterized as p(postF*year*studyarea + d2toF*year*studyarea).

[No., the model rank; Pred., the prediction the model was intended to represent; φ, survival; ΔQAICc, the change in QAICc relative to the top-ranked model; 

k, the number of parameters in the model; wi, the model weight; QDeviance, quasi-deviance; *, a multiplicative relationship; +, an additive relationship]

No. Pred. Model (φ) ΔQAICc k wi QDeviance

1

1 7 postF + d1toF*preflood + d1toF*postflood + d1toF*floodtrend + studyarea*d1toF*age 0.00 28 0.33 2,724.19

2 6 postF + studyarea*d1toF*preflood + studyarea*d1toF*postflood + studyarea*d1toF*floodtrend 

+ studyarea*d1toF*age

0.35 31 0.28 2,718.48

3 11 postF + d1toF*preflood + d1toF*firstyear + d1toF*otheryear + studyarea*d1toF*age 2.02 28 0.12 2,726.22

4 5 postF + studyarea*d1toF + preflood*d1toF + postflood*d1toF + studyarea*d1toF*age 2.34 28 0.11 2,726.53

5 19 postF + studyarea*d1toF*preflood + studyarea*d1toF*firstyear + studyarea*d1toF*otheryear + 

studyarea*d1toF*age

3.13 31 0.07 2,721.27

6 4 postF + studyarea*d1toF + studyarea*preflood*d1toF + studyarea*postflood*d1toF + 

studyarea*d1toF*age

3.66 29 0.05 2,725.83

7 3 postF + studyarea*d1toF + year*d1toF + studyarea*d1toF*age 4.10 33 0.04 2,718.19

8 2 postF + studyarea*year*d1toF + studyarea*d1toF*age 6.64 36 0.01 2,714.66

9 8 postF + studyarea*d1toF + studyarea*d1toF*firstyear + studyarea*d1toF*age 11.81 29 <0.00 2,733.99

10 9 postF + d1toF + d1toF*firstyear + studyarea*d1toF*age 14.23 27 <0.00 2,740.44

11 1 postF + studyarea *d1toF + studyarea*d1toF*age 14.79 27 <0.00 2,741.01

1

QAICc for top supported model is 5,983.47.
22 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Table 12. Daily detection probability estimates (p) and 

associated 95-percent confidence intervals for uniquely 

marked pre-fledge (less than 18 days) and fledged (18+ days) 

least tern chicks resighted/recaptured on the Garrison Reach 

(GRR) for all years of the study. All estimates were generated 

by taking the geometric means of age-dependent estimates 

from model [φ(postF + d1toF + year*d1toF + d1toF*age), 

p(postF*year*age + age*year*d2toF)], (fig. 13, model 1).

[LCL, lower confidence level; UCL, upper confidence level]

Stage Year Estimate 95% LCL 95% UCL

Pre-fledge 2006 0.272 0.234 0.313

2007 0.366 0.309 0.426

2012 0.195 0.163 0.231

2013 0.188 0.146 0.240

2014 0.134 0.094 0.189

Fledged 2006 0.181 0.146 0.222

2007 0.132 0.096 0.179

2012 0.144 0.109 0.186

2013 0.079 0.047 0.127

2014 0.036 0.018 0.070

Adult Survival

We included 441 uniquely marked piping plovers 

observed on the Garrison Reach and 392 plovers observed on 

Lake Sakakawea in our study-area-specific analysis of annual 

adult apparent survival and detection (table 14). Apparent 

survival of adult piping plovers was higher for individuals 

observed on the Garrison Reach (φGRR = 0.759 ± 0.019 SE) 

than for those observed on Lake Sakakawea (φSAK = 0.658 

± 0.031 SE) (fig. 1). The annual probability of detecting a 

uniquely marked piping plover adult was higher on the Garrison Reach (pGRR = 0.923 ± 0.019 SE) than on Lake Sakakawea 

(pSAK = 0.604 ± 0.043 SE).

We included 810 piping plovers in our analysis of adult 

survival for both study areas combined (table 14). The overall 

estimate of annual adult apparent survival across both study 

areas was generated using model [φ(constant), p(constant)] 

and was 0.721 ± 0.024 SE (fig. 1). Adult apparent annual 

survival estimates and associated 95-percent confidence intervals by study area were generated using model [φ(studyarea), 

p(studyarea)], and reflected higher adult survival on the Garrison Reach than on Lake Sakakawea (φGRR = 0.759 ± 0.019 SE, 

φSAK = 0.658 ± 0.031 SE). The annual probability of detecting 

a uniquely marked piping plover across both study areas was 

0.846 ± 0.029 SE.

There were no adult piping plovers marked or resighted 

in the pre-flood years and thus the results of our analyses 

reflect annual adult survival after the flood. 

Figure 13. Cumulative estimates of least tern chick survival to fledging (18 days) along the Garrison 

Reach by year and for the pre-flood and post-flood periods. Pre- and post-flood period indicated with 

horizontal line.

Pre-flood Post-flood

2006 2008 2010 2012 2014

0

0.2

0.4

0.6

0.8

1.0

Year

Cumulative survival

Garrison least terns

Horizontal extent represents the

years of data collection

Vertical position represents the mean

across years of data collection

EXPLANATION

95-percent confidence interval
Results 23

Breeding Population Size

On the Garrison Reach, our estimate of BPOP for piping 

plovers was higher in the two pre-flood years than in the year 

immediately following the flood (table 15). However, our 

estimate of BPOP for piping plovers steadily increased in 2013 

and 2014; the 2014 BPOP estimate was the highest estimate of 

BPOP for piping plovers during any of the years of the study.

