The airline said it now anticipates revenue per available seat mile for the first quarter of this year to be flat to down slightly year over year, compared with earlier expectations of it to be flat or up slightly.
RBC Capital Markets analyst Walter Spracklin said the lower revenue guidance for WestJet was not material, but could hurt sentiment on the stock in the short term.
"With what appears to be strong traffic growth from a system-wide perspective, the key will be how yields are correspondingly tracking," Spracklin wrote in a report to clients.
WestJet shares were up eight cents at $25.58 in trading on the Toronto Stock Exchange.
The revised outlook by WestJet came as Canada's two biggest airlines said their flights last month had a few more empty seats compared with a year ago as they both added capacity faster than traffic grew.
Air Canada (TSX:AC.B) and WestJet both reported a lower system load factor for February compared with a year ago.
Air Canada saw its measure of how full its flights were slip to 79 per cent compared with 79.8 per cent a year ago, while WestJet's load factor fell to 84.6 per cent from 86.1 per cent in February 2013.
The decrease at Air Canada came as the airline increased capacity 4.9 per cent, as measured by available seat miles, compared with February 2012. However, passenger traffic as measured by revenue passenger miles increased 3.8 per cent.
WestJet (TSX:WJA) increased capacity by 9.2 per cent, while traffic gained 7.3 per cent compared with a year ago.
Air Canada's system-wide results include the regional airlines from which it buys capacity and Air Canada Rouge.
"Despite severe winter weather conditions at our main hub in Toronto and across much of North America, Air Canada generated greater traffic for the month of February in all markets the airline serves, led by increased traffic in the U.S. transborder market," Air Canada president and chief executive Calin Rovinescu said in a statement.
Domestic traffic at Air Canada was up 2.8 per cent, while traffic on U.S. flights was up 7.6 per cent from year ago. Traffic on flights across the Atlantic was up 1.8 per cent, the Pacific saw a 3.7 per cent increase and flights to Latin America and the Caribbean gained 3.0 per cent compared with a year ago.
Spracklin noted the Air Canada results were better than expected for the month.
"These are good numbers from Air Canada with a nice pick-up in demand across segments — in particular in the Pacific region," Spracklin wrote.
"This bodes well with regards to new capacity Air Canada is rolling out and anticipated pricing strength on international routes, in our view. We also are encouraged by the ramp in domestic traffic growth and the continued strong demand coming out of U.S. transborder routes."
Air Canada shares were up 12 cents at $6.60 while WestJet stock gained 36 cents to $25.86.