Appendix 1. Selection table for detection models used to predict the number of observations recorded per survey period. This represented the first stage of our two-tier model selection process and was intended to determine which variables most influenced detection probability. Model covariates included Sandhill Crane migration related variables derived from weekly aerial surveys (see Caven et al. 2019 for methods) of the Central Platte River Valley such as the number of days from the peak of migration that observations were made (Days to Peak), daily Sandhill Crane abundance estimated using Kalman smoothing (SACR-Kalman) and linear (SACR-Linear) missing value imputation as well as date-related variables (day of calendar year – Date and the quadratic transformation of day of year – Date (quadratic)), flow (river flow via USGS gage station 06770200, Kearney, NE), and observer (Obs.). All models outperforming the null model are presented in this table. Models were ranked via Akaike Information Criterion corrected for small sample sizes (AICc) and are presented with coefficient estimates and significance levels (***p < 0.001, **p < 0.01, *p < 0.05, ^p < 0.10) for individual covariates, as well as model degrees of freedom (df), log-likelihood (Log Lik.), AICc, ΔAICc, and model weight (Wt.). Covariates in models outperforming the null model with ΔAIC < 2.0 and a model weight > 0.10 included Days to Peak, Flow, and Date (quadratic) and were advanced to the second tier of model selection.

Model†	Days to Peak	SACR-Kalman	SACR-Linear	Date	Date (quadratic)	Flow	Obs.	df	Log Lik.	AICc	ΔAICc	Wt.
Mig. & Flow	-0.03806***					-0.01530**		4	-402.5	813.5	0.000	0.433
Date & Flow					+***	-0.01243**		5	-401.4	813.6	0.100	0.413
Date					+***			4	-404.3	817.1	3.640	0.070
Mig. & Flow			9.88E-07**			-0.01306**		4	-405.1	818.7	5.200	0.032
Mig.	-0.03035**							3	-406.5	819.3	5.820	0.024
Mig. & Flow		7.37E-07*				-0.01259*		4	-406.3	821.0	7.550	0.010
Mig.			8.06E-07*					3	-408.0	822.3	8.800	0.005
Date & Flow				0.01002*		-0.00972^		4	-407.6	823.7	10.190	0.003
Mig.		5.77E-07*						3	-408.9	824.1	10.590	0.002
Obs.							+**	8	-403.1	824.2	10.750	0.002
Date				0.00915^				3	-409.3	824.9	11.400	0.001
Flow						-0.00894^		3	-409.3	824.9	11.420	0.001
Null								2	-410.7	825.6	12.060	0.001
† “Mig.” = “Migration” or migration related variables derived weekly from aerial surveys conducted by the Crane Trust. “Flow” indicates the flow recorded at the nearest USGS gaging station near Kearney, Nebraska USA (06770200). “Date” indicates variables related to the day of year. “Obs.” indicates observers performing surveys.