On Lake Sakakawea, our piping plover BPOP estimates 

did not reflect similar patterns to what was seen on the Garrison Reach. The BPOP estimate was highest in 2007 (a preflood year), but BPOP estimates were equivalent during 2008 

(pre-flood) and 2013 (post-food). BPOP estimates were lowest 

during 2014 (a post-flood year) and second-lowest in 2012 

(the year immediately following the flood) (table 16).

Our estimates for least tern breeding population sizes did 

not reflect any annual variability among the years of the study. 

However, breeding population sizes per sampling unit were 

more variable during 2012–13 than any of the other years in 

the study (table 14).

Table 13. Ranked set of Cormack-Jolly-Seber models used to estimate daily least tern chick survival to 30 days following the third 

stage of model selection: examination of the effect of the 2011 Missouri River flood. Models ranked according to lowest Quasi-Akaike’s 

Information Criterion (QAICc) value corrected for small sample size and overdispersion (ĉ = 1.25). For all models, detection (p) was 

parameterized as p(postF*year*age + age*year*d2toF).

[No., the model rank; Pred., the prediction the model was intended to represent; φ, survival; ΔQAICc, the change in QAICc relative to the top-ranked model; k, 

the number of parameters in the model; wi, the model weight; QDeviance, quasi-deviance; *, a multiplicative relationship; +, an additive relationship; 

<, less than]

No. Pred. Model (φ) ΔQAICc k wi QDeviance

1

1 3 postF + d1toF + year*d1toF + d1toF*age 0.00 27 0.94 2,390.83

2 9 postF + d1toF*firstyear + d1toF*age 6.40 24 0.04 2,403.38

3 11 postF + d1toF*preflood + d1toF*firstyear + d1toF*otheryear + 

d1toF*age

7.27 25 0.02 2,402.20

4 7 postF + d1toF*preflood + d1toF*postflood + d1toF*floodtrend + 

d1toF*age

13.23 25 < 0.00 2,408.17

5 5 postF + preflood*d1toF + postflood*d1toF + d1toF*age 24.80 24 < 0.00 2,421.78

6 1 postF + d1toF + d1toF*age 28.83 23 < 0.00 2,427.85

1

QAICc of top-ranked model was 6,546.67.

Mark and Recapture of Least Terns

The daily detection probability of least tern chicks was 

lower in the years in which only metal and alphanumeric 

bands were used to uniquely mark chicks (2013–14) than in 

years in which terns were marked with unique combinations of 

color bands and metal bands (table 12).

During 2012–15 we recaptured a total of 54 least terns 

that were originally banded with color band only combinations 

during 2012 (table 17). Of these, only 8 (15 percent) would 

have been uniquely identifiable without recapture (that is, by 

sight only) in the year following their original banding. During 2012–15, we recaptured 36 least terns originally banded 

with alphanumeric bands in a year subsequent to their original 

banding. None of these individuals were missing alphanumeric 

bands. 

We included a total of 183 uniquely marked least terns in 

our analysis of adult survival (table 13). The annual apparent survival of uniquely marked adult least terns was 0.868 

± 0.041 SE. The annual probability of detecting a uniquely 

marked least tern was 0.590 ± 0.048 SE. There were no adult 

least terns marked or resighted in the pre-flood years and thus 

the results of our analyses reflect annual adult survival after 

the flood. 
24 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Table 14. Sample of piping plover and least tern adults used to estimate annual apparent survival. 

Plovers and terns were captured and banded during 2012–14, and resighted by U.S. Geological 

Survey crews along the Garrison Reach and Lake Sakakawea during 2012–15.

[Post-flood refers to years following the 2011 Missouri River flood. GRR, Garrison Reach; SAK, Lake Sakakawea]

Species Study area

Post-flood

Total

2012 2013 2014 2015

Piping plover GRR 93 200 341 266 441

SAK 127 187 178 138 392

Total 216 375 498 410 810

Least tern Total 59 73 124 89 183

Figure 14. Estimates of piping plover adult annual apparent survival by study area and for both study areas.

EXPLANATION

95-percent confidence interval

Estimates of piping plover adult apparent survival for all areas

Estimates of piping plover adult apparent survival by study area

0.4

0.6

0.8

1.0

Study area

Annual apparent survival

GRR SAK
Results 25

Table 15. Estimates of minimum breeding population (MINBPOP) size and breeding population 

size (BPOP) for piping plovers (PIPL) and least terns (LETE) nesting along the Garrison Reach 

before (2006–7) and after (2012–14) the 2011 Missouri River flood. Also presented are the 95-percent 

confidence intervals (CI) associated with the MINBPOP estimates and coefficient of variations (CV). 

MINBPOP values are not adjusted for imperfect detection and thus are a biased estimate of BPOP. 

Detection ratios (MINBPOP/BPOP) ranged from 0.71 to 0.98 for LETE in 2006–7 and from 0.51 to 0.79 

for PIPL in 2006–7. We used 2012–14 estimates of nest success and extrapolated from figure 30 

from Shaffer and others (2013) to determine detection ratios for PIPL and LETE in 2012–14; BPOPs 

calculated using these extrapolations are designated with a “~.”

[MINBPOP, minimum breeding population; BPOP, breeding population size; %, percent; CI, confidence interval; 

CV, coefficient of variations; ~, BPOPs calculated using extrapolations from Shaffer and others (2013)]

Species Year MINBPOP BPOP

95% CI

CV

Lower Upper

Piping plover 2006 220 374–432 188 252 7.3

2007 277 352–411 216 337 10.8

2012 175 ~180–210 98 252 22.0

2013 225 ~280–320 140 392 37.1

2014 317 ~400–450 292 342 3.9

Least tern 2006 189 231–266 168 210 5.6

2007 183 206–231 159 206 6.3

2012 220 ~240–260 98 420 45.5

2013 172 ~180–190 100 332 46.5

2014 181 ~200–210 136 228 13.0

Table 16. Estimates of minimum breeding population (MINBPOP) size and breeding population 

size (BPOP) for piping plovers (PIPL) nesting along Lake Sakakawea before (2007–8) and after 

(2012–14) the 2011 Missouri River flood. Also presented are the 95-percent confidence intervals (CI) 

associated with the MINBPOP estimates and coefficient of variations (CV). MINBPOP values are not 

adjusted for imperfect detection and thus are a biased estimate of BPOP. Detection ratios (MINBPOP 

BPOP) ranged from 0.50 to 0.67 for PIPL in 2007–8. We used 2012–14 estimates of nest success and 

extrapolated from figure 30 from Shaffer and others (2013) to determine detection ratios for PIPL in 

2012–14; BPOPs calculated using these extrapolations are designated with a “~.”

[MINBPOP, minimum breeding population; BPOP, breeding population size; %, percent; CI, confidence interval; 

CV, coefficient of variations; ~, BPOPs calculated using extrapolations from Shaffer and others (2013)]

Species Year MINBPOP BPOP

95% CI

CV

Lower Upper

Piping plover 2007 658 1065–1311 364 952 22.3

2008 590 884–1046 325 856 22.5

2012 452 ~500–570 125 779 36.2

2013 531 ~830–1000 338 724 18.2

2014 228 ~370–470 104 352 27.2
26 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Table 17. Number of individual least terns banded between 2012 

and 2014 that were missing color bands (Color) or alphanumeric 

bands (Alpha) based on the number of years between recapture.

[n, the number of individuals that were missing bands when recaptured; N, the 

total number of individuals captured]

Years

Color band Alpha band

n N n N

1 5 12 0 31

2 10 10 0 5

3 31 32 0 0

Discussion

The high river flows during the 2011 Missouri River 

flood contributed to vegetative scour on existing sandbars and 

led to the creation of new and larger sandbars along the Garrison Reach. In the years following the flood, newly exposed 

sand was eroded and vegetation growth occurred in some 

areas; however, generally there was an abundance of sandbar 

habitat that remained available to terns and plovers at the Garrison Reach. 

Early in the 2011 nesting season, Lake Sakakawea 

reached maximum water level (565 m mean sea level [MSL]) 

for the first time since the 1997 flood and the water level 

remained high for the entire season. By May 15, 2012 (the 

typical beginning of the plover nesting season), the water level 

of Lake Sakakawea had dropped to 559.4 m MSL. On average, the water level of Lake Sakakawea increased more than 

1.5 m during the plover nesting season (May 15 to June 30) 

(Anteau and others, 2012b). However, during the 2012 

nesting season, the water level only increased 0.6 m, which 

resulted in shorelines that were freshly scoured of vegetation 

and remained mostly uninundated during the nesting season 

(Anteau and others, 2014a). During 2013, the water level of 

Lake Sakakawea was 557 m MSL (May 15) and had increased 

2.5 m by the end of the nesting period (June 30). During 2014, 

Lake Sakakawea started the nesting season at 560 m MSL and 

the water level increased 2.1 m. The following is a comparison of water-level dynamics of Lake Sakakawea from 2006 

to 2009 and 2012 to 2014: (1) 2012 stands out as the best 

habitat conditions (2006–14) because a large amount of new 

habitat was exposed and remained uninundated throughout 

the season (Anteau and others, 2012a, 2014b); (2) 2013 was 

similar to 2007 and 2008, when new habitat was exposed in 

early spring and then later mostly inundated throughout the 

nesting season; and (3) 2014 was similar to that of 2009, when 

no new habitat was exposed in spring and existing habitat was 

partially inundated during the nesting season. The following sections address the five goals previously listed in the 

“Objectives” section. 

1. Estimate Survival to Hatch of Least Tern and 

Piping Plover Nests and Compare to Pre-Flood 

Estimates

The cumulative survival of piping plover nests to hatch 

(35 days) was best explained by models accounting for annual 

differences in survival probability that were not suggestive 

of any overall change in nest survival for all years following the flood; however, on Lake Sakakawea and the Garrison 

Reach, the highest cumulative nest survival estimates were in 

the year immediately following the flood. Our results indicate 

that although nest survival in 2012 may have been higher than 

usual, in part due to the effect of the flood, the influence of this 

event on nest survival for piping plovers seemed to attenuate 

over time. 

Piping plovers preferentially select nest sites with gravel 

or cobble substrate and demonstrate higher nest survival on 

sites with sparse vegetation (Gaines and Ryan, 1988). This 

tendency is explained by the increased crypsis of nests on 

cobble or gravel substrate in combination with the decreased 

outward visibility associated with dense vegetation (Anteau 

and others, 2012a). As a result, we might expect nest success 

to be highest in the first year after the flood because unvegetated habitat would be most available in both study areas. On 

the Garrison Reach, the flood resulted in the creation of new 

sandbars, which would have been ideal for piping plover nesting because they would have been unvegetated and covered 

with uneroded nesting substrate but also because they would 

have added new areas for predators to search or would have 

facilitated more space between nests. However, Catlin and 

others (2011) observed no difference in plover nest success 

between engineered sandbars, which were similar to newly 

created natural sandbars, and the older and more densely 

vegetated natural sandbars located on the Gavins Point Reach. 

Without active management, sandbars can become revegetated 

and eroded within only a few years (Catlin, 2009). By 2013, 

the sandbars that would have been newly created or scoured in 

2012 had already begun to erode and revegetate. 

Interestingly, the daily survival rates of plover nests initiated on the Garrison Reach in pre-flood years varied with seasonal date and nest age, whereas nests initiated post-flood did 

not. Prior to the flood, piping plover nest survival increased 

as the season progressed and as the age of the nest increased, 

which is likely in part due to a large proportion of early nests 

being inundated by high spring flows on the Garrison Reach 

during the pre-flood period (Shaffer and others, 2013). The 

extremely high flood stage produced post-flood sandbars that 

were substantially higher in elevation than pre-flood sandbars, 

which may have afforded post-flood nesting birds greater 

security from flow-induced nest loss. 

On Lake Sakakawea, the drop in reservoir levels between 

2011 and 2012 resulted in scoured shorelines, which is 

believed to increase the ability of piping plovers to detect 

approaching terrestrial nest predators (Anteau and others, 

2012a). Lake Sakakawea is often subjected to extreme 
Discussion 27

mid-season water-level rises that inundate nesting habitat 

and often destroy plover nests during incubation. In most 

years during which we have worked on Lake Sakakawea, 

nest flooding due to reservoir water-level rise was a predictable and marked source of nest mortality (Anteau and others, 

2012a); however, in 2012 there was no significant mid-season 

reservoir level rise. The lack of mid-season water-level rise, 

coupled with the low nesting densities and extensive nesting 

habitat available, made conditions ideal for piping plover nests 

to survive until hatching. Based on model estimates informed 

by 2006–9 nesting data, Anteau and others (2012a) predicted 

that when nest inundation was unlikely (such as 2012), plovers 

could have as much as 62–65 percent nest success (assuming 

35 and 31 days exposure, respectively). Our estimate for nest 

survival in 2012 was generally similar to that earlier prediction 

and more consistent with plovers nesting elsewhere (compare 

to Anteau and others, 2012a). In contrast, water levels during 

the spring of 2009 and 2014 were higher than the previous 

summer, thus there was no new habitat exposed. Moreover, 

in those years water levels increased more than 2 m during the nesting period, resulting in marked decreases in nest 

survival (Anteau and others, 2012a). During 2007, 2008, and 

2013, there were significant increases in water level during 

the nesting period, although these nesting periods started with 

newly exposed shorelines that allowed for water-level increase 

without inundating all of the available nesting habitat. In any 

case, intensity and timing of mid-summer increases in water 

levels is the largest known factor affecting plover nest survival 

on reservoirs (Anteau and others, 2012a), and our nest survival 

predictions were similar among groups of years that had similar water levels and water-level dynamics. 

As was the case for piping plovers, daily survival of 

least tern nests was best explained by models accounting for 

annual variability in nest survival unrelated to flood effects. 

However, where piping plover nest survival was highest in the 

year immediately following the flood, least tern nest survival 

was higher than piping plover nest survival in all years, with 

the highest cumulative nest survival occurring in the preflood year 2007. On the surface, these results indicate that the 

nesting habitat created by the flood was no better or worse 

for least terns than the nesting habitat available prior to the 

flood. Although both species clearly benefit from the creation 

of sandbars for nesting, the size of the least tern population 

coupled with the differences in their nest site selection criteria 

may explain this lack of effect (Sherfy and others, 2012b; 

Stucker and others, 2012). Terns are colonially nesting species 

which means that they tend to nest in groups, and thus the 

same number of nesting terns does not necessarily require the 

same amount of “nesting” habitat as would nesting plovers; 

for example, low-lying natural sandbars typically support 

colonies of 7–8 pairs of terns (Stucker and others, 2013). 

Stucker and others (2013) determined that nest survival was 

lower on natural sandbars relative to those engineered on the 

Gavins Point Reach. This effect was in part attributed to the 

higher elevation of these sandbars and associated decreased 

presence of wet-sand substrate (Stucker and others, 2013). 

Following the flood at the Garrison Reach, the elevation of 

sandbars was generally higher than pre-flood elevations on the 

same locations. 

2. Estimate Survival to Fledging Age of Least 

Tern and Piping Plover Chicks and Compare to 

Pre-Flood Estimates

For the Garrison Reach and Lake Sakakawea study areas, 

cumulative survival to fledging (25 days) for piping plovers 

exhibited significant annual variability. This variability was 

best explained by models that, along with supporting a difference between pre-flood and post-flood years, represented 

scenarios in which the probability of survival following the 

flood followed a log-linear trend. Chick survival on Lake 

Sakakawea and the Garrison Reach responded in different 

directions. On the Garrison Reach, post-flood piping plover 

chick survival was lowest in 2012 and increased by year until 

2014; whereas on Lake Sakakawea, the highest survival estimates for piping plover chicks occurred immediately after the 

flood, in 2012. In this region, sources of piping plover chick 

mortality typically include predation, weather events, and 

water-level rise. 

On Lake Sakakawea, rises in water levels that occur during the summer inundate potential brooding habitat for plover 

chicks (Anteau and others, 2014b,c). Although in most cases 

summer water-level rises simply reduce the amount of suitable 

brooding habitat, in some cases summer rises can completely 

inundate islands with plover broods prior to their fledging. 

Considering water-level dynamics during each year, it is not 

surprising that chick survival was near lowest during 2008 

because the water level of Lake Sakakawea during that year 

increased 3.4 m during the nesting period (May 15 to June 30) 

and increased another 2.0 m during July (U.S. Army Corps of 

Engineers, 2017), when many chicks are present. However, 

it seems that water-level dynamics are less likely implicated 

in the low chick survival observed during 2013 because, 

although water level increased 2.4 m during the nesting season, it only increased 0.1 m during July (U.S. Army Corps of 

Engineers, 2017). The greatest chick survival estimate at Lake 

Sakakawea was during 2012, when water level only increased 

slightly during the nesting season (0.6 m) and actually 

decreased 0.1 m during July (U.S. Army Corps of Engineers, 

2017). These years provide some circumstantial evidence for 

the importance of water-level dynamics during the breeding 

season on chick survival; however, there seem to be other 

important but unexplained factors affecting chick survival, as 

exemplified during 2013. 

In contrast to the Garrison Reach, the shorelines of Lake 

Sakakawea have large amounts of cobble substrate and other 

structures (for example, dead trees, rocks, sticks, and so forth) 

that can serve as cover for piping plover chicks (Anteau and 

others, 2014b). Additionally, whether due to a low density of 

ground-nesting species or predators themselves, shorelines 

along Lake Sakakawea do not seem to experience the same 
28 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

degree of predator pressure during nesting as on the Garrison 

Reach (Anteau and others, 2012a); however, chick survival 

estimates at Lake Sakakawea are generally lower than those 

on the Garrison Reach. We previously determined that island 

habitat on Lake Sakakawea had much lower fledging rates, 

possibly because of greater use of those areas by gulls and 

to some extent higher plover brood densities (Anteau and 

others, 2014c). Anteau and others (2014c) reported only weak 

evidence to support density dependence in survival of chicks 

to fledging age. It is plausible that summer increases in water 

levels are a component of mechanisms that drive density 

dependence through the decrease in available habitat. During 

2012 there was a vast amount of brooding habitat available 

throughout the summer. As predicted by Anteau and others 

(2014c), density-dependent processes would likely not be 

a strong factor affecting chick survival at Lake Sakakawea 

during 2012; however, we suspect that water-level dynamics 

during the nesting and brooding periods likely interact with 

the vulnerability of plovers to predation because they affected 

density of broods and how they utilized the habitat (Murphy 

and others, 2003; Cohen and others, 2009; Anteau and others, 

2014b,c; Wiltermuth and others, 2015). Further analysis and 

work is needed to better understand vulnerability of plover 

chicks to predation under the changing habitat conditions 

observed at Lake Sakakawea. 

On the Garrison Reach, 2012 was the post-flood year 

with the highest piping plover nest survival and the lowest 

chick survival . We did not notice any major differences on 

the Garrison Reach between 2012 and 2013–14 that could 

help explain this lower chick survival; however, the overall 

population size of breeding piping plovers was lower in 2012 

than any other year, and this was reflected in lower nesting densities during this year than in other years. Although 

high nesting densities may negatively affect piping plover 

chick survival and growth (Anteau and others, 2014c; Catlin 

and others, 2014), there is little evidence to support density 

dependent processes, including inter-brood aggression, at the 

densities observed in most breeding areas (Anteau and others, 

2014c). Although predation of chicks could be a mechanism 

of density dependence, predation rates could be independent 

of chick density due to dynamics in predator communities and 

abundance that are driven by many external factors (Sargeant 

and others, 1993); however, the Garrison Reach generally has 

much less substrate available for hiding broods in comparison 

to habitat at Lake Sakakawea. This would have been particularly true during 2012, when sandbars would have been newly 

created (and thus lacking vegetation) or newly scoured. It is 

possible that the lower nesting densities during 2012, coupled 

with little hiding substrate, made piping plovers more easily seen by predators. This speculation indicates that habitat 

features that help chicks hide from predation could have been 

a limiting factor that year. 

The cumulative survival of least terns to fledging 

(18 days) on the Garrison Reach was best explained by a 

model that contained annual differences in survival but not 

necessarily in reference to whether or not those years occurred 

pre- or post-flood. Least tern chick survival was highest during 

2012, the year immediately following the flood. We observed 

no substantial differences in habitat abundance or structure 

between 2012 and the other post-flood years other than the 

generally low amount of vegetation that could have been used 

for cover during 2012. 

Although one can make some indirect inferences on 

annual foraging conditions for piping plovers and consequences for survival based on weather and water-level dynamics (for example, Anteau, 2012a; Brudney and others, 2013; 

Anteau and others, 2014c), the same is more difficult for least 

terns. Least terns are piscivorous, semi-precocial, and dependent on the foraging success of their parents until they fledge 

(Sherfy and others, 2012a). Additionally, predictions of forage 

abundance for terns based on environmental characteristics 

are likely more tenuous than for plovers because fish generally 

live longer and occupy higher trophic levels than the invertebrate foods of plovers. Chick growth and survival are both 

affected by the availability of appropriately sized fish to feed 

least tern young (Atwood and Kelly, 1984; Massey and others, 

1992; Dugger, 1997). Although Stucker and others (2012) 

found no difference in fish forage base between natural and 

constructed sandbars, they did note that there were significant 

interannual differences in fish species abundance along the 

Gavins Point Reach and that greater fish abundances occurred 

at shallower depths. Thus, it is possible the among-year differences in least tern chick survival could reflect differences 

in the availability of their fish forage base, perhaps in relation 

to the abundance of shallow-water habitat. Because shallowwater habitats are associated with sandbar complexes, it is 

possible that one effect of the 2011 flood was the creation of 

additional shallow-water habitats, which could have led to an 

increase in the number of least tern foraging areas, foraging 

success, or prey fish habitat abundance, any combination of 

which could have led to higher chick provisioning rates and 

ultimately survival.

3. Estimate Annual Adult Survival of Piping 

Plovers and Compare to Pre-Flood Estimates

There were no estimates of annual adult survival available for the pre-flood period for piping plovers breeding on 

Lake Sakakawea or the Garrison Reach. Instead, we can 

compare our apparent survival estimates to those published 

for plovers breeding in similar habitat types under conditions 

similar to those that would have existed on both study areas 

prior to the flood. 

True survival of piping plovers nesting along riverine shoreline and sandbar habitat on Gavins Point Reach, 

Nebraska, and Lewis and Clark Lake, South Dakota, during 

2005–11 was estimated to be 0.76 (± 0.049 SE) (Catlin and 

others, 2015). Catlin and others (2015) reported adult survival 

estimates declined between 2005 and 2011 and seemed to be 

lowest in 2010 (that is, the year prior to the flood). The conditions experienced by piping plovers nesting on the Garrison 
Discussion 29

Reach in pre-flood years are probably most similar to those 

experienced in this region. Our annual apparent survival estimate for piping plovers nesting on the Garrison Reach during 

2012–14 was 0.75 (95-percent confidence interval: 0.72–0.79). 

Because apparent survival rates are a composite of survival 

and fidelity, our estimates indicate that the survival of adults 

breeding on the Garrison Reach is equivalent to that on the 

Gavins Point Reach (if fidelity is near 100 percent) or slightly 

higher. 

Of available estimates, the annual apparent survival of 

piping plovers nesting along reservoir shoreline habitat of 

Lake Diefenbaker, Saskatchewan, Canada, during 2002–8 

likely reflects conditions most similar to those experienced 

by piping plovers breeding on Lake Sakakawea. Both lakes 

are large reservoir impoundments that experience intra- and 

interannual water-level changes and are located within dispersal distance of wetlands used for nesting by piping plovers. 

During 2002–8, piping plovers nesting on Lake Diefenbaker 

experienced apparent annual survival rates of 0.76 (95-percent 

confidence interval: 0.72–0.82) (Roche and others, 2010). 

During 2012–14, plovers nesting on Lake Sakakawea experienced annual apparent survival rates of 0.65 (95-percent confidence interval: 0.59–0.72), indicating a significantly lower 

apparent survival rate than on Lake Diefenbaker.

Based on available data, it is unclear whether the apparent survival estimates for plovers on Lake Sakakawea are 

comparatively lower than those of Lake Diefenbaker due to 

differences in permanent emigration rates or true survival. 

Lake Sakakawea is positioned within close proximity to 

several other breeding habitats: a river reach, Lake Oahe, and 

hundreds of wetlands that are used by piping plovers, and 

there is evidence of dispersal among these areas both within 

and among years (Murphy and others, 2003; McCauley and 

others, 2015; Roche and others, 2016). If piping plovers nesting on Lake Sakakawea had a higher tendency to disperse than 

plovers nesting on the Garrison Reach, then a corresponding effect on post-flood apparent survival could occur. For 

example, if plovers emigrated out of Lake Sakakawea and the 

Garrison Reach and remained in the regions they emigrated 

to during 2012–14, this emigration would be captured in our 

estimate of apparent survival; however, like Lake Sakakawea, 

Lake Diefenbaker is located near nesting habitat to which plovers could permanently disperse. Accordingly, our continued 

research efforts will be better suited to address these questions 

of survival compared to emigration. 

4. Estimate Annual Breeding Population of 

Least Terns and Piping Plovers and Compare to 

Pre-Flood Estimates

The Garrison Reach and Lake Sakakawea study areas 

exhibited lower piping plover breeding population sizes in the 

year immediately following the flood (2012). On the Garrison 

Reach, the population size increased such that by 2014, levels 

were higher than in the years prior to the flood; however, on 

Lake Sakakawea, our estimate of breeding population size was 

only at pre-flood levels during 2013, with 2014 breeding population sizes significantly lower and equivalent to sizes in 2012. 

Possible explanations for the smaller breeding population sizes 

in the year immediately following the flood include a combination of the emigration of breeding adults to other northern 

Great Plains nesting locations, low reproductive success during 2010–11, and increased adult mortality during 2010–12. 

During 2011, the amount of water in the Missouri River 

system left none of the traditional nesting habitat for piping plovers available for use on the Garrison Reach or Lake 

Sakakawea. Presumably in response to loss of their traditional 

nesting areas, plovers were observed using non-traditional 

nesting habitats such as parking lots. Mark-recapture work 

in the Canadian prairies has revealed that when flooding 

occurred on reservoirs like Lake Diefenbaker, plovers moved 

to nest in nearby non-flooded locations in the Prairie Coteau 

(Roche and others, 2012). Lake Sakakawea and the Garrison 

Reach are on the western boundary of the North Dakota alkali 

lakes region, which is an area also used for nesting by piping 

plovers; therefore, plovers that traditionally nested on flooded 

sections of the Garrison Reach and Lake Sakakawea potentially had other nearby options available for nesting in 2011. 

During 2011, counts of adult piping plovers in the alkali lakes 

region during the June census were lower than in the previous 

year but higher than they would be during the next 3 years 

(Brennan and others, 2014). Unfortunately, there was no markrecapture work done during this time so it is unknown whether 

the increased population sizes in the alkali lakes were due to 

piping plovers emigrating out of the Garrison Reach and Lake 

Sakakawea study areas. If plovers did emigrate into these 

other areas during 2011 and experienced reproductive success, it is possible they would have continued to nest in these 

areas even when nesting habitat again became available on 

the Missouri River due to the association between reproductive success and site fidelity (Roche and others, 2012; Catlin 

and others, 2015). There would then be a lag time between the 

emergence of nesting habitat on the Missouri river and use of 

this habitat by nesting piping plovers. 

Because there was little nesting habitat available on the 

reservoir or river during the flood, the number of chicks that 

fledged from these two study areas was probably negligible. 

Moreover, in the 3 years leading up to the flood event, water 

levels on Lake Sakakawea were not conducive to fledging of 

plover chicks (Anteau and others, 2012a, 2014c; Wiltermuth 

and others, 2015). During all 3 years leading up to the flood 

event, water levels increased more than 2.3 m during the 

nesting period and more than 1.1 m during the brood rearing 

period (U.S. Army Corps of Engineers, 2017). Piping plovers 

do not necessarily return to nest in the same locations they 

hatch from, but based on preliminary analysis of dispersal data 

for this study and published estimates for elsewhere in the 

Missouri River system, there is some degree of site fidelity at 

least to a “study area” (Catlin and others, 2015). For example, 

of the 95 plovers resighted during 2015 that had hatched on 

the Garrison Reach in 2014, 70 of them were observed on the 
30 Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study

Garrison Reach. Similarly, of the 15 plovers observed during 

2015 that had hatched on Lake Sakakawea in 2014, 13 of them 

were observed on Lake Sakakawea. Given the tendency of 

plovers to return to nest in the study area they hatched from, 

it seems reasonable to conclude that breeding population sizes 

would have, in part, been lower during 2012 because there 

would be no 2011 cohort to recruit into the breeding population; however, this explanation alone does not account for the 

difference in breeding population sizes.

Another hypothesis for the lower numbers of adult 

breeding plovers present at the Garrison Reach and Lake 

Sakakawea during 2012 is that they suffered increased mortality between our study intervals. Plovers winter on beaches 

of the Gulf of Mexico (Elliott-Smith and Haig, 2004). The 

Deepwater Horizon oil spill occurred in the Gulf of Mexico 

during spring and summer 2010, which could have had direct 

(for example, oiling) or indirect effects (for example, reduced 

forage, increased disturbance) on mortality during winter of 

2010/2011 and the following spring migration (Henkel and 

others, 2012). During spring 2011, in addition to the Missouri 

River being flooded, much of the wetland habitats used by 

plovers were also flooded (K. Brennan, U.S. Fish and Wildlife Service, oral commun.). Although this obviously reduced 

breeding habitat for adults, it likely also forced plovers to 

forage in novel habitats. The oil spill on the wintering grounds 

and flooding conditions at nearly all of the nesting areas during the following spring in the northern Great Plains may have 

had synergistic effects that increased adult mortality during 

late 2010 through 2011.

There is also some evidence to indicate that displacement 

from a territory or traditional nesting location can result in 

lowered survival rates. This phenomenon has been observed 

in a number of bird species and is generally attributed to an 

organism’s lack of knowledge about its new environment, 

which in turn leads to a greater risk of predation and lowered 

foraging success (Brown and others, 2008). There is some evidence to indicate that the same phenomenon may affect piping 

plovers following nest habitat flooding (Cohen and GrattoTrevor, 2011; Roche and others, 2012). By mid-summer 2007, 

all traditional nesting habitat used by piping plovers on Big 

Quill Lake in Saskatchewan was completely covered by water 

and piping plovers were observed nesting in non-traditional 

habitat such as upland areas including pastures (Gratto-Trevor, 

pers. obs.). Survival analyses during this period revealed 

that adult piping plovers that had nested at Big Quill Lake 

before and (or) during the flood years had lower apparent 

survival following the year of the flood (Roche and others, 

2012). These lowered apparent survival rates did not seem 

to be explainable by increased permanent emigration during 

this period because survival rates remained lower in postflood years even when accounting for changes in emigration 

probability (Cohen and Gratto-Trevor, 2011); however, like 

the reduced recruitment between 2011 and 2012, it is highly 

unlikely that increased adult mortality alone could explain 

population size changes. 

In complete contrast with piping plovers, our pre- and 

post-flood estimates of least tern breeding population sizes 

indicate there was no detectable change following the flood. 

There are several key demographic differences between plovers and terns that may account for why one species exhibited 

a response where the other did not. Least terns have higher 

survival rates and generally live longer than plovers (Massey 

and others, 1992; Renken and Smith, 1995a). In contrast to 

a previously published study on least tern adult site fidelity 

(Renken and Smith, 1995b), mark-recapture data collected 

on the Garrison Reach and Lake Sakakawea during 2012–15 

indicated that breeding adult least terns displayed high fidelity 

to their study area. For example, out of 101 uniquely marked 

least terns observed nesting in 2015, only 8 were observed 

nesting in a different study area than they had nested in 2014. 

Unlike piping plovers, there is not much information 

available on dispersal tendencies in least terns. It is unclear 

whether the terns that normally would have nested in 2011 

simply remained on the river foraging and skipped nesting 

or whether they continued northward to find habitat on the 

Yellowstone River or other tributaries. Unlike piping plovers, 

which would have had to disperse tens of kilometers to find 

new nesting habitat during the flood year, least terns would 

have had to disperse much farther distances. The detection of 

long-distance dispersal is difficult because it requires concurrent work over a large area (Koenig and others, 1996); however, incidental sightings of banded least terns over the years 

have indicated that they are capable of very large interannual 

movements as adults. For example, Renken and Smith (1995b) 

report that at least four least terns banded as adults were 

observed during subsequent breeding seasons in Indiana and 

South Dakota, more than 300 km from their original nesting 

locations. 

Another major difference between the two species is 

in the area of natal fidelity and recruitment into the breeding population. Although piping plovers have a tendency to 

return to the same general region in which they hatched the 

year previously (Saunders and others, 2014; Catlin and others, 

2015), least terns show no such tendency (Renken and Smith, 

1995b). Importantly, least terns do not seem to recruit into 

breeding populations the year after they hatch like piping plovers do (Massey and Atwood, 1981), which means that even if 

the 2011 flood caused a decrease in recruitment to the Garrison Reach, it would not have been detected until 2013. Our 

estimates of least tern breeding population size were slightly 

lower in 2013 than 2012 but did not exhibit the same magnitude of difference as the change in piping plover breeding 

population size during the same period.

There is limited information about least tern natal dispersal in part because most of them simply do not return to 

the region in which they hatched (Renken and Smith, 1995b). 

Least terns banded on the Garrison Reach during 2012–15 
References Cited 31

have been recaptured/resighted nesting in regions hundreds 

or thousands of kilometers from their hatch location (for 

example, Nebraska, Japan) and Boyd and Thompson (1985) 

reported capturing a nesting adult in Kansas that had been 

originally banded as a juvenile in Texas (more than 1,200 km). 

Such observations indicate least terns are capable of recruiting 

into very different breeding populations than those from which 

they hatched. Given this tendency, it is unlikely that most of 

the least terns currently nesting on the Garrison Reach of the 

Missouri River actually hatched on this same stretch of river. 

The lack of change in the breeding population size following 

the flood is consistent with this assumption; recruitment would 

not necessarily have been different during 2012 because most 

of the terns recruiting into the population were not produced 

along the Garrison Reach.

5. Evaluate Feasibility of Mark-Resight 

Techniques to Estimate Site Fidelity and Annual 

Survival of Adult Least Terns

During 2012–14 we were able to successfully estimate 

chick survival to fledging and annual apparent survival for 

least terns. We determined that overall detection probability 

of least tern chicks was highest when combinations of color 

bands and stainless steel metal bands were used. This banding 

scheme resulted in higher daily detection probability of least 

tern chicks before and after fledging. A downside of using 

color bands on chicks is that due to the size of least tern chick 

legs, the number of combinations available for use is limited 

without placing bands above the tibiotarsal joints if chicks are 

banded at less than 8 days. After this point, the chicks can be 

recaptured and the color bands placed below the joint, but this 

requires an extra level of effort on the part of researchers and 

increased handling of the chicks.

We determined that banding with color bands and metal 

bands was not ideal for estimating annual survival of least 

terns after they had fledged. Least terns exhibit greater band 

loss than we see when marking piping plovers. This is true for 

metal bands, which are often slipped off the foot while terns 

are still chicks, and for color bands; we also noted band loss of 

heat-sealed color bands on adult terns. Additionally, because 

least terns do not return to nest until 2 years after hatching, 

any tern marked as a chick would have an even longer time 

period to lose color bands than is typically the case for piping plovers, ultimately making them unidentifiable without 

recapture. Beginning in 2013, we started banding all terns 

with yellow darvic alphanumeric leg bands on the lower leg 

opposite of the leg carrying the stainless steel metal band. 

Although these bands proved more difficult to resight on least 

tern chicks, they exhibited much higher band retention than 

typical color bands and in several cases remained on terns that 

had lost their stainless steel metal band. 

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For more information about this publication, contact:

Director, USGS Northern Prairie Wildlife Research Center 

8711 37th Street Southeast

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701–253–5500

For additional information, visit: https://www.usgs.gov/centers/npwrc

Publishing support provided by the 

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Anteau and others—Demographic Responses of Least Terns and Piping Plovers to the 2011 Missouri River Flood—A Large-Scale Case Study—Open-File Report 2018–1176

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20181